National Ambient Air Quality Standards for Particulate Matter
[Federal Register: January 17, 2006 (Volume 71, Number 10)]
[Proposed Rules]
[Page 2619-2708]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr17ja06-21]
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ENVIRONMENTAL PROTECTION AGENCY
40 CFR Part 50
[OAR-2001-0017; FRL-8015-8]
RIN 2060-AI44
National Ambient Air Quality Standards for Particulate Matter
AGENCY: Environmental Protection Agency (EPA).
ACTION: Proposed rule.
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SUMMARY: Based on its review of the air quality criteria and national
ambient air quality standards (NAAQS) for particulate matter (PM), EPA
proposes to make revisions to the primary and secondary NAAQS for PM to
provide requisite protection of public health and welfare,
respectively, and to make corresponding revisions in monitoring
reference methods and data handling conventions for PM.
With regard to primary standards for fine particles (particles
generally less than or equal to 2.5 micrometers ([mu]m) in diameter,
PM2.5), EPA proposes to revise the level of the 24-hour
PM2.5 standard to 35 micrograms per cubic meter ([mu]g/
m3), providing increased protection against health effects
associated with short-term exposure (including premature mortality and
increased hospital admissions and emergency room visits) and to retain
the level of the annual PM2.5 standard at 15 [mu]g/
m3, continuing protection against health effects associated
with long-term exposure (including premature mortality and development
of chronic respiratory disease). The EPA solicits comment on
alternative levels of the 24-hour PM2.5 standard (down to 25
[mu]g/m3 and up to 65 [mu]g/m3) and the annual
PM2.5 standard (down to 12 [mu]g/m3), and on
alternative approaches for selecting the standard levels.
With regard to primary standards for particles generally less than
or equal to 10 [mu]m in diameter (PM10), EPA proposes to
revise the 24-hour PM10 standard in part by establishing a
new indicator for thoracic coarse particles (particles generally
between 2.5 and 10 [mu]m in diameter, PM10-2.5), qualified
so as to include any ambient mix of PM10-2.5 that is
dominated by resuspended dust from high-density traffic on paved roads
and PM generated by industrial sources and construction sources, and
excludes any ambient mix of PM10-2.5 that is dominated by
rural windblown dust and soils and PM generated by agricultural and
mining sources. The EPA proposes to set the new PM10-2.5
standard at a level of 70 [mu]g/m3, continuing to provide a
generally equivalent level of protection against health effects
associated with short-term exposure (including hospital admissions for
cardiopulmonary diseases, increased respiratory symptoms and possibly
premature mortality). Also, EPA proposes to revoke, upon finalization
of a primary 24-hour standard for PM10-2.5, the current 24-
hour PM10 standard in all areas of the country except in
areas where there is at least one monitor located in an urbanized area
(as defined by the U.S. Bureau of the Census) with a minimum population
of 100,000 that violates the current 24-hour PM10 standard
based on the most recent three years of data. In addition, EPA proposes
to revoke the current annual PM10 standard upon promulgation
of this rule. The EPA solicits comment on alternative approaches for
selecting the level of a 24-hour PM10-2.5 standard, on
alternative approaches based on retaining the current 24-hour
PM10 standard, and on revoking and not replacing the 24-hour
PM10 standard.
With regard to secondary PM standards, EPA proposes to revise the
current standards by making them identical to the suite of proposed
primary standards for fine and coarse particles, providing protection
against PM-related public welfare effects including visibility
impairment, effects on vegetation and ecosystems, and materials damage
and soiling. Also, EPA solicits comment on adding a new sub-daily
PM2.5 standard to address visibility impairment.
DATES: Written comments on this proposed decision must be received by
April 17, 2006.
ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-
OAR-2001-0017 by one of the following methods:
? http://www.regulations.gov: 
Follow the on-line
instructions for submitting comments.
? E-mail: a-and-r-Docket@epa.gov.
? Fax: 202-566-1749.
? Mail: Docket ID No. EPA-HQ-OAR-2001-0017, Environmental
Protection Agency, Mailcode: 6102T, 1200 Pennsylvania Avenue, NW.,
Washington, DC 20460. Please include a total of two copies.
? Hand Delivery: Environmental Protection Agency, EPA West
Building, Room B102, 1301 Constitution Avenue, NW., Washington, DC.
Such deliveries are only accepted during the Docket's normal hours of
operation, and special arrangements should be made for deliveries of
boxed information.
Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-
2001-0017. The EPA's policy is that all comments received will be
included in the public docket without change and may be made available
online at http://www.regulations.gov, including any personal
information provided, unless the comment includes information claimed
to be Confidential Business Information (CBI) or other information
whose disclosure is restricted by statute. Do not submit information
that you consider to be CBI or otherwise protected through http://
www.regulations.gov or e-mail. The http://www.regulations.gov
Web site is an ``anonymous access'' system, which means EPA will not
know your identity or contact information unless you provide it in the body
of your comment. If you send an e-mail comment directly to EPA without
going through http://www.regulations.gov your e-mail address
will be automatically captured and included as part of the comment that is
placed in the public docket and made available on the Internet. If you
submit an electronic comment, EPA recommends that you include your name
and other contact information in the body of your comment and with any
disk or CD-ROM you submit. If EPA cannot read your comment due to
technical difficulties and cannot contact you for clarification, EPA
may not be able to consider your comment. Electronic files should avoid
the use of special characters, any form of encryption, and be free of
any defects or viruses. For additional information about EPA's public
docket visit the EPA Docket Center homepage at
http://www.epa.gov/epahome/dockets.htm.
Docket: All documents in the docket are listed in the http://
www.regulations.gov index. Although listed in the index, some
information is not publicly available, e.g., CBI or other information
whose disclosure is restricted by statute. Certain other material, such
as copyrighted material, will be publicly available only in hard copy.
Publicly available docket materials are available either electronically
in http://www.regulations.gov or in hard copy at the Air and
Radiation Docket and Information Center, EPA/DC, EPA West, Room B102, 1301
Constitution Ave., NW., Washington, DC. The Public Reading Room is open
from 8:30 a.m. to 4:30 p.m. Monday through Friday, excluding legal
holidays. The telephone number for the Public Reading Room is 202-566-
1744 and the telephone number for the Air and Radiation Docket and
Information Center is 202-566-1742.
Public Hearings: The EPA intends to hold public hearings around the
end of
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February in Philadelphia, Chicago, and San Francisco, and will announce
in a separate Federal Register notice the date, time, and address of
the public hearings on this proposed decision.
FOR FURTHER INFORMATION CONTACT: Dr. Erika Sasser, mail code C539-01,
Air Quality Strategies and Standards Division, Office of Air Quality
Planning and Standards, U.S. Environmental Protection Agency, Research
Triangle Park, North Carolina 27711, telephone: (919) 541-3889, e-mail:
sasser.erika@epa.gov.
SUPPLEMENTARY INFORMATION:
General Information
A. What Should I Consider As I Prepare My Comments for EPA?
1. Submitting CBI. Do not submit this information to EPA through
http://www.regulations.gov or e-mail. Clearly mark the part or
all of the information that you claim to be CBI. For CBI information in a
disk or CD-ROM that you mail to EPA, mark the outside of the disk or CD-ROM
as CBI and then identify electronically within the disk or CD-ROM the
specific information that is claimed as CBI. In addition to one
complete version of the comment that includes information claimed as
CBI, a copy of the comment that does not contain the information
claimed as CBI must be submitted for inclusion in the public docket.
Information so marked will not be disclosed except in accordance with
procedures set forth in 40 CFR part 2.
2. Tips for Preparing Your Comments. When submitting comments,
remember to:
? Identify the rulemaking by docket number and other
identifying information (subject heading, Federal Register date and
page number).
? Follow directions--The agency may ask you to respond to
specific questions or organize comments by referencing a Code of
Federal Regulations (CFR) part or section number.
? Explain why you agree or disagree; suggest alternatives
and substitute language for your requested changes.
? Describe any assumptions and provide any technical
information and/or data that you used.
? If you estimate potential costs or burdens, explain how
you arrived at your estimate in sufficient detail to allow for it to be
reproduced.
? Provide specific examples to illustrate your concerns, and
suggest alternatives.
? Explain your views as clearly as possible, avoiding the
use of profanity or personal threats.
? Make sure to submit your comments by the comment period
deadline identified.
Availability of Related Information
A number of documents are available on EPA Web sites. The Air
Quality Criteria for Particulate Matter (Criteria Document) (two
volumes, EPA/600/P-99/002aF and EPA/600/P-99/002bF, October 2004) is
available on EPA's National Center for Environmental Assessment Web
site. To obtain this document, go to http://www.epa.gov/ncea, and click
on ``Particulate Matter''. The Staff Paper, human health risk
assessment, and several other related technical documents are available
on EPA's Office of Air Quality Planning and Standards (OAQPS)
Technology Transfer Network (TTN) Web site. The Staff Paper is
available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_sp.html
, and the risk assessment and technical documents are available
at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_td.html. These
and other related documents are also available for inspection and
copying in the EPA docket identified above.
Table of Contents
The following topics are discussed in today's preamble:
I. Background
A. Legislative Requirements
B. Review of Air Quality Criteria and Standards for PM
C. Related Control Programs to Implement PM Standards
D. Overview of Current PM NAAQS Review
II. Rationale for Proposed Decisions on Primary PM2.5 Standards
A. Health Effects Related to Exposure to Fine Particles
1. Mechanisms
2. Nature of Effects
3. Integration and Interpretation of the Health Evidence
4. Sensitive Subgroups for PM2.5-Related Effects
5. PM2.5-Related Impacts on Public Health
B. Quantitative Risk Assessment
1. Overview
2. Scope and Key Components
3. Risk Estimates and Key Observations
C. Need for Revision of the Current Primary PM2.5 Standards
D. Indicator of Fine Particles
E. Averaging Time of Primary PM2.5 Standards
F. Form of Primary PM2.5 Standards
1. 24-Hour PM2.5 Standard
2. Annual PM2.5 Standard
G. Level of Primary PM2.5 Standards
1. 24-Hour PM2.5 Standard
2. Annual PM2.5 Standard
H. Proposed Decisions on Primary PM2.5 Standards
III. Rationale for Proposed Decisions on the Primary PM10 Standards
A. Health Effects Related to Exposure to Thoracic Coarse Particles
1. Mechanisms
2. Nature of Effects
3. Integration and Interpretation of the Health Evidence
4. Sensitive Subgroups for Effects of Thoracic Coarse Particle Exposure
5. Impacts on Public Health from Thoracic Coarse Particle Exposure
B. Quantitative Risk Assessment
C. Need for Revision of the Current Primary PM10 Standards
D. Indicator of Thoracic Coarse Particles
E. Averaging Time of Primary PM10-2.5 Standard
F. Form of Primary PM10-2.5 Standard
G. Level of Primary PM10-2.5 Standard
H. Proposed Decisions on Primary PM10-2.5 Standard
IV. Rationale for Proposed Decisions on Secondary PM Standards
A. Visibility Impairment
1. Visibility Impairment Related to Ambient PM
2. Need for Revision of the Current Secondary PM Standards for
Visibility Protection
3. Indicator of PM for Secondary Standard to Address Visibility
Impairment
4. Averaging Time of a Secondary PM2.5 Standard for
Visibility Protection
5. Elements of a Secondary PM2.5 Standard for
Visibility Protection
B. Other PM-related Welfare Effects
1. Nature of Effects
2. Need for Revision of Current Secondary PM Standards to
Address Other PM-related Welfare Effects
C. Proposed Decision on Secondary PM Standards
V. Interpretation of the NAAQS for PM
A. Proposed Amendments to Appendix N--Interpretation of the
National Ambient Air Quality Standards for PM2.5
1. General
2. PM2.5 Monitoring and Data Reporting Considerations
3. PM2.5 Computations and Data Handling Conventions
4. Secondary Standard
5. Conforming Revisions
B. Proposed Appendix P--Interpretation of the National Ambient
Air Quality Standards for PM10-2.5
1. General
2. PM2.5 Data Reporting Considerations
3. PM10-2.5 Computations and Data Handling Conventions
4. Exceptional Events
VI. Reference Methods for the Determination of Particulate Matter as
PM2.5 and PM10-2.5
A. Proposed Appendix O: Reference Method for the Determination
of Coarse Particulate Matter (as PM10-2.5) in the Atmosphere
1. Purpose of the New Reference Method
2. Rationale for Selection of the New Reference Method
3. Consideration of Other Methods for the Federal Reference Method
4. Consideration of Automated Method
5. Relationship of Proposed FRM to Transportation Equity Act Requirements
6. Use of the Proposed Federal Reference Method
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7. Basic Requirements of the Proposed Federal Reference Method Sampler
8. Other Important Aspects of the Proposed Federal Reference
Method Sampler
B. Proposed Amendments to Appendix L--Reference Method for the
Determination of Fine Particulate Matter (as PM2.5) in
the Atmosphere
VIII. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review
B. Paperwork Reduction Act
C. Regulatory Flexibility Act
D. Unfunded Mandates Reform Act
E. Executive Order 13132: Federalism
F. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
G. Executive Order 13045: Protection of Children from
Environmental Health and Safety Risks
H. Executive Order 13211: Actions That Significantly Affect
Energy Supply, Distribution or Use
I. National Technology Transfer Advancement Act
J. Executive Order 12898: Federal Actions To Address
Environmental Justice in Minority Populations and Low-Income
Populations
References
I. Background
A. Legislative Requirements
Two sections of the Clean Air Act (CAA) govern the establishment
and revision of the NAAQS. Section 108 (42 U.S.C. 7408) directs the
Administrator to identify and list ``air pollutants'' that ``in his
judgment, may reasonably be anticipated to endanger public health and
welfare'' and whose ``presence * * * in the ambient air results from
numerous or diverse mobile or stationary sources'' and to issue air
quality criteria for those that are listed. Air quality criteria are
intended to ``accurately reflect the latest scientific knowledge useful
in indicating the kind and extent of identifiable effects on public
health or welfare which may be expected from the presence of [a]
pollutant in ambient air * * *.''
Section 109 (42 U.S.C. 7409) directs the Administrator to propose
and promulgate ``primary'' and ``secondary'' NAAQS for pollutants
listed under section 108. Section 109(b)(1) defines a primary standard
as one ``the attainment and maintenance of which in the judgment of the
Administrator, based on such criteria and allowing an adequate margin
of safety, are requisite to protect the public health.'' \1\ A
secondary standard, as defined in section 109(b)(2), must ``specify a
level of air quality the attainment and maintenance of which, in the
judgment of the Administrator, based on such criteria, is requisite to
protect the public welfare from any known or anticipated adverse
effects associated with the presence of [the] pollutant in the ambient
air.'' \2\
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\1\ The legislative history of section 109 indicates that a
primary standard is to be set at ``the maximum permissible ambient
air level * * * which will protect the health of any [sensitive]
group of the population,'' and that for this purpose ``reference
should be made to a representative sample of persons comprising the
sensitive group rather than to a single person in such a group'' [S.
Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970)].
\2\ Welfare effects as defined in section 302(h) [42 U.S.C.
7602(h)] include, but are not limited to, ``effects on soils, water,
crops, vegetation, man-made materials, animals, wildlife, weather,
visibility and climate, damage to and deterioration of property, and
hazards to transportation, as well as effects on economic values and
on personal comfort and well-being.''
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In setting standards that are ``requisite'' to protect public
health and welfare, as provided in section 109(b), EPA's task is to
establish standards that are neither more nor less stringent than
necessary for these purposes. In establishing ``requisite'' primary and
secondary standards, EPA may not consider the costs of implementing the
standards. See generally Whitman v. American Trucking Associations, 531
U.S. 457, 465-472, 475-76 (2001).
The requirement that primary standards include an adequate margin
of safety was intended to address uncertainties associated with
inconclusive scientific and technical information available at the time
of standard setting. It was also intended to provide a reasonable
degree of protection against hazards that research has not yet
identified. Lead Industries Association v. EPA, 647 F.2d 1130, 1154
(D.C. Cir 1980), cert. denied, 449 U.S. 1042 (1980); American Petroleum
Institute v. Costle, 665 F.2d 1176, 1186 (D.C. Cir. 1981), cert.
denied, 455 U.S. 1034 (1982). Both kinds of uncertainties are
components of the risk associated with pollution at levels below those
at which human health effects can be said to occur with reasonable
scientific certainty. Thus, in selecting primary standards that include
an adequate margin of safety, the Administrator is seeking not only to
prevent pollution levels that have been demonstrated to be harmful but
also to prevent lower pollutant levels that may pose an unacceptable
risk of harm, even if the risk is not precisely identified as to nature
or degree. The CAA does not require the Administrator to establish a
primary NAAQS at a zero-risk level or at background concentration
levels (see Lead Industries Association v. EPA, supra, 647 F.2d at 1156
n. 51), but rather at a level that reduces risk sufficiently so as to
protect public health with an adequate margin of safety.
In addressing the requirement for an adequate margin of safety, EPA
considers such factors as the nature and severity of the health effects
involved, the size of the sensitive population(s) at risk, and the kind
and degree of the uncertainties that must be addressed. The selection
of any particular approach to providing an adequate margin of safety is
a policy choice left specifically to the Administrator's judgment. Lead
Industries Association v. EPA, supra, 647 F.2d at 1161-62.
Section 109(d)(1) of the CAA requires that ``not later than
December 31, 1980, and at 5-year intervals thereafter, the
Administrator shall complete a thorough review of the criteria
published under section 108 and the national ambient air quality
standards * * * and shall make such revisions in such criteria and
standards and promulgate such new standards as may be appropriate * *
*.'' Section 109(d)(2) requires that an independent scientific review
committee ``shall complete a review of the criteria * * * and the
national primary and secondary ambient air quality standards * * * and
shall recommend to the Administrator any new * * * standards and
revisions of existing criteria and standards as may be appropriate * *
*.'' This independent review function is performed by the Clean Air
Scientific Advisory Committee (CASAC) of EPA's Science Advisory Board.
B. Review of Air Quality Criteria and Standards for PM
Particulate matter is the generic term for a broad class of
chemically and physically diverse substances that exist as discrete
particles (liquid droplets or solids) over a wide range of sizes.
Particles originate from a variety of anthropogenic stationary and
mobile sources as well as from natural sources. Particles may be
emitted directly or formed in the atmosphere by transformations of
gaseous emissions such as sulfur oxides (SOX), nitrogen
oxides (NOX), and volatile organic compounds (VOC). The
chemical and physical properties of PM vary greatly with time, region,
meteorology, and source category, thus complicating the assessment of
health and welfare effects.
The last review of PM air quality criteria and standards was
completed in July 1997 with notice of a final decision to revise the
existing standards (62 FR 38652, July 18, 1997). In that decision, EPA
revised the PM NAAQS in several respects. While EPA determined that the
PM NAAQS should continue to focus on particles less than or equal to 10
[mu]m in
[[Page 2623]]
diameter (PM10), EPA also determined that the fine and
coarse fractions of PM10 should be considered separately.
The EPA added new standards, using PM2.5 as the indicator
for fine particles (with PM2.5 referring to particles with a
nominal mean aerodynamic diameter less than or equal to 2.5 [mu]m), and
retained PM10 standards for the purpose of regulating the
coarse fraction of PM10 (referred to as thoracic coarse
particles or coarse-fraction particles; generally including particles
with a nominal mean aerodynamic diameter greater than 2.5 [mu]m and
less than or equal to 10 [mu]m, or PM10-2.5). The EPA
established two new PM2.5 standards: an annual standard of
15 [mu]g/m3, based on the 3-year average of annual
arithmetic mean PM2.5 concentrations from single or multiple
community-oriented monitors; and a 24-hour standard of 65 [mu]g/
m3, based on the 3-year average of the 98th percentile of
24-hour PM2.5 concentrations at each population-oriented
monitor within an area. Also, EPA established a new reference method
for the measurement of PM2.5 in the ambient air and adopted
rules for determining attainment of the new standards. To continue to
address thoracic coarse particles, EPA retained the annual
PM10 standard, while revising the form, but not the level,
of the 24-hour PM10 standard to be based on the 99th
percentile of 24-hour PM10 concentrations at each monitor in
an area. The EPA revised the secondary standards by making them
identical in all respects to the primary standards.
Following promulgation of the revised PM NAAQS, petitions for
review were filed by a large number of parties, addressing a broad
range of issues. In May 1999, a three-judge panel of the U.S. Court of
Appeals for the District of Columbia Circuit issued an initial decision
that upheld EPA's decision to establish fine particle standards,
holding that ``the growing empirical evidence demonstrating a
relationship between fine particle pollution and adverse health effects
amply justifies establishment of new fine particle standards.''
American Trucking Associations v. EPA, 175 F.3d 1027, 1055-56 (D.C.
Cir. 1999) (rehearing granted in part and denied in part, 195 F.3d 4
(D.C. Cir. 1999), affirmed in part and reversed in part, Whitman v.
American Trucking Associations, 531 U.S. 457 (2001). The Panel also
found ``ample support'' for EPA's decision to regulate coarse particle
pollution, but vacated the 1997 PM10 standards, concluding
in part that PM10 is a ``poorly matched indicator for coarse
particulate pollution'' because it includes fine particles. Id. at
1053-55. Pursuant to the court's decision, EPA removed the vacated 1997
PM10 standards from the Code of Federal Regulations (CFR)
(69 FR 45592, July 30, 2004) and deleted the regulatory provision (at
40 CFR 50.6(d)) that controlled the transition from the pre-existing
1987 PM10 standards to the 1997 PM10 standards
(65 FR 80776, December 22, 2000). The pre-existing 1987 PM10
standards remained in place. Id. at 80777.
More generally, the three-judge panel held (with one dissenting
opinion) that EPA's approach to establishing the level of the standards
in 1997, both for PM and for ozone NAAQS promulgated on the same day,
effected ``an unconstitutional delegation of legislative authority.''
Id. at 1034-40. Although the panel stated that ``the factors EPA uses
in determining the degree of public health concern associated with
different levels of ozone and PM are reasonable,'' it remanded the rule
to EPA, stating that when EPA considers these factors for potential
non-threshold pollutants ``what EPA lacks is any determinate criterion
for drawing lines'' to determine where the standards should be set.
Consistent with EPA's long-standing interpretation, the panel also
reaffirmed prior rulings holding that in setting NAAQS EPA is ``not
permitted to consider the cost of implementing those standards.'' Id.
at 1040-41.
Both sides filed cross appeals on these issues to the United States
Supreme Court, and the Court granted certiorari. In February 2001, the
Supreme Court issued a unanimous decision upholding EPA's position on
both the constitutional and cost issues. Whitman v. American Trucking
Associations, 531 U.S. 457, 464, 475-76. On the constitutional issue,
the Court held that the statutory requirement that NAAQS be
``requisite'' to protect public health with an adequate margin of
safety sufficiently guided EPA's discretion, affirming EPA's approach
of setting standards that are neither more nor less stringent than
necessary. The Supreme Court remanded the case to the Court of Appeals
for resolution of any remaining issues that had not been addressed in
that court's earlier rulings. Id. at 475-76. In March 2002, the Court
of Appeals rejected all remaining challenges to the standards, holding
under the traditional standard of judicial review that EPA's
PM2.5 standards were reasonably supported by the
administrative record and were not ``arbitrary and capricious.''
American Trucking Associations v. EPA, 283 F.3d 355, 369-72 (D.C. Cir. 2002).
In October 1997, EPA published its plans for the current periodic
review of the PM criteria and NAAQS (62 FR 55201, October 23, 1997),
including the 1997 PM2.5 standards and the 1987
PM10 standards. As part of the process of preparing an
updated Air Quality Criteria Document for Particulate Matter
(henceforth, the ``Criteria Document''), EPA's National Center for
Environmental Assessment (NCEA) hosted a peer review workshop in April
1999 on drafts of key Criteria Document chapters. The first external
review draft Criteria Document was reviewed by CASAC and the public at
a meeting held in December 1999. Based on CASAC and public comment,
NCEA revised the draft Criteria Document and released a second draft in
March 2001 for review by CASAC and the public at a meeting held in July
2001. A preliminary draft of a staff paper, Review of the National
Ambient Air Quality Standards for Particulate Matter: Assessment of
Scientific and Technical Information (henceforth, the ``Staff Paper'')
prepared by EPA's Office of Air Quality Planning and Standards (OAQPS)
was released in June 2001 for public comment and for consultation with
CASAC at the same public meeting. Taking into account CASAC and public
comments, a third draft Criteria Document was released in May 2002 for
review at a meeting held in July 2002.
Shortly after the release of the third draft Criteria Document, the
Health Effects Institute (HEI) \3\ announced that researchers at Johns
Hopkins University had discovered problems with applications of
statistical software used in a number of important epidemiological
studies that had been discussed in that draft Criteria Document. In
response to this significant issue, EPA took steps in consultation with
CASAC to encourage researchers to reanalyze affected studies and to
submit them expeditiously for peer review by a special expert panel
convened at EPA's request by HEI. The results of this reanalysis and
peer-review process were subsequently incorporated into a fourth draft
Criteria Document, which was released in June 2003 and reviewed by
CASAC and the public at a meeting held in August 2003.
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\3\ The HEI is an independent research institute, jointly
sponsored by EPA and a group of U.S. manufacturers and marketers of
motor vehicles and engines, that conducts health effects research on
major air pollutants related to motor vehicle emissions.
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The first draft Staff Paper, based on the fourth draft Criteria
Document, was released at the end of August 2003, and was reviewed by
CASAC and the public at a meeting held in November 2003.
[[Page 2624]]
During that meeting, EPA also consulted with CASAC on a new framework
for the final chapter (integrative synthesis) of the Criteria Document
and on ongoing revisions to other Criteria Document chapters to address
previous CASAC comments. The EPA held additional consultations with
CASAC at public meetings held in February, July, and September 2004,
leading to publication of the final Criteria Document in October 2004.
The second draft Staff Paper, based on the final Criteria Document, was
released at the end of January 2005, and was reviewed by CASAC and the
public at a meeting held in April 2005. The CASAC's advice and
recommendations to the Administrator, based on its review of the second
draft Staff Paper, were further discussed during a public
teleconference held in May 2005 and are provided in a June 6, 2005
letter to the Administrator (Henderson, 2005a). The final Staff Paper,
issued in June, 2005, takes into account the advice and recommendations
of CASAC and public comments received on the earlier drafts of this
document. The Administrator subsequently received additional advice and
recommendations from the CASAC, specifically on potential standards for
thoracic coarse particles in a teleconference on August 11, 2005, and
in a letter to the Administrator dated September 15, 2005 (Henderson,
2005b).\4\
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\4\ The EPA has posted on its Web site (http://www.epa.gov/ttn/naaqs/
standards/pm/s_pm_index.html) a second edition of the Staff
Paper which was prepared for the purpose of including as an
attachment this September 2005 letter from CASAC.
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The schedule for completion of this review is governed by a consent
decree resolving a lawsuit filed in March 2003 by a group of plaintiffs
representing national environmental organizations. The lawsuit alleged
that EPA had failed to perform its mandatory duty, under section
109(d)(1), of completing the current review within the period provided
by statute. American Lung Association v. Whitman (No. 1:03CV00778,
D.D.C. 2003). An initial consent decree was entered by the court in
July 2003 after an opportunity for public comment. The consent decree,
as modified by the court, provides that EPA will sign for publication
notices of proposed and final rulemaking concerning its review of the
PM NAAQS no later than December 20, 2005 and September 27, 2006, respectively.
C. Related Control Programs to Implement PM Standards
States are primarily responsible for ensuring attainment and
maintenance of ambient air quality standards once EPA has established
them. Under section 110 of the CAA (42 U.S.C. 7410) and related
provisions, States are to submit, for EPA approval, State
implementation plans (SIPs) that provide for the attainment and
maintenance of such standards through control programs directed to
sources of the pollutants involved. The States, in conjunction with
EPA, also administer the prevention of significant deterioration (PSD)
program (42 U.S.C. 7470-7479) for these pollutants. In addition,
Federal programs provide for nationwide reductions in emissions of
these and other air pollutants through the Federal Mobile Source
Control Program under title II of the CAA (42 U.S.C. 7521-7574), which
involves controls for automobile, truck, bus, motorcycle, nonroad or
off-highway, and aircraft emissions; the new source performance
standards under section 111 (42 U.S.C. 7411); and the national emission
standards for hazardous air pollutants under section 112 (42 U.S.C. 7412).
As described in a recent EPA report, The Particle Pollution Report:
Current Understanding of Air Quality and Emissions through 2003 (EPA,
2004b), State and Federal programs have made substantial progress in
reducing ambient concentrations of PM10 and
PM2.5. For example, PM10 concentrations have
decreased 31 percent nationally since 1988. Regionally, PM10
concentrations decreased most in areas with historically higher
concentrations--the Northwest (39 percent decline), the Southwest (33
percent decline), and southern California (35 percent decline). Direct
emissions of PM10 have decreased approximately 25 percent
nationally since 1988.
Programs aimed at reducing direct emissions of particles have
played an important role in reducing PM10 concentrations,
particularly in western areas. Some examples of PM10
controls include paving unpaved roads and using best management
practices for agricultural sources of resuspended soil. Additionally,
EPA's Acid Rain Program has substantially reduced sulfur dioxide
(SO2) emissions from power plants since 1995 in the eastern
United States, contributing to lower PM concentrations. Of the 87 areas
that were designated nonattainment for PM10 in the early
1990s, 64 now meet those standards. In cities that have not attained
the PM10 standards, the number of days above the standards
is down significantly.
Nationally, PM2.5 concentrations have declined by 10
percent from 1999 to 2003. Generally, PM2.5 concentrations
have also declined the most in regions with the highest
concentrations--the Southeast (20 percent decline), southern California
(16 percent decline), and the Industrial Midwest (9 percent decline).
With the exception of the Northeast, the remaining regions posted
modest declines in PM2.5 concentrations from 1999 to 2003.
Direct emissions of PM2.5 have decreased by 5 percent
nationally over the past 5 years.
National programs that affect regional emissions have contributed
to lower sulfate concentrations and, consequently, to lower
PM2.5 concentrations, particularly in the Industrial Midwest
and Southeast. National ozone-reduction programs designed to reduce
emissions of volatile organic compounds (VOCs) and nitrogen oxides
(NOX) also have helped reduce carbon and nitrates, both of
which are components of PM2.5. Nationally, SO2
emissions have declined 9 percent, NOX emissions have
declined 9 percent, and VOC emissions have declined by 12 percent from
1999 to 2003. In eastern States affected by the Acid Rain Program,
sulfates decreased 7 percent over the same period.
Over the next 10 to 20 years, national and regional regulations
will make major reductions in ambient PM2.5 levels. The
Clean Air Interstate Rule (CAIR) and the NOX SIP Call will
reduce SO2 and NOX emissions from electric
generating units and industrial boilers across the eastern half of the
U.S., regulations to implement the current ambient air quality
standards for PM2.5 will require direct PM2.5 and
PM2.5 precursor controls in nonattainment areas, and new
national mobile source regulations affecting heavy-duty diesel engines,
highway vehicles, and other mobile sources will reduce emissions of
NOX, direct PM2.5, SO2, and VOCs. The
EPA estimates that these regulations for stationary and mobile sources
will cut SO2 emissions by 6 million tons annually in 2015
from 2001 levels. Emissions of NOX will be cut by 9 million
tons annually in 2015 from 2001 levels. Emissions of VOCs will drop by
3 million tons, and direct PM2.5 emissions will be cut by
200,000 tons in 2015, compared to 2001 levels.
Modeling done by EPA indicates that by 2010, 18 of the 39 areas
currently not attaining the PM2.5 standards will come into
attainment just based on regulatory programs already in place,
including CAIR, the Clean Diesel Rules, and other Federal measures.
Four more PM2.5 areas are projected to attain the standards
by 2015 based on the implementation of these programs. All areas in the
eastern U.S. will have lower PM2.5 concentrations in 2015
relative to present-day conditions. In most cases,
[[Page 2625]]
the predicted improvement in PM2.5 ranges from 10 percent to
20 percent.
D. Overview of Current PM NAAQS Review
This action presents the Administrator's proposed decisions on the
review of the current primary and secondary PM2.5 and
PM10 standards. Primary standards for fine particles and for
thoracic coarse particles are addressed separately below in sections II
and III, respectively, consistent with the decision made by EPA in the
last review and with the conclusions in the Criteria Document and Staff
Paper that fine and thoracic coarse particles should continue to be
considered as separate subclasses of PM pollution. Thus, the principal
focus of this current review of the air quality criteria and primary
standards for PM is on evidence of health effects and risks related to
exposures to fine particles and to thoracic coarse particles. Secondary
standards for fine and coarse-fraction particles are addressed below in
section IV.
Past and current decisions to address fine particles and thoracic
coarse particles separately are based in part on long-established
information on differences in sources, properties, and atmospheric
behavior between fine and coarse particles (EPA, 2005a, section 2.2).
Fine particles are produced chiefly by combustion processes and by
atmospheric reactions of various gaseous pollutants, whereas thoracic
coarse particles are generally emitted directly as particles as a
result of mechanical processes that crush or grind larger particles or
the resuspension of dusts. Sources of fine particles include, for
example, motor vehicles, power generation, combustion sources at
industrial facilities, and residential fuel burning. Sources of
thoracic coarse particles include, for example, resuspension of
traffic-related emissions such as tire and brake lining materials,
direct emissions from industrial operations, construction and
demolition activities, and agricultural and mining operations. Fine
particles can remain suspended in the atmosphere for days to weeks and
can be transported thousands of kilometers, whereas thoracic coarse
particles generally deposit rapidly on the ground or other surfaces and
are not readily transported across urban or broader areas. The approach
in this review to continue to address fine and thoracic coarse
particles separately is reinforced by new information that advances our
understanding of differences in human exposure relationships and
dosimetric patterns characteristic of these two subclasses of PM
pollution, as well as the apparent independence of health effects that
have been associated with them in epidemiologic studies (EPA, 2004,
section 3.2.3). See also American Trucking Associations v. EPA, 175 F.
3d at 1053-54, 1055-56 (EPA justified in establishing separate
standards for fine and thoracic coarse particles).
Today's proposed decisions separately addressing fine and coarse
particles are based on a thorough review in the Criteria Document of
the latest scientific information on known and potential human health
and welfare effects associated with exposure to these subclasses of PM
at levels typically found in the ambient air. These proposed decisions
also take into account: (1) Staff assessments in the Staff Paper of the
most policy-relevant information in the Criteria Document and as well
as a quantitative risk assessment; (2) CASAC advice and
recommendations, as reflected in the CASAC's letters to the
Administrator, discussions of drafts of the Criteria Document and Staff
Paper at public meetings, and separate written comments prepared by
individual members of the CASAC PM Review Panel \5\ (henceforth,
``CASAC Panel''), and (3) public comments received during the
development of these documents, either in connection with CASAC
meetings or separately.
---------------------------------------------------------------------------
\5\ The CASAC PM Review Panel is comprised of the seven members
of the chartered CASAC, supplemented by fifteen subject-matter
experts appointed by the Administrator to provide the types of
scientific expertise relevant to this review of the PM NAAQS.
---------------------------------------------------------------------------
The EPA is aware that a number of new scientific studies on the
health effects of PM have been published since the 2002 cutoff date for
inclusion in the Criteria Document. As in the last PM NAAQS review, EPA
intends to conduct a review and assessment of any significant new
studies published since the close of the Criteria Document, including
studies submitted during the public comment period in order to ensure
that, before making a final decision, the Administrator is fully aware
of the new science that has developed since 2002. In this assessment,
EPA will examine these new studies in light of the literature evaluated
in the Criteria Document. This assessment and a summary of the key
conclusions will be placed in the rulemaking docket. A preliminary list
of potentially significant new studies identified to date has been
compiled and placed in the rulemaking docket for this proposal, and EPA
solicits comment on other relevant studies that may be added to this
list. This list includes a wide array of different types of studies
that are potentially relevant to various issues discussed in the
following sections, including issues related to the elements of the
standards under review.
Throughout this preamble a number of conclusions, findings, and
determinations by the Administrator are noted. It should be understood
that these are all provisional and proposed in nature. While they
identify the reasoning that supports this proposal, they are not
intended to be final or conclusive in nature. The EPA invites comments
on all issues involved with this proposal, including all such proposed
judgments, conclusions, findings, and determinations.
II. Rationale for Proposed Decisions on Primary PM2.5 Standards
As discussed more fully below, the rationale for the proposed
revisions of the primary PM2.5 NAAQS includes consideration
of: (1) Evidence of health effects related to short- and long-term
exposures to fine particles; (2) insights gained from a quantitative
risk assessment; and (3) specific conclusions regarding the need for
revisions to the current standards and the elements of PM2.5
standards (i.e., indicator, averaging time, form, and level) that,
taken together, would be requisite to protect public health with an
adequate margin of safety.
In developing this rationale, EPA has drawn upon an integrative
synthesis of the entire body of evidence of associations between
exposure to ambient fine particles and a broad range of health
endpoints (EPA, 2004, Chapter 9), focusing on those health endpoints
for which the Criteria Document concludes that the associations are
likely to be causal. This body of evidence includes hundreds of studies
conducted in many countries around the world, using various indicators
of fine particles. In its assessment of the evidence judged to be most
relevant to making decisions on elements of the primary
PM2.5 standards, EPA has placed greater weight on U.S. and
Canadian studies using PM2.5 measurements, since studies
conducted in other countries may well reflect different demographic and
air pollution characteristics.
As with virtually any policy-relevant scientific research, there is
uncertainty in the characterization of health effects attributable to
exposure to ambient fine particles. As discussed below, however, an
unprecedented amount of new research has been conducted since the last
review, with important new information coming from epidemiologic,
toxicologic, controlled human exposure,
[[Page 2626]]
and dosimetric studies. Moreover, the newly available research studies
evaluated in the Criteria Document have undergone intensive scrutiny
through multiple layers of peer review and extended opportunities for
public review and comment. While important uncertainties remain, the
review of the health effects information has been extensive and
deliberate. In the judgment of the Administrator, this intensive
evaluation of the scientific evidence has provided an adequate basis
for regulatory decision making at this time. This review also provides
important input to EPA's research plan for improving our future
understanding of the relationships between exposures to ambient fine
particles and health effects.
A. Heath Effects Related to Exposure to Fine Particles
This section outlines key information contained in the Criteria
Document (Chapters 6-9 and the Staff Paper (Chapter 3) on known or
potential effects associated with exposure to fine particles and their
major constituents. The information highlighted here summarizes: (1)
New information available on potential mechanisms for health effects
associated with exposure to fine particles and constituents; (2) the
nature of the effects that have been associated with ambient fine
particles or fine particle constituents; (3) an integrative assessment
of the evidence on fine particle-related health effects; (4)
subpopulations that appear to be sensitive to effects of exposure to
fine particles; and (5) the public health impact of exposure to ambient
fine particles.
As was true in the last review, evidence from epidemiologic studies
plays a key role in the Criteria Document's evaluation of the
scientific evidence. Some highlights of the new epidemiologic evidence
include:
(1) New multi-city studies that use uniform methodologies to
investigate the effects of various indicators of PM on health with data
from multiple locations with varying climate and air pollution mixes,
contributing to increased understanding of the role of various
potential confounders, including gaseous co-pollutants, on observed
associations with fine particles. These studies provide more precise
estimates of the magnitude of an effect of exposure to PM, including
fine particles, than most smaller-scale individual city studies.
(2) More studies of various health endpoints evaluating
associations between effects and fine particles and thoracic coarse
particles (discussed below in section III), as well as ultrafine
particles or specific components (e.g., sulfates, nitrates, metals,
organic compounds, and elemental carbon) of fine particles.
(3) Numerous new studies of cardiovascular endpoints, with
particular emphasis on assessment of cardiovascular risk factors or
physiological changes.
(4) Studies relating population exposure to fine particles and
other pollutants measured at centrally located monitors to estimates of
exposure to ambient pollutants at the individual level. Such studies
have led to a better understanding of the relationship between ambient
fine particles levels and personal exposures to fine particles of
ambient origin.
(5) New analyses and approaches to addressing issues related to
potential confounding by gaseous co-pollutants, possible thresholds for
effects, and measurement error and exposure misclassification.\6\
---------------------------------------------------------------------------
\6\ ``Confounding'' occurs when a health effect that is caused
by one risk factor is attributed to another variable that is
correlated with the causal risk factor; epidemiologic analyses
attempt to adjust or control for potential confounders (EPA, 2004,
section 8.1.3.2; EPA, 2005a, section 3.6.4). A ``threshold'' is a
concentration below which it is expected that effects are not
observed (EPA, 2004, section 8.4.7; EPA, 2005a, section 3.6.6).
``Gaseous co-pollutants'' generally refer to other commonly-occuring
air pollutants, specifically O3, CO, SO2 and
NO2. ``Measurement error'' refers to uncertainty in the
air quality measurements, while ``exposure misclassification''
includes uncertainty in the use of ambient pollutant measurements in
characterizing population exposures to PM (EPA, 2004, section 8.4.5;
EPA, 2005a, section 3.6.2)
---------------------------------------------------------------------------
(6) Preliminary attempts to evaluate the effects of fine particles
from different sources (e.g., motor vehicles, coal combustion,
vegetative burning, crustal \7\ ), using factor analysis or source
apportionment methods with fine particle speciation data.
---------------------------------------------------------------------------
\7\ ``Crustal'' is used here to describe particles of geologic
origin, which can be found in both fine- and coarse-fraction PM.
---------------------------------------------------------------------------
(7) Several new ``intervention studies'' providing evidence for
improvements in respiratory or cardiovascular health with reductions in
ambient concentrations of particles and gaseous co-pollutants.
In addition, the body of evidence on PM-related effects has greatly
expanded with findings from studies on potential mechanisms or pathways
by which particles may result in the effects identified in the
epidemiologic studies. These studies include important new dosimetry,
toxicologic and controlled human exposure studies, as highlighted below:
(8) Animal and controlled human exposure studies using concentrated
ambient particles (CAPs), new indicators of response (e.g., C-reactive
protein and cytokine levels, heart rate variability), and animal models
simulating sensitive human subpopulations. The results of these studies
are relevant to evaluation of plausibility of the epidemiologic
evidence and provide insights into potential mechanisms for PM-related
effects.
(9) Dosimetry studies using new modeling methods that provide
increased understanding of the dosimetry of different particle size
classes and in members of potentially sensitive subpopulations, such as
people with chronic respiratory disease.
1. Mechanisms
In the last review, EPA considered the lack of demonstrated
biologic mechanisms for the varying effects observed in epidemiologic
studies to be an important caution in its integrated assessment of the
health evidence. Much new evidence is now available on potential
mechanisms or pathways for PM-related effects, ranging from effects on
the respiratory system to indicators of cardiovascular response; these
new findings are discussed in depth in Chapter 7 of the Criteria
Document. While questions remain, the new findings have advanced our
understanding of the complex and different patterns of particle
deposition and clearance in the respiratory tract and provide insights
into potential mechanisms for PM-related effects and support the
plausibility of the findings of epidemiologic studies.
Although there are differences among the size fractions of
particles, fine particles, including accumulation mode and ultrafine
particles, and thoracic coarse particles can all penetrate into and be
deposited in the tracheobronchial and alveolar regions of the
respiratory tract (i.e., the ``thoracic'' regions).\8\ Penetration into
the tracheobronchial and alveolar regions is greater for accumulation
mode particles than for coarse or ultrafine particles, since coarse and
ultrafine particles are more efficiently removed from the air in the
extrathoracic region than are accumulation-mode fine particles; the
evidence from dosimetric studies is
[[Page 2627]]
reviewed in detail in Chapter 6 of the Criteria Document.
---------------------------------------------------------------------------
\8\ Particles are often classified in modes based on their
distribution by characteristics such as mass, surface area, and
particle number. ``Coarse mode'' particles are those with diameters
mostly greater than the minimum in the particle mass distribution,
which generally occurs between about 1 and 3 [mu]m. ``Accumulation
mode'' particles are those with diameters from about 0.1 [mu]m to
between about 1 and 3 [mu]m. Ultrafine particles are generally those
with diameters below about 0.1 [mu]m (EPA, 2004, pages 2-14).
---------------------------------------------------------------------------
Fine particles have varying physical or chemical characteristics
that may influence health responses. Physical characteristics that may
be of importance are solubility or physical state of the particles
(e.g., solid, liquid). Fine particle components include metals, acids,
organic compounds, biogenic constituents, sulfate and nitrate salts,
elemental carbon, and reactive components such as peroxides; size and
surface area of the particles can also influence health responses. By
way of illustration, Mauderly et al. (1998) discussed particle
components or characteristics hypothesized to contribute to health,
producing an illustrative list of 11 components or characteristics of
interest for which some evidence existed. The list included: (1)
Particle mass concentration, (2) particle size/surface area, (3)
ultrafine particles, (4) metals, (5) acids, (6) organic compounds, (7)
biogenic particles, (8) sulfate and nitrate salts, (9) peroxides, (10)
soot, and (11) co-factors, including effects modification or
confounding by co-occurring gases and meteorology. The authors stressed
that this list is neither definitive nor exhaustive, and note that ``it
is generally accepted as most likely that multiple toxic species act by
several mechanistic pathways to cause the range of health effects that
have been observed'' (Mauderly et al., 1998). The range of health
outcomes linked with fine particle exposures is also broad, including
effects on the cardiovascular and respiratory systems, and potential
links with developmental effects in children (e.g., low birth weight)
and death from lung cancer. It appears unlikely that the complex mixes
of particles that are present in ambient air would act alone through
any single pathway of response. Accordingly, it is plausible that
several physiological responses might occur in concert to produce
reported health endpoints.
As discussed in section 7.10 of the Criteria Document, the
potential pathways for direct effects on the respiratory system include
lung injury and inflammation, increased airway reactivity and asthma
exacerbation, and increased susceptibility to respiratory infections.
New toxicologic or controlled human exposure studies have reported some
evidence of inflammatory responses in animals, as well as increased
susceptibility to infections. Toxicologic studies also report evidence
of lung injury, inflammation, or altered host defenses with exposure to
ambient particles or particle constituents. Some toxicologic evidence,
particularly from results of studies using diesel exhaust particle
exposures, also indicates that PM can aggravate asthmatic symptoms or
increase airway reactivity.
Potential pathways for fine particle-related effects also include
systemic effects that are secondary to effects in the respiratory
system. These include impairment of lung function leading to cardiac
effects, pulmonary inflammation and cytokine production leading to
systemic hemodynamic effects, lung inflammation leading to increased
blood coagulability, and lung inflammation leading to hematopoiesis
effects. While more limited than for direct pulmonary effects, some new
toxicologic studies suggest that injury or inflammation in the
respiratory system can lead to changes in heart rhythm, reduced
oxygenation of the blood, changes in blood cell counts, and changes in
the blood that can increase the risk of blood clot formation, a risk
factor for heart attacks and strokes. In addition, health studies have
suggested potential pathways for effects on the heart that include
effects related to uptake of particles or particle constituents in the
blood, and effects on the autonomic control of the heart and
circulatory system. In the last review, little or no evidence was
available from toxicologic studies on potential cardiovascular effects.
More recent studies have provided some initial evidence that particles
can have direct cardiovascular effects. Particle deposition in the
respiratory system also could lead to cardiovascular effects, such as
fine particle-induced pulmonary reflexes resulting in changes in the
autonomic nervous system that then could affect heart rhythm. Also,
inhaled fine particles could affect the heart or other organs if
particles or particle constituents are released into the circulatory
system from the lungs; some new evidence indicates that the smaller
ultrafine particles or their soluble constituents can move directly
from the lungs into systemic circulation.
The potential mechanisms and/or general pathways for effects
discussed above are primarily effects related to short-term rather than
long-term exposure to fine particles; for the most part, air pollution
toxicologic studies are not designed to assess long-term exposure
effects. While repeated occurrences of some short-term insults, such as
inflammation, might contribute to long-term effects, it is likely that
wholly different mechanisms are involved in the development of chronic
health responses. Some mechanistic evidence is available, however, for
potential carcinogenic or genotoxic effects of ambient fine particles
and combustion products of coal, wood, diesel, and gasoline (discussed
in section 7.8 of the Criteria Document).
Overall, the findings indicate that different health responses are
linked with different particle characteristics and that both individual
components and complex particle mixtures appear to be responsible for
many biologic responses relevant to fine particle exposures. In
evaluating the new body of evidence, the Criteria Document states:
``Thus, there appear to be multiple biologic mechanisms that may be
responsible for observed morbidity/mortality due to exposure to ambient
PM. It also appears that many biologic responses are produced by PM
whether it is composed of a single component or a complex mixture''
(EPA, 2004, p. 7-206).
2. Nature of Effects
In the last review, evidence from health studies indicated that
exposure to PM (using various indicators) was associated with premature
mortality and indices of morbidity including respiratory hospital
admissions and emergency room visits, school absences, work loss days,
restricted activity days, effects on lung function and symptoms,
morphological changes, and altered host defense mechanisms.\9\ As
reviewed in Chapter 8 of the Criteria Document, recent epidemiologic
studies have continued to report associations between short-term
exposure to fine particles or fine particle indicators, and effects
such as premature mortality, hospital admissions or emergency
department visits for respiratory disease, and effects on lung function
and symptoms. In addition, recent epidemiologic studies have provided
some new evidence linking short-term fine particle exposures to effects
on the cardivascular system, including cardiovascular hospital
admissions and more subtle indicators of cardiovascular health. Long-
term exposure to PM2.5 and sulfates has also been associated
with mortality from cardiopulmonary diseases and lung cancer, and
effects on the respiratory system such as decreased lung function or
the development of chronic respiratory disease. The
[[Page 2628]]
evidence for such effects is summarized below.
\9\ Historical reports of dramatic pollution episodes,
considered in the 1987 review of the PM NAAQS, provided clear
evidence of mortality associated with high levels of PM and other
pollutants, such as the air pollution episode that occurred in
London in 1952 (EPA, 1996a, pp. 12-28 to 12-31).
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[[Page 2629]]
a. Effects Associated With Short-Term Exposure to Fine Particles
Numerous epidemiologic studies have demonstrated statistical
associations between short-term exposure to fine particles and health
outcomes ranging from total mortality to respiratory symptoms, as
discussed below. Figure 1 summarizes results from both multi-city and
single-city epidemiologic studies using short-term exposures to
PM2.5, including all U.S. and Canadian studies that used
direct measurements of PM2.5 and for which effect estimates
and confidence intervals were reported.\10\ The central effect estimate
is indicated by a diamond for each study result, with the vertical bar
representing the 95 percent confidence interval around the estimate. In
the discussions that follow, an individual study result is considered
to be statistically significant if the 95 percent confidence interval
does not include zero. Positive effect estimates indicate increases in
the health outcome with PM2.5 exposure. In considering these
results as a whole, it is important to consider not only whether
statistical significance at the 95 percent confidence level is reported
in individual studies, but also the general pattern of results,
focusing in particular on studies with greater statistical power that
report relatively more precise results.
---------------------------------------------------------------------------
\10\ In the following discussion of specific studies, results
from single-pollutant models are referred to, as shown in Figure 1,
unless otherwise noted.
---------------------------------------------------------------------------
i. Mortality
Since the last review, a large number of new time-series studies of
the relationship between short-term exposure to PM, including
PM2.5, and mortality have been published, including several
multi-city studies that are responsive to the recommendations from the
last review. As discussed in section 8.2 of the Criteria Document,
these include studies that have been conducted in single cities or
locations in the U.S. or Canada, as well as Mexico City and locations
in Europe, South America, Asia, and Australia.
Several recent multi-city studies have been published since the
last review that are of particular relevance for this review. The
results of multi-city studies on associations between PM10
and mortality across 90 U.S. cities (Dominici, 2003) and across ten
U.S. cities (Schwartz, 2003b), while not specifically on fine
particles, have provided important new information to help address
uncertainties regarding a number of issues, including model
specification, potential confounding by co-pollutants and the form of
concentration-response functions (EPA, 2004, section 8.2.2.3). Two
multi-city studies have included measurements of PM2.5; one
was conducted in six U.S. cities (Schwartz et al., 2003a; Klemm and
Mason, 2003) and the other in eight Canadian cities (Burnett and
Goldberg, 2003). In the last review, results from one multi-city study
(the Six Cities study) were available, in which the authors reported
significant associations for total mortality with PM2.5 and
PM10, but not with PM10-2.5. Reanalyses of Six
Cities data have reported results consistent with the findings of the
original study, with statistically significant increases for total
mortality with short-term exposure to PM2.5 (Schwartz,
2003a; Klemm and Mason, 2003). In a study using data from the eight
largest Canadian cities, positive associations were reported for
PM2.5, PM10, and PM10-2.5 with
mortality, and the association with PM2.5 was statistically
significant (Burnett and Goldberg, 2003).
Single-city studies of mortality associations with short-term
exposures to fine particles have also been conducted in areas across
U.S. and Canada as well as in Europe, Australia and Mexico (some using
fine particle indicators such as British Smoke). In general, it can be
seen in Figure 1 that the effect estimates for associations between
mortality and short-term exposure to PM2.5 are positive and
a number are statistically significant, particularly when focusing on
the results of studies with greater precision. For total nonaccidental
mortality, the effect estimates from the multi-city and single-city
studies with greater precision generally fall in a range of 2 to 6
percent increases per 25 [mu]g/m3 PM2.5.\11\
Somewhat larger effect estimates have been reported for associations
with cardiovascular or respiratory mortality than with total
nonaccidental mortality although the confidence intervals may also be
larger, especially for respiratory mortality since respiratory deaths
comprise only a small proportion of total deaths (EPA, 2005a, p. 3-15).
Some studies evaluated seasonal variation in effects, and there is no
consistent pattern in results. The Criteria Document concludes that the
results of recent epidemiologic studies are generally consistent with
findings available in the previous review (EPA, 2004, p. 8-305).
---------------------------------------------------------------------------
\11\ In general, the results of studies conducted over shorter
time periods and/or smaller areas have a broader range or effect
estimates with larger standard errors, as shown in Figure 1.
---------------------------------------------------------------------------
In addition, associations have been reported between mortality and
short-term exposure to a number of fine particle components, including
sulfates, nitrates, metals, organic compounds and elemental carbon
(EPA, 2004, Section 8.2.2.5.2), as well as gaseous precursors such as
SO2 and NO2 and other gaseous pollutants such as
CO. Further, three recent studies have used PM2.5 speciation
data to evaluate the effects of air pollutant combinations or mixtures
using factor analysis or source apportionment methods to evaluate
potential associations between mortality and PM2.5 from
different source categories. These studies reported that short-term
exposures to fine particles from combustion sources, including motor
vehicle emissions, coal combustion, oil burning and vegetative burning,
were associated with increased mortality (EPA, 2004, Section
8.2.2.5.3). However, different patterns of associations between various
components or source categories of fine particles and total or
cardiovascular mortality are seen in different studies (EPA, 2004, p.
8-70, Tables 8-3, 8-4).
ii. Respiratory Morbidity
As discussed in Section 8.4.6.4 of the Criteria Document, recent
epidemiologic studies have provided further evidence for fine particle
effects on morbidity, including effects such as hospital admissions or
emergency department for respiratory diseases, respiratory symptoms and
lung function changes.
(a) Hospital Admissions or Emergency Department Visits for Respiratory
Diseases
In the last review, results were available from one study that
reported associations between PM2.5 and hospitalization for
respiratory diseases; these findings were also supported by a number of
studies using other fine particle indicators. Numerous studies had also
reported statistically significant associations between hospital
admissions or emergency department visits for respiratory diseases
short-term exposures with various indicators ambient PM, especially
PM10, in areas where fine particles are the predominant
fraction of PM10, such as locations in the Eastern U.S. and
in Ontario, Canada (EPA, 1996a, p. 13-39).
The body of evidence has been expanded with numerous new studies in
the U.S. and other countries that have reported associations between
PM2.5 and hospitalization or emergency department visits
(discussed more fully in Section 8.3.2 of the Criteria Document). As
shown in Figure 1, all U.S. and Canadian studies report
[[Page 2630]]
associations between PM2.5 and hospitalization for all
respiratory causes that are positive and statistically significant. A
number of studies have also reported findings for hospital admissions
for individual disease categories (COPD, pneumonia, and asthma) that
are positive, but not always statistically significant, perhaps due to
smaller sample sizes for the specific respiratory diseases. The effect
estimates for respiratory hospital admissions tend to fall in the range
of 5 to 15 percent per 25 [mu]g/m3 PM2.5.\12\ In
addition, several studies have reported positive, statistically
significant associations between exposure to PM2.5 and
emergency department visits for respiratory diseases. The effect
estimates for these associations range up to about 25 percent per 25
[mu]g/m3 PM2.5 (EPA, 2005a, pp. 3-20, 3-21).
---------------------------------------------------------------------------
\12\ Some studies have evaluated seasonal variation in effects,
and no consistent pattern is apparent in the results. For example,
stronger associations were reported between PM2.5 and
asthma hospitalization in the warmer season in Seattle (Sheppard et
al., 2003) but in the cooler season in Los Angeles (Nauenberg and
Basu, 1999).
---------------------------------------------------------------------------
(b) Respiratory Symptoms and Lung Function Changes
Associations between short-term exposure to PM2.5 and
symptoms in U.S. and Canadian studies are presented in Figure 1. As
discussed in Section 8.3.3 of the Criteria Document, a number of new
studies have reported significant associations between short-term
exposure to PM and increased respiratory symptoms (e.g., cough, wheeze,
shortness of breath) and decreased lung function in people with asthma.
In studies of nonasthmatic subjects, there were generally positive
associations between short-term PM2.5 exposures and
respiratory symptoms that often were not statistically significant and
the results for changes in lung function were somewhat inconsistent.
The Criteria Document concludes that the findings of these studies
suggest associations with fine PM in reduced lung function and
increased respiratory symptoms. For example, significant associations
were reported between ambient PM2.5 and lower respiratory
symptoms in children in a number of U.S. cities (Schwartz and Neas,
2000), and significant associations were found with reduced lung
function in Philadelphia (Neas et al., 1999). These findings are
supported by results from numerous studies conducted in Europe and
Central and South America. The Criteria Document finds that the recent
epidemiologic findings are consistent with those of the previous review
in showing associations with both respiratory symptom incidence and
decreased lung function (EPA, 2004, Section 8.4.6.4).
iii. Cardiovascular Morbidity
In the last review, none of the available studies had evaluated
associations between exposure to PM and cardiovascular morbidity,
though some studies had reported associations with cardiopulmonary
morbidity. In this area, the evidence on PM-related effects has been
greatly expanded. Numerous recent studies, including multi-city
analyses, have reported significant associations between short-term
exposures to PM and health endpoints related to cardiovascular
morbidity, including hospitalization or emergency department visits for
cardiovascular diseases, incidence of myocardial infarction, cardiac
arrhythmia, changes in heart rate or heart rate variability and changes
in cardiac health indicators such as fibrinogen or C-reactive protein
(EPA, 2004, section 9.2.3.2.1).
(a) Hospital Admissions and Emergency Department Visits for
Cardiovascular Diseases
Several recent studies, including multi-city analyses, have
reported significant associations between short-term exposures to
various PM indicators and hospital admissions or emergency department
visits for cardiovascular diseases. Among the studies using
PM2.5 measurements are a number of single-city analyses of
hospitalization or emergency department visits for cardiovascular
diseases. As shown in Figure 1, studies conducted in Los Angeles,
Toronto and Detroit have reported associations with hospital admissions
or emergency department visits for all cardiovascular diseases that are
positive and statistically significant or nearly so (Burnett et al.,
1997; Ito, 2003; Moolgavkar, 2003). As was true for respiratory
diseases, the results for specific diseases (ischemic heart disease,
dysrhythmia, congestive heart disease or heart failure, and stroke) are
positive but often not statistically significant. The effect estimates
reported for associations with hospitalization for cardiovascular
diseases range from about 1 to 10 percent per 25 [mu]g/m3
PM2.5 (EPA, 2004, p. 8-310); effect estimates reported for
associations with emergency department visits are generally somewhat
larger.
(b) Cardiovascular Health Indicators
In addition to the greatly expanded body of evidence on
hospitalization or emergency department visits for cardiovascular
diseases, new epidemiologic studies have also reported associations
with more subtle physiological changes in the cardiovascular system
with short-term exposures to PM, particularly PM10 and
PM2.5 (EPA, 2004, p. 9-67). Associations between short-term
exposures to ambient PM (often using PM10) have been
reported with measures of changes in cardiac function such as
arrhythmia, alterations in electrocardiogram (ECG) patterns, heart rate
or heart rate variability changes, although the Criteria Document urges
caution in drawing conclusions regarding the effects of PM on heart
rhythm, recognizing the need for further research to more firmly
establish and understand links between particles and these more subtle
endpoints. Recent studies have also reported increases in blood
components or biomarkers such as increased levels of C-reactive protein
and fibrinogen. Several of these studies report significant
associations between various cardiovascular endpoints and short-term
PM2.5 exposures, including one in which statistically
significant associations were reported between onset of myocardial
infarction and short-term PM2.5 exposures averaged over 2
and 24 hours (EPA, 2004, p. 8-165; Peters et al., 2001). In this study,
the effect estimates for the two averaging periods are quite similar in
magnitude suggesting that for certain health outcomes very short-term
fine particle concentration fluctuations are important (EPA, 2004, p.
9-42; Peters et al., 2001). These new epidemiologic findings provide
important insight into potential biologic mechanisms that could
underlie associations between short-term PM exposure and cardiovascular
mortality and hospitalization that have been reported previously.
b. Effects Associated With Long-Term Exposure to Fine Particles
In the last review, results were available from several cohort
studies that suggested associations between long-term exposure to PM
(using various indicators) and both mortality and respiratory
morbidity. Two studies of adult populations (the Six Cities and ACS
studies) reported associations between increases in mortality and long-
term exposure to PM2.5, and results of a 24-city study
indicated that long-term exposure to fine particles was associated with
increased respiratory illness in children.
As discussed below, the new evidence available in the current
review includes an extensive reanalysis of data from the Six Cities and
ACS studies, new analyses using updated data from the ACS and
California Seventh Day
[[Page 2631]]
Adventist (AHSMOG) studies, and a new analysis using data from a cohort
of veterans. In addition, new studies have been published on the
association between long-term exposure to fine particles and
respiratory morbidity using data from a cohort of schoolchildren in
Southern California. In general, the newly available evidence has
supported earlier findings, and the results of reanalyses have
increased confidence in the associations reported in previous
prospective cohort studies.
i. Mortality
In the 1996 Criteria Document, statistically significant
associations between long-term exposure to both PM2.5 and
sulfates and mortality were reported in studies from the Six Cities and
ACS cohorts (Dockery et al., 1993; Pope et al., 1995). These studies
reported effect estimates of 6.6 percent (95 percent CI: 3.5, 9.8)
increases in total mortality per 10 [mu]g/m3
PM2.5 in the ACS study and 13 percent (95 percent CI: 4.2,
23) increases in total mortality per 10 [mu]g/m3
PM2.5 in the Six Cities study, with somewhat larger effect
estimates reported for cardiopulmonary mortality (EPA, 2004, p. 8-117).
A number of reviewers raised questions about the adequacy of
adjustments for potential confounders and other issues (61 FR 65642,
December 13, 1996). Subsequently, as discussed in more detail in
Section 8.2.3 of the Criteria Document, the Health Effects Institute
conducted a major reanalysis of the data from the Six Cities and ACS
studies by a group of independent investigators to address questions
and uncertainties raised about these prospective cohort studies. The
reanalysis included two major components, a replication and validation
study and a sensitivity analysis. In the first part of the reanalysis,
the investigators validated the data used by the original investigators
in both studies, and they were able to replicate the original results.
The results confirmed the original investigators' findings of
associations with both total and cardiorespiratory mortality, and the
authors reported that the results were not dependent on the computer
programs used in the original analyses (EPA, 2004, p. 8-91; Krewski et
al., 2000, p. 91).
The second component of the reanalysis project evaluated an array
of different models and variables to determine whether the original
results would remain robust to different analytic assumptions. This
included controlling for other individual level variables, such as
cigarette smoking, alcohol consumption, obesity and occupational
exposures to dusts or other pollutants, and evaluation of the
sensitivity of results to the addition of a range of additional city-
level variables such as population change, income, education levels,
and access to health care. The sensitivity analysis included assessment
of effects in different subgroups of the population. The investigators
also evaluated the sensitivity of the results to the inclusion of
gaseous co-pollutants, and tested the effects of different statistical
modeling approaches, including methods to adjust for spatial patterns,
such as the correlation in pollutant levels between cities.
The authors found that adjustment for individual-level variables
did not alter the results for the association between long-term
PM2.5 or sulfate exposure and mortality (Krewski et al.,
2000, p. 218). In addition, in most (but not all) cases the
associations between mortality and long-term exposure to
PM2.5 and sulfates were unchanged when additional city-level
variables were added to the models (Krewski et al., 2000, p. 233).
Analyses to assess the potential modification of effects in different
subgroups of the population found, for the most part, little difference
in effects for different subgroups. However, education level was found
to modify the estimated effect of fine particles, in that associations
were statistically significant for those subgroups with lower education
levels, whereas the effect estimates from associations for the subgroup
with better than high school education were appreciably smaller and
were statistically insignificant. The authors suggest that educational
attainment may be a marker for lower socioeconomic status and thus
greater vulnerability to fine particle-related effects (EPA, 2004, p.
8-94; Krewski et al., 2000, p. 232).\13\
---------------------------------------------------------------------------
\13\ In multivariate models, the association found between
mortality and long-term PM2.5 exposure was little changed
with addition of education level to the model (Krewski et al., 2000,
p. 184). This indicates that education level was not a confounder in
the relationship between fine particles and mortality, but the
relationship between fine particles and mortality is larger in the
population subsets with lower education in this study and not
statistically significant in the population subset with the highest
education (EPA, 2004, p. 8-100).
---------------------------------------------------------------------------
In single-pollutant models, none of the gaseous co-pollutants was
significantly associated with mortality except SO2. Further
reanalysis included multi-pollutant models with the gaseous pollutants,
and the associations between mortality and both fine particles and
sulfates were unchanged in these models, except when SO2 was
included, which decreased the size of the effect estimates for
PM2.5 to one-sixth of its original value and for sulfates to
less than one-third of its original value (EPA, 2004, p. 8-136; Krewski
et al., 2000, pp. 183-184).\14\ However, the regional association of
SO2 and PM2.5 was relatively high, such that the
effects of the separate pollutants could not be distinguished. The
authors conclude that these findings support the notion that increased
mortality may be attributable to more than one component of ambient air
pollution, and that throughout the reanalyses, fine particles,
sulfates, and SO2 demonstrated positive associations with
mortality (Krewski et al., 2000, p. 233-234). As discussed more
generally in the Criteria Document, this result may be reflecting the
relatively high correlation between PM2.5 levels and
SO2 levels that would be expected in cities across the
industrial Midwest and northeastern states, the role that
SO2 has as a precursor to sulfate components in the mix of
PM2.5, and/or the likelihood that SO2 is part of
the causal pathway linking exposure to PM2.5 to adverse
health outcomes (EPA, 2004, section 8.1.3.2).
---------------------------------------------------------------------------
\14\ For a 24.5 [mu]g/m3 change in PM2.5,
the relative risk for the association between mortality and
PM2.5 alone was 1.20 (95 percent CI: 1.11-1.29), and
after adjustment for SO2 it was 1.03 (95 percent CI:
0.95-1.13). The relative risk for SO2 alone was 1.49 (95
percent CI: 1.36-1.64) and after adjustment for PM2.5 was
1.46 (95 percent CI: 1.32-1.63) (Krewski et al., 2000, p. 184). The
relative risk for sulfates alone was 1.28 (95 percent CI: 1.18-1.40)
and after adjustment for SO2 it was 1.14 (95 percent CI:
1.04-1.25) (Krewski et al., 2000, p. 184). These relative risks for
PM2.5 are equivalent to effect estimates of 7.5 percent
and 1.2 percent increases in mortality per 10 [mu]g/m3,
in single-pollutant and two-pollutant models, respectively.
---------------------------------------------------------------------------
Finally, Krewski and colleagues used several methods to address
spatial patterns in the data; for example, concentrations of air
pollutants may be correlated between cities within a region. These
analyses were primarily based on sulfate concentrations, since more
cities had data for sulfates than for fine particles. Addressing
spatial patterns in the data generally reduced the size of the
association between sulfates and mortality, but the models all
continued to show associations between mortality risk and long-term
sulfate exposures, although not all were statistically significant
(Krewski et al., 2000, p. 228). Overall, considering the results of the
extensive set of replication and sensitivity analyses, the authors
report that the reanalysis confirmed the association between mortality
and fine particle and sulfate exposures (EPA, 2004, p. 8-95; Krewski et
al., 2000).
In addition, extended analyses were conducted for the ACS cohort
study that included follow-up health data and air quality data from the
new fine particle
[[Page 2632]]
monitoring network for 1999-2000. In this study of the expanded ACS
cohort, significant associations were reported between long-term
exposure to fine particles (using various averaging periods for air
quality concentrations) and premature mortality from all causes,
cardiopulmonary diseases, and lung cancer (Pope et al., 2002; EPA,
2004, 8-102). This extended analysis included the use of more recent
data on fine particle concentrations, as well as data on gaseous co-
pollutant concentrations, though no multi-pollutant model results are
presented. Further evaluation of the influence of other covariates
(e.g., dietary intake data, occupational exposure) used methods similar
to those in the reanalysis described above, and new statistical
approaches were used for modeling the PM-mortality relationship as well
as adjusting for spatial correlation (EPA, 2004, section 8.2.3.2.2).
The investigators reported that the associations found with fine
particle and sulfate concentrations were not markedly affected by
adjustment for numerous socioeconomic variables, demographic factors,
environmental variables, indicators of access to health services or
personal health variables (e.g., dietary factors, alcohol consumption,
body mass index). Similar to the results of Krewski et al. (2000),
education level was found to be a modifier in the relationship between
fine particles and mortality, in that associations were statistically
significant for those subgroups with lower education levels, whereas
effect estimates from associations for those with better than a high
school education were close to zero and were statistically insignificant.
There are also new analyses using updated data from the AHSMOG
cohort. These include estimated PM2.5 concentrations from
visibility data, along with new health information from continued
follow-up of the Seventh Day Adventist cohort. Positive associations
were reported for mortality with PM2.5 in males, but the
estimates were generally not statistically significant (Abbey et al.,
1999; McDonnell et al., 2000; EPA, 2004, pp. 8-110 and 8-117). In
addition, one new set of analyses was done using subsets of PM exposure
and mortality time periods and data from a Veterans Administration (VA)
cohort of hypertensive men. The investigators report inconsistent and
largely nonsignificant associations between PM exposure (including,
depending on availability, TSP, PM10, PM2.5,
PM15 and PM15-2.5) and mortality (EPA, 2004, pp.
8-110 to 8-111; Lipfert et al., 2000b).
The Criteria Document and Staff Paper place greatest weight on the
findings of the Six Cities and ACS studies (including reanalyses and
extended analyses) that include measured fine particle data (in
contrast with AHSMOG effect estimates based on TSP or visibility
measurements), have study populations more similar to the general
population than the VA study cohort, and have been replicated and
examined through exhaustive reanalysis (EPA, 2005a, at 5-22; see also
EPA, 2004, at 8.2.3.2.5.). In these studies, effect estimates for
deaths from all causes fall in a range of 6 to 13 percent increased
risk per 10 [mu]g/m3 PM2.5, while effect
estimates for deaths from cardiopulmonary causes fall in a range of 6
to 19 percent per 10 [mu]g/m3 PM2.5. For lung
cancer mortality, the effect estimate was a 13 percent increase per 10
[mu]g/m3 PM2.5 in the results of the extended
analysis from the ACS cohort (Pope et al., 2002; CD, Table 8-12).
The prospective cohort studies have used air quality measurements
averaged over long periods of time, such as several years, to
characterize the long-term ambient levels in the community. The
exposure comparisons are basically cross-sectional in nature, and do
not provide evidence concerning any temporal relationship between
exposure and effect (EPA, 2004, p. 9-42). As discussed in the Criteria
Document, it is not easy to differentiate the role of historic
exposures from more recent exposures, leading to potential exposure
measurement error that is increased if average PM concentrations change
over time differentially between areas (EPA, 2004, p. 5-118). Several
new studies have used different air quality periods for estimating
long-term exposure and tested associations with mortality for the
different exposure periods. As discussed in section 3.6.5.4 of the
Staff Paper, these analyses indicate that averaging PM concentrations
over a longer time period results in stronger associations, and that
the longer series of data is likely a better indicator of cumulative
exposure. Thus, in evaluating these findings, EPA has focused on the
results of analyses using fine particle or sulfate measurements for the
longer exposure periods in the studies.
ii. Respiratory Morbidity
In the last review, several studies had reported that long-term PM
exposure was linked with increased respiratory disease and decreased
lung function. One study, using data from 24 U.S. and Canadian cities
(``24 Cities'' study), reported associations with these effects and
long-term exposure to fine particles or acidic particles, but not with
PM10 exposure (Dockery et al., 1996; Raizenne et al., 1996).
More specifically, statistically significant associations were reported
between long-term exposure to fine particles and decreases in several
measures of lung function evaluated at a single point in time (Raizenne
et al., 1996). In addition, positive but not statistically significant
associations were reported between long-term exposure to fine particles
and prevalence of a range of respiratory conditions (e.g., asthma,
bronchitis, chronic cough) (Dockery et al., 1996).
In the current review, new studies conducted in the U.S. have been
based on data from cohorts of schoolchildren in 12 Southern California
Communities and an adult cohort of Seventh Day Adventists (AHSMOG)
(EPA, 2004, section 8.3.3.2). Information specifically on associations
with long-term PM2.5 exposures are available from the
Southern California children's cohort study. Early findings from cross-
sectional analyses done at the beginning of the study suggested
associations between long-term PM2.5 exposures and
respiratory morbidity, but the findings were generally not
statistically significant.\15\ Later publications from this cohort have
reported associations with lung function growth in children over four-
year follow-up periods. In a study of a cohort of children followed
from 4th to 7th grade, some measures of decreases in lung function
growth were statistically significantly associated with increasing
exposure to PM2.5, whereas in a second cohort of 4th
graders, the associations generally did not reach statistical
significance (Gauderman et al., 2002). Decreases in measures of lung
function growth were also reported for cohorts of older children, but
the associations did not reach statistical significance (Gauderman et
al., 2000). The Criteria Document finds that these studies ``provide
the best evidence'' on effects of long-term fine particle exposure
(EPA, 2004, p. 8-314). However, this is the only cohort study to have
evaluated associations with decreases in lung function growth in
children over time. Considered together, the Criteria Document finds
that the evidence from these studies indicates that long-term
PM2.5 exposures may
[[Page 2633]]
result in chronic respiratory effects (EPA, 2004, p. 8-314).
---------------------------------------------------------------------------
\15\ In an initial report on the prevalence of respiratory
illnesses reported at the beginning of the study, positive
associations, though not statistically significant, were reported
between long-term PM2.5 exposure and risk of bronchitis
and cough only in the subset of children with asthma (McConnell et
al., 1999), and no significant associations with long-term
PM2.5 exposure were reported for the full cohort (Peters
et al., 1999a). In addition, long-term PM2.5 exposure was
associated with decreases in some lung function measurements made at
that time, but the associations were only statistically significant
for females (Peters et al., 1999b).
---------------------------------------------------------------------------
3. Integration and Interpretation of the Health Evidence
In evaluating the evidence from epidemiologic studies, the Criteria
Document and Staff Paper focused on well-recognized criteria, including
the strength of associations; robustness of reported associations to
the use of alternative model specifications, potential confounding by
co-pollutants, and exposure misclassification related to measurement
error; consistency of findings in multiple studies of adequate power,
and in different persons, places, circumstances and times; the nature
of concentration-response relationships; and information from so-called
natural experiments or intervention studies. These evaluations
addressed key methodological issues that are relevant to interpretation
of evidence from epidemiologic studies. Further, findings from
epidemiologic studies were integrated with experimental (e.g.,
dosimetric and toxicologic) studies, in considering the extent of
coherence and biological plausibility of effects observed in
epidemiologic studies. This integrative assessment provided the basis
for the judgments made in the Criteria Document and Staff Paper about
the extent to which causal inferences can be made about observed
associations between health endpoints and PM2.5 (as well as
other indicators or constituents of ambient PM), acting alone and/or in
combination with other pollutants. Key elements of these evaluations
are briefly summarized below.
(1) For short-term exposures to fine particles, in considering the
magnitude and statistical strength of the associations, there is a
pattern of positive and often statistically significant associations
for cardiovascular and respiratory health outcomes with short-term
exposure to PM10 and PM2.5. Of particular note
are several multi-city studies that have yielded relative risk
estimates for associations between short-term exposure to various
indices of PM and mortality or morbidity. Although small in size, the
effect estimates from multi-city studies have great precision due to
the statistical power of the studies. New analyses of pre-existing
cohorts with studies of long-term exposure to fine particles are
available that confirm and strengthen conclusions from the previous
review, although the effect estimates are sensitive to education level,
co-pollutant effects of SO2, and spatial correlation, as
discussed above.
(2) The Criteria Document and Staff Paper have evaluated the
robustness of epidemiologic associations in part by considering the
effect of differences in statistical model specification, potential
confounding by co-pollutants and exposure error on PM-health
associations (EPA, 2004, section 9.2.2.2; EPA, 2005a, sections 3.4.2
and 3.6).
As discussed in section 8.4.2 of the Criteria Document and section
3.6.3 of the Staff Paper, the influence of alternative modeling
strategies on epidemiologic study results was assessed, with a
particular focus on the recent set of analyses to address statistical
modeling questions in epidemiologic studies for short-term PM
exposures. Numerous recent studies used a certain type of statistical
method (i.e., generalized additive methods (GAM)) in widely used
statistical software (Splus), and it was discovered that the default
program settings could potentially result in biased effect estimates
for associations between pollutants and health outcomes. Results from a
number of epidemiologic studies were reanalyzed to address this
problem. These reanalyses also more broadly included the use of
alternative statistical models and alternative methods of control for
time-varying effects, such as weather or season (HEI, 2003). In
general, the results of the reanalyses to address the use of default
program settings in the Splus software showed little change in effect
estimates for some studies; in others the effect estimates were reduced
in size, though it was observed that the reductions were often not
substantial (EPA, 2004, p. 9-35). For example, in comparing results for
numerous studies of mortality associations with PM10, the
Criteria Document found that the extent of reduction in effect
estimates resulting from reanalysis was smaller than the variation in
effect estimate size across studies (EPA, 2004, p. 8-229 and Figure 8-
15). A review panel commentary on the set of reanalysis studies (using
various PM indicators) notes that most studies were considered to show
``little or no change'' in results with initial reanalyses to address
questions about the use of modeling specifications in the statistical
software package (HEI, 2003, pp. 258-259).
In addition, the reanalyses also refocused attention in general on
the control for relationships between health effects and weather
variables in time-series epidemiologic studies; such issues had been
also discussed at length in the 1996 Criteria Document (EPA, 2004,
section 8.4.3.5). The reanalysis results showed greater sensitivity to
the modeling approach used to account for temporal effects and weather
variables than to correcting the initial problem with default settings
in the use of GAM in Splus software (EPA, 2004, p. 8-236). For example,
in the review panel commentary, sixteen of the reanalyzed studies were
considered to have ``little or no change'' in results of initial
reanalyses, while only two studies showed ``substantial'' changes
(Goldberg and Burnett, 2003; some results in Ito, 2003; HEI, 2003, pp.
258-259). In contrast, four of the eight studies that were reanalyzed
with additional methods to adjust for time-related variables were
considered to show ``substantial'' changes in effect estimate size
(HEI, 2003, p. 262).
The recent time-series epidemiologic studies evaluated in the
Criteria Document have included some degree of control for variations
in weather and seasonal variables. As summarized in the HEI review
panel commentary, selecting a level of control to adjust for time-
varying factors, such as temperature, in time-series epidemiologic
studies involves a trade-off. For example, if the model does not
sufficiently adjust for the relationship between the health outcome and
temperature, some effects of temperature could be falsely ascribed to
the pollution variable. Conversely, if an overly aggressive approach is
used to control for temperature, the result would possibly
underestimate the pollution-related effect and compromise the ability
to detect a small but true pollution effect (EPA, 2004, p. 8-236; HEI,
2003, p. 266). The selection of approaches to address such variables
depends in part on prior knowledge and judgments made by the
investigators, for example, about weather patterns in the study area
and expected relationships between weather and other time-varying
factors and health outcomes considered in the study. While recognizing
the need for further exploration of alternative modeling approaches for
time-series analyses, the Criteria Document found that the studies
included in this part of the reanalysis in general continued to
demonstrate associations between PM and mortality and morbidity beyond
those attributable to weather variables alone (EPA, 2004, pp. 8-340, 8-
341). Further, considering the full set of reanalyses, the Criteria
Document concludes that associations between short-term exposure to PM
and various health outcomes are generally robust to the use of
alternative modeling strategies, again recognizing that further
evaluation of alternative modeling strategies was warranted (EPA, 2004,
p. 9-48).
[[Page 2634]]
For long-term exposure to fine particles, the reanalysis and
extended analyses of data from prospective cohort studies, discussed
above in section II.A.2, have shown that reported associations between
mortality and long-term exposure to fine particles are robust to
alternative modeling strategies (Krewski et al., 2000). As stated in
the reanalysis report, ``The risk estimates reported by the Original
Investigators were remarkably robust to alternative specifications of
the underlying risk models, thereby strengthening confidence in the
original findings'' (Krewski et al., 2000, p. 232). In extended
analysis, Krewski et al. (2000) identified model sensitivities related
to education level and spatial correlation, as well as to co-pollutant
effects of SO2, as discussed below.
The Criteria Document also included extensive evaluation of the
sensitivity of PM-health responses to confounding by gaseous co-
pollutants (EPA, 2004, section 8.4.3, Figures 8-16 to 8-19). Results of
new multi-city short-term exposure studies, that combine data from
locations with different mixes of pollutants, provide important new
results. Using PM10, the NMMAPS results indicated that
associations with mortality were not confounded by co-pollutant
concentrations across 90 U.S. cities (Dominici, 2003),\16\ and a
similar lack of confounding was observed in a mortality study across 10
U.S. cities (Schwartz, 2003b) (EPA, 2004, Figure 8-16). That is, in
these studies, the size of the effect estimates are little changed and
the associations remain statistically significant in multi-pollutant
models including one or more of the gaseous co-pollutants. Similar
results are seen in some single-city studies using PM2.5 for
some health outcomes in which the single-pollutant model association
was statistically significant (EPA, 2004, Figures 8-16 to 8-18),
including the association with mortality in Santa Clara County, CA
(Fairley, 2003); associations with hospital admissions in Detroit (for
heart failure and pneumonia in Ito, 2003) and Seattle (for asthma in
Sheppard et al., 2003); and associations with cardiovascular-related
biomarkers in Boston (Gold et al., 2000). The size of the effect
estimates were little changed in other studies as well in which the
single-pollutant model associations were not statistically significant
(e.g., for some health outcomes in Ito, 2003; for mortality in Chock et
al., 2000). In yet other studies, however, for some combinations of
pollutants in some areas, substantial reductions in the size of the
effect estimates for PM2.5 were observed; notably,
Moolgavkar (2003) reports substantial reductions in effect estimates
when CO is included in models for mortality and hospitalization in Los
Angeles, and Thurston et al. (1994) and Delfino et al. (1998) report
substantial reductions when O3 is included in models for
hospital admissions in Toronto and emergency department visits in
Montreal, respectively.\17\ It is recognized that collinearity between
co-pollutants can make interpretation of such multi-pollutant model
results difficult (EPA, 2004, p. 8-253). Further, associations between
long-term exposure to PM2.5 and mortality were not generally
sensitive to inclusion of co-pollutants, with the notable exception of
the inclusion of SO2 in multipollutant models used in the
reanalysis of the ACS study, as discussed above in section II.A.2 (EPA,
2004, p. 8-136). Overall, the Criteria Document concluded that these
studies indicate that effect estimates for associations between
mortality and morbidity and various PM indices are generally robust to
confounding by co-pollutants, while recognizing that disentangling the
effects attributable to various pollutants within an air pollution
mixture is challenging (EPA, 2004, p. 9-37).
---------------------------------------------------------------------------
\16\ In the HEI Review Panel commentary on the results of the
NMMAPS multi-city analyses, the Panel stated that the results did
not show a confounding effect of other pollutants, observing that
the PM10 effects on mortality were not changed by
addition of either O3, SO2, NO2 or
CO to the models (HEI, 2000, p. 77).
\17\ The correlation coefficients between concentrations of
PM2.5 and the noted co-pollutants in these studies were
high; the coefficient with CO in Los Angeles was 0.58, and the
coefficients with O3 were 0.58 and 0.72 in Montreal and
Toronto, respectively.
---------------------------------------------------------------------------
Finally, as discussed in section 3.6.2, a number of recent studies
have evaluated the influence of exposure error on PM-health
associations. This includes both consideration of error in measurements
of PM and other co-pollutants, and the degree to which measurements
from an individual monitor reflect exposures to the surrounding
community. As further discussed in section 3.6.2, several studies have
shown that fairly extreme conditions (e.g., very high correlation
between pollutants and no measurement error in the ``false'' pollutant)
are needed for complete ``transfer of causality'' of effects from one
pollutant to another (EPA, 2004, p. 9-38). In comparing fine and
thoracic coarse particles, the Criteria Document observes that exposure
error is likely to be more important for associations with
PM10-2.5 than with PM2.5, since there is
generally greater error in PM10-2.5 measurements,
PM10-2.5 concentrations are less evenly distributed across a
community, and less likely to penetrate into buildings (EPA, 2004, p.
9-38). Therefore, while the Criteria Document concludes that
associations reported with PM10, PM2.5 and
PM10-2.5 are generally robust, it recognizes that factors
related to exposure error may result in reduced precision for
epidemiologic associations with PM10-2.5 (EPA, 2004, p. 9-46).
(3) Consistency refers to the persistent finding of an association
between exposure and outcome in multiple studies of adequate power in
different persons, places, circumstances and times (CDC, 2004). The
1996 Criteria Document reported associations between short-term PM
exposure and mortality or morbidity from studies conducted in locations
across the U.S. as well as in other countries, and concluded that the
epidemiologic data base had ``general internal consistency'' (EPA,
1996a, p. 13-30). New multi-city studies have allowed evaluation of
consistency in effect estimates across geographic locations, using
uniform statistical modeling approaches; the results suggest that
effect estimates differ from one location to another, but the extent of
variation is not clear. For example, the Canadian 8-city study reported
no evidence of heterogeneity in city-specific results in the initial
study findings; however, in the reanalysis to address model
specification issues, the findings suggested more evidence of
heterogeneity in associations between mortality and short-term
PM2.5 exposure (Burnett and Goldberg, 2003; EPA, 2004, p. 9-
39). The Criteria Document discussed a number of factors that would be
likely to cause variation in PM-health outcomes in different
populations and geographic areas in section 9.2.2.3, including
indicators of exposure to traffic-related pollution, population
characteristics that affect susceptibility or exposure differences,
distribution of PM sources, or geographic features that would affect
the spatial distribution of PM (EPA, 2004, p. 9-41). In addition, the
use of data collected on a 1-in-6 or 1-in-3 day schedule results in
reduced statistical power, resulting in less precision for estimated
effect estimates for the individual cities and increased potential
variability in results (EPA, 2004, p. 9-40). Overall, the Criteria
document concluded that ``[f]ocusing on the studies with the most
precision, it can be concluded that there is much consistency in
epidemiologic evidence regarding associations between short-term and
long-term exposures to fine
[[Page 2635]]
particles and cardiopulmonary mortality and morbidity.'' (EPA, 2004, p.
9-47).
(4) The form of concentration-response relationships (e.g., linear,
sigmoid) and the potential existence of thresholds was one of the
important research questions remaining in the previous review. The
Criteria Document recognized that it is reasonable to expect that there
likely are biologic thresholds for different health effects in
individuals or groups of individuals with similar innate
characteristics and health status (EPA, 2004, Section 9.2.2.5).
Individual thresholds would presumably vary substantially from person
to person due to individual differences in genetic-level susceptibility
and pre-existing disease conditions (and could even vary from one time
to another for a given person). Thus, it would be difficult to detect a
distinct threshold at the population level, below which no individual
would experience a given effect, especially if some members of a
population are unusually sensitive even down to very low
concentrations. The person-to-person difference in the relationship
between personal exposure to PM of ambient origin and the concentration
observed at a monitor may also add to the variability in observed
concentration-response relationships, further obscuring potential
population thresholds within the range of observed concentrations (CD,
p. 9-43, 9-44).
Several new epidemiologic studies have used different modeling
methods to address this question, and most have been unable to detect
threshold levels in the relationship between short-term PM exposure
(generally using PM10) and mortality; in fact, one single-
city analysis suggests that statistical methods would allow detection
of a threshold in the epidemiologic data if a clear threshold existed.
However, a few analyses in individual cities have provided suggestions
of some potential threshold levels, generally at fairly low ambient
concentrations. One single-city study used PM2.5 and
PM10-2.5 measurements in Phoenix and reported that there was
suggestive evidence of a threshold for the association between
mortality and short-term exposure to PM2.5 in the range of
20-25 [mu]g/m3 (Smith et al., 2000; EPA, 2004, p. 8-322).
The shape of the concentration-response function for long-term
exposure to PM2.5 with mortality was evaluated using data
from the ACS cohort. In the ACS reanalysis, the authors report that the
concentration-response functions for PM2.5 and all-cause and
cardiopulmonary mortality demonstrate near-linear increasing trends
through the range of particle levels observed in the fine particle
cohort (Krewski, p. 160). However, the HEI Review Committee concluded
that these results show no clear evidence either for or against overall
linearity (Krewski, p. 265). In the extended ACS study, the authors
reported that the associations for all-cause, cardiovascular and lung
cancer mortality ``were not significantly different from linear
associations'' (Pope, et al., 2002).
Thus, evaluation of the health effects data summarized in the
Criteria Document provides no evidence to support selecting any
particular population threshold for PM2.5. The Staff Paper
also recognized, however, that it is reasonable to expect that, for
individuals, there may be thresholds for specific health responses and
that it is possible that such thresholds exist toward the lower end of
these ranges (or below these ranges) but cannot be detected due to
variability in susceptibility across a population. Even in those few
studies with suggestive evidence of such thresholds, the potential
thresholds are at fairly low concentrations (EPA, 2004, sections 8.4.7
and 9.2.2.5).
(5) Few studies are available that assess the extent to which
reductions in ambient PM actually lead to reductions in health effects
attributable to PM. As discussed in sections 8.2.3.4 and 9.2.2.6 of the
Criteria Document, several epidemiologic studies were done in the Utah
Valley area over a time period when a major source of PM was closed,
resulting in markedly decreased PM10 concentrations. An
epidemiologic study reported that respiratory hospital admissions
decreased during the plant closure time period (EPA, 2004, p. 8-131;
Pope et al., 1989). Newly available controlled human exposure and
animal toxicology studies, using particles extracted from stored
PM10 sampling filters from the Utah Valley, have shown
inflammatory responses that are greater with extracts of particles
collected during the time period of source operation than when the
source was closed, suggesting that the PM from the steel mill was more
harmful than other ambient PM on an equal mass basis (EPA, 2004, p. 9-
73). Epidemiologic studies in Dublin, Ireland and Hong Kong also
provides evidence for reduced relative risks for mortality when PM
(measured as BS or PM10) and SO2 were reduced as
the result of interventions aimed at reducing air pollution. The
Criteria Document concluded that this small group of studies add
further support to the results of the hundreds of other epidemiologic
studies linking ambient PM exposure to an array of health effects, and
provide strong evidence that reducing emissions of PM and gaseous
pollutants has beneficial public health impacts (EPA, 2004, p. 9-45 to
9-46).
(6) Several issues related to fine particle exposure time periods
were assessed in the Criteria Document, as summarized in section 3.6.5
of the Staff Paper. As discussed above in this section, these include
the exposure time periods used in long-term exposure studies as well as
health outcome associations with very short time periods (e.g., 2-hour
average). An additional issue is the time period (``lag'') between fine
particle exposure and health outcome that is reported in short-term
exposure study results. In these epidemiologic studies, associations
are often tested for a range of lag periods, for example, with PM
concentrations from the same day as the effect, and one or more days
preceding the effect. In evaluating these results, it is important to
consider the pattern of results that is seen across the series of lag
periods. If there is an apparent pattern of results across the
different lags, with positive associations reported for a series of
consecutive lag periods, then selecting the single-day lag with the
largest effect from a series of positive associations is likely to
underestimate the overall effect size, since single-day lag effect
estimates do not fully capture the risk that may be distributed over
adjacent or other days (EPA, 2004, sections 8.4.4 and 9.2.2.4). For
many epidemiologic studies, the authors have reported just such a
pattern of associations across several consecutive lag periods (EPA,
2004, p. 8-279). However, if there is no apparent pattern or reported
effects vary across lag days, any result for a single day may well be
biased (CD, p. 9-42).
Some new studies have used a ``distributed lag'' model approach,
that captures an effect of PM over a series of days following
exposure.\18\ Where effects are found for a series of lag periods, a
distributed lag model will more accurately characterize the effect
estimate size. A number of recent studies that have investigated
associations with distributed lags provide effect estimates for health
responses that persist over a period of time (days to weeks) after the
exposure period. Effect estimates from distributed lag models are thus
often, but not always, larger in size that those for single-day lag
periods (EPA, 2004, p. 8-281).
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\18\ The available studies have generally used PM10,
but not PM2.5 or PM10-2.5.
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[[Page 2636]]
The Criteria Document concludes that it is likely that the most
appropriate lag period for a study will vary depending on the health
outcome and the specific pollutant under study. For example, for a
health outcome such as a delayed asthma response, the lag period of a
day or several days might be expected between exposure and outcome;
however, some cardiovascular responses might be expected to occur
within a very short time period (e.g., an hour) after exposure (EPA,
2004, p. 8-279). As shown in Figures 8-24 to 8-28, the Criteria
Document notes a pattern of stronger associations between
PM10 and mortality or cardiovascular hospitalization with
shorter lag periods (e.g., same-day or 1-day lagged PM10).
For other effects, however, such as respiratory symptoms, asthma
emergency department visits or hospitalization, stronger effects were
reported with PM concentrations averaged over several days (EPA, 2004,
pp. 8-273 to 8-279). Thus, the Criteria Document concludes that one
would expect to see different best-fitting lags for different health
effects, based on potentially different biological mechanisms as well
as individual variability in responses (EPA, 2004, p. 8-342). For some
health outcomes, it is reasonable to expect associations to be observed
with PM exposures on the same day or with very short lag periods, but
not longer lag periods. In other cases, multi-day average exposure
periods or distributed lag models would more appropriately estimate
potential PM-related health risks.
(7) Looking more broadly to integrate epidemiologic evidence with
that from exposure-related, dosimetric and toxicologic studies, EPA has
considered the coherence of the evidence and the extent to which the
new evidence provides insights into mechanisms by which PM, especially
fine particles, may be affecting human health. Progress made in gaining
insights into potential mechanisms lends support to the biologic
plausibility of results observed in epidemiologic studies. For
cardiovascular effects, the convergence of important new epidemiologic
and toxicologic evidence (especially from studies using concentrated
ambient particles) builds support for the plausibility of causal
associations, especially between fine particles and physiological
endpoints indicative of increased risk of cardiovascular disease and
changes in cardiac rhythm. This finding is supported by new
cardiovascular effects research focused on fine particles that has
notably advanced our understanding of potential mechanisms by which
PM2.5 exposure, especially in susceptible individuals, could
result in changes in cardiac function or blood parameters that are risk
factors for cardiovascular disease. For respiratory effects,
toxicologic studies have provided evidence that supports plausible
biologic pathways for fine particles, including inflammatory responses,
increased airway responsiveness, or altered responses to infectious
agents. Further, coherence across a broad range of cardiovascular and
respiratory health outcomes is supported by evidence from epidemiologic
and toxicologic studies done in the same location, for example, in the
series of studies conducted in or evaluating ambient PM from Boston and
the Utah Valley (EPA, 2004, 7-42 to 43, 7-46 to 47, and 9-45).
Toxicologic studies have suggested that some combustion-related
particles, including particles from wood burning and diesel engine
exhaust, but not others such as coal fly ash, may have carcinogenic
effects (EPA, 2004, Section 7.8.4). This evidence supports the
plausibility of the observed relationship between fine particles and
lung cancer mortality. Evidence for PM-related infant mortality and
developmental effects poses an emerging concern, but the current
information is still very limited in support of the plausibility of
potential ambient PM relationships. More generally, toxicologic animal
studies often test effects of exposures to individual chemical
components, and thus the physical and chemical characteristics may
differ from those of particles in ambient air to which humans are
exposed. These and other differences in toxicologic and epidemiologic
study designs complicate the assessment of coherence in results from
across disciplines (EPA, 2004, section 9.2.3.1; Schlesinger and Cassee,
2003).
Overall, the Criteria Document finds that much more evidence is now
available related to the coherence and plausibility of effects than in
the last review. For short-term exposures, integration of evidence from
epidemiologic and toxicologic studies indicates both coherence and
plausibility of effects on the cardiovascular and respiratory systems,
especially for fine particles (EPA, 2004, p. 9-79). There is evidence
supporting coherence and plausibility for the observed associations
between long-term exposures to fine particles and lung cancer mortality
(EPA, 2004, p. 9-78).
(8) In summary, as discussed in the Staff Paper (section 3.5) and
the Criteria Document (section 9.2.2), the extensive body of
epidemiologic evidence now available continues to support likely causal
associations between PM2.5 and a broad range of mortality
and morbidity health outcomes based on an assessment of the strength of
the evidence, including the strength and robustness of reported
associations and the consistency of the results. While the limitations
and uncertainties in the available evidence suggest caution in
interpreting the epidemiologic studies at the lower levels of air
quality observed in the studies, the evidence now available provides
strong support that both short-term and long-term exposures to fine
particles are plausibly associated with a broad range of effects on the
respiratory and cardiovascular systems. The Criteria Document
concludes: ``the epidemiological evidence continues to support likely
causal associations between PM2.5 and PM10 and
both mortality and morbidity from cardiovascular and respiratory
diseases, based on an assessment of strength, robustness, and
consistency in results.'' (EPA, 2004, p. 9-48). In its integrative
assessment, the Criteria Document finds that health evidence from
various disciplines provides a strong and coherent basis for concluding
that both short-term and long-term exposure to fine particles is
associated with health effects ranging from subtle changes in lung
function to premature mortality.
4. Sensitive Subgroups for PM2.5-Related Effects
As described in the PM Criteria Document, the term susceptibility
refers to innate (e.g., genetic or developmental) or acquired (e.g.,
personal risk factors, age) factors that make individuals more likely
to experience effects with exposure to pollutants. A number of
population subgroups have been identified as potentially susceptible to
health effects as a result of PM exposure, including people with
existing heart and lung diseases, including diabetes, and older adults
and children. In addition, new attention has been paid to the concept
of some population groups having increased vulnerability to pollution-
related effects due to factors such as socioeconomic status or factors
that result in particularly elevated exposure levels, such as residence
near sources such as roadways (EPA, 2004, p. 9-81).
A good deal of evidence indicates that people with existing heart
or lung diseases are more susceptible to PM-related effects. In
addition, new studies have suggested that people with diabetes, who are
at risk for cardiovascular disease, may have
[[Page 2637]]
increased susceptibility to PM exposures. As discussed in Section
9.2.4.1 of the Criteria Document, this body of evidence includes
findings from epidemiologic studies that associations with mortality or
morbidity are greater in those with preexisting conditions, as well as
evidence from toxicologic studies using animal models of
cardiopulmonary disease. In addition, dosimetric evidence indicates
that deposition of particles is increased, and can be focused in ``hot
spots'' in the respiratory tract, in people with chronic respiratory
diseases.
Two age groups, older adults and the very young, are also
potentially at greater risk for PM-related effects. Epidemiologic
studies have generally not shown striking differences between adult age
groups. However, some epidemiologic studies have suggested that serious
health effects, such as premature mortality, are greater among older
populations (EPA, 2005a, p. 8-328). In addition, preexisting
respiratory or cardiovascular conditions are more prevalent in older
adults than younger age groups; thus there is some overlap between
potentially susceptible groups of older adults and people with heart or
lung diseases.
Epidemiologic evidence has reported associations with emergency
hospital admissions for respiratory illness and asthma-related symptoms
in children. Several factors may make children susceptible to PM-
related effects, including the greater ventilation rate per kilogram
body weight in children, greater prevalence of chronic asthma, and the
fact that children are more likely to be active outdoors and thus have
greater exposures. In addition, there is a more limited body of new
evidence from epidemiologic studies for potential PM-related health
effects in infants, using various PM indicators. Results from this body
of evidence, though mixed, are suggestive of possible effects; more
research is needed to further elucidate the potential risks of PM
exposure for these health outcomes (EPA, 2004, p. 8-222).
In summary, there are several population groups that may be
especially susceptible or vulnerable to PM-related effects. These
groups include those with preexisting heart and lung diseases, older
adults and children. Emerging evidence indicates that people from lower
socioeconomic strata or who have particularly elevated exposures may be
more vulnerable to PM-related effects.
5. PM2.5-Related Impacts on Public Health
As just discussed, there are several population groups that may be
especially susceptible or vulnerable to effects from exposure to PM.
These population subgroups, such as young children or older adults, and
people with pre-existing heart or lung diseases, constitute a large
portion of the U.S. population. For example, approximately 22 million
people, or 11 percent of the U.S. population, have received a diagnosis
of heart disease, about 20 percent of the population has hypertension
and about 9 percent of adults and 11 percent of children in the U.S.
have been diagnosed with asthma. In addition, about 26 percent of the
U.S. population is under 18 years of age,\19\ and about 12 percent is
65 years of age or older (EPA, 2004, Table 9-4). EPA recognizes that
combining fairly small risk estimates and small changes in PM
concentrations with large groups of the U.S. population would result in
large public health impacts.
---------------------------------------------------------------------------
\19\ Health studies that have suggested that children are
susceptible to PM-related effects include varying age ranges, for
example, for hospital admissions in children up to 18 years of age,
or respiratory symptoms in panels of 4th and 5th grade children.
---------------------------------------------------------------------------
One issue that is important for interpreting the public health
implications of the associations reported between mortality and short-
term exposure to PM is whether mortality is occurring only in very
frail individuals (sometimes referred to as ``harvesting''), resulting
in loss of just a few days of life expectancy. A number of new analyses
assess the likelihood of such ``harvesting'' occurring in the short-
term exposure studies. Overall, the Criteria Document concludes from
the time-series studies that there appears to be no strong evidence to
suggest that short-term exposure to PM is only shortening life by a few
days (EPA, 2004, Section 8.4.10). In addition to the evidence from
short-term exposure studies discussed above, one new report used the
mortality risk estimates from the ACS prospective cohort study to
estimate potential loss of life expectancy from PM-related mortality in
a population. The authors estimated that the loss of population life
expectancy associated with long-term exposure to PM2.5 was
on the order of a year or so (EPA, 2004, p. 8-334). The Criteria
Document recognizes that these calculations were based on studies in
adult populations, and potential population life shortening would be
increased if the new, albeit limited, evidence from infant mortality
studies was considered (EPA, 2004, p. 8-335). The Criteria Document
also observes that the risk estimates reported for long-term fine
particle exposures and lung cancer mortality are in about the same
range as the risk seen for a nonsmoker living with a smoker (EPA, 2004,
p. 9-94).
Large subgroups of the U.S. population are included in
subpopulations considered to be potentially sensitive to effects
related to fine particle exposures (EPA, 2004, section 9.2.5.1). While
individual epidemiologic effect estimates may be small in size, the
public health impact of the mortality and morbidity associations can be
quite large. In addition, it appears that mortality risks are not
limited to the very frail. Taken together, these results suggest that
exposure to ambient PM, especially PM2.5, can have
substantial public health impacts (EPA, 2004, p. 9-93).
B. Quantitative Risk Assessment
This section discusses the approach used to develop quantitative
risk estimates associated with exposures to PM2.5 building
upon a more limited risk assessment that was conducted during the last
review.\20\ At that time, EPA conducted a very limited risk assessment
covering a portion of two cities (i.e., Philadelphia County and
Southeast Los Angeles County) for which ambient PM2.5 data
were available. For short-term exposure mortality and morbidity health
effects, the prior assessment relied on either pooled analyses that
combined the results from several studies of individual cities or
individual single- and multi-city studies, none of which included the
two urban counties for which risks were estimated, to estimate
concentration-response relationships for these two cities. EPA
recognized that the lack of city-specific relative risks introduced
substantial uncertainties in the risk estimates due to inherent
differences (e.g., different population characteristics, PM size
distributions) that might influence the concentration-response
relationships. For long-term exposure mortality, the prior assessment
relied on the concentration-response relationship reported in the
original ACS study (Pope et al., 1995). Additional important
uncertainties noted at the time of that assessment with respect to all
health effects included: (1) The absence of clear evidence regarding
mechanisms of
[[Page 2638]]
action for the various effects of interest, (2) uncertainties about the
shape of the concentration-response relationships; and (3) concern
about whether the use of ambient PM2.5 fixed-site monitoring
data adequately reflected the relevant population exposures to PM that
are responsible for the reported health effects (61 FR 65650).
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\20\ The methodology, scope, and results from the risk
assessment conducted in the last review are described in Chapter 6
of the 1996 Staff Paper (EPA, 1996b) and in several technical
reports (Abt Associates, 1996; Abt Associates, 1997a,b) and
publications (Post et al., 2000; Deck et al., 2001).
---------------------------------------------------------------------------
In light of the substantial uncertainties in the prior risk
estimates, EPA placed greater weight on the overall conclusions derived
from the health effect studies--that ambient PM was likely causing or
contributing to significant adverse effects at levels below those
permitted by the then-existing PM10 standards--than on the
specific concentration-response functions and quantitative risk
estimates derived from them. Nevertheless, EPA judged that the
assessment provided reasonable estimates as to the possible extent of
risk for those effects given the available information (62 FR at 38656).
1. Overview
The updated risk assessment conducted as part of this review
includes estimates of (1) risks of mortality, morbidity, and symptoms
associated with recent ambient PM2.5 levels; (2) risk
reductions and remaining risks associated with just meeting the current
suite of PM2.5 NAAQS; and (3) risk reductions and remaining
risks associated with just meeting various alternative PM2.5
standards in a number of example urban areas. This risk assessment is
more fully described and presented in the Staff Paper (EPA, 2005a,
Chapter 4) and in a technical support document, Particulate Matter
Health Risk Assessment for Selected Urban Areas (Abt Associates,
2005a). The scope and methodology for this risk assessment were
developed over the last few years with considerable input from the
CASAC PM Panel and the public.\21\ The information presented in these
documents included specific criteria for the selection of health
endpoints and studies to include in the assessment. It also addressed
which alternative statistical models (e.g., for control of time-varying
factors such as weather and for various lags) to include in the
assessment, recognizing that some of the health studies presented
results from a large number of alternative models. In an advisory
letter sent by CASAC to the Administrator documenting its advice in May
2002 (Hopke, 2002), CASAC concluded that the general methodology and
framework to be used in the assessment were appropriate.
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\21\ In June 2001, OAQPS released a draft document, PM NAAQS
Risk Analysis Scoping Plan (EPA, 2001), for CASAC consultation and
public comment, which described staff's general plan for this
assessment. In January 2002, OAQPS released a more detailed draft
document, Proposed Methodology for Particulate Matter Risk Analyses
for Selected Urban Areas (Abt Associates, 2002), for CASAC review
and public comment, which described staff's plans to assess (a)
PM2.5-related risks for several health endpoints,
including mortality, hospital admissions, and respiratory symptoms
and (b) PM10-2.5-related risks for hospital admissions
and respiratory symptoms (as discussed below in Section III.B).
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The goals of the PM2.5 risk assessment were: (1) To
provide estimates of the potential magnitude of mortality and morbidity
effects associated with current PM2.5 levels, and with
meeting the current suite of PM2.5 NAAQS and alternative
PM2.5 standards, in specific urban areas; (2) to develop a
better understanding of the influence of various inputs and assumptions
on the risk estimates; and (3) to gain insights into the distribution
of risks and patterns of risk reductions associated with meeting
alternative suites of PM2.5 standards. EPA recognizes that
there are many sources of uncertainty and variability inherent in the
inputs to this assessment and that there is a high degree of
uncertainty in the resulting PM2.5 risk estimates. While
some of these uncertainties have been addressed quantitatively in the
form of estimated confidence ranges around central risk estimates,
other uncertainties and the variability in key inputs are not reflected
in these confidence ranges, but rather have been addressed through
separate sensitivity analyses or characterized qualitatively.
2. Scope and Key Components
The risk assessment estimates risks of various health effects
associated with exposure to ambient PM2.5 in nine urban
areas selected to illustrate the public health impacts associated with
a recent year of air quality and potential reductions in risk
associated with just meeting the current suite of PM2.5
standards and alternative suites of standards. The selection of urban
areas was largely determined by identifying areas in the U.S. for which
acceptable epidemiological studies were available that estimated
concentration-response relationships for PM2.5, which were
then used in assessing the risks. Thus, unlike the prior risk
assessment, the current risk assessment for short-term exposure
mortality and morbidity health effects used concentration-response
relationships reported in studies that included the urban areas for
which risks were estimated. Based on a review of the evidence evaluated
in the Criteria Document and Staff Paper, as well as the criteria
discussed in Chapter 4 of the Staff Paper, the following broad
categories of health endpoints were included in the risk assessment for
PM2.5 associated with short-term exposure: Total (non-
accidental), cardiovascular, and respiratory mortality; hospital
admissions for cardiovascular and respiratory causes; and respiratory
symptoms not requiring hospitalization. Also included in the
PM2.5 risk assessment were total, cardiopulmonary, and lung
cancer mortality associated with long-term exposure.
The available long-term exposure mortality concentration-response
functions are all based on cohort studies, in which a cohort of
individuals is followed over time. Based on the evaluation contained in
the Criteria Document and EPA's assessment of the complete data base
addressing mortality associated with long-term exposure to
PM2.5, studies based on the following two cohorts were
identified as being particularly relevant for the PM2.5 risk
assessment: (1) The Six Cities study cohort (referred to as Krewski et
al. (2000)--Six Cities) and (2) the ACS cohort (referred to Krewski et
al. (2000)--ACS), which includes a much larger number of individuals
from many more cities. In addition, Pope et al. (2002) extended the
follow-up period for the ACS cohort to sixteen years and published
findings on the relation of long-term exposure to PM2.5 and
all-cause mortality as well as cardiopulmonary and lung cancer
mortality (referred to as Pope et al. (2002)--ACS extended).\22\
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\22\ The use of these particular cohort studies to estimate
health risks associated with long-term exposure to PM2.5
is consistent with the views expressed in the National Academy of
Sciences (2002) report, ``Estimating the Public Health Benefits of
Proposed Air Pollution Regulations,'' and the Science Advisory Board
Clean Air Act Compliance Council review of the proposed methodology
to estimate the health benefits associated with the Clean Air Act
(SAB, 2004).
---------------------------------------------------------------------------
The available short-term exposure morbidity and mortality
concentration-response functions used in the risk assessment are all
from time series studies. The risk assessment included only those
health endpoints for which the the Criteria Document concluded that
there is likely to be a causal relationship with short-term exposure to
PM2.5 based on the overall weight of the evidence from the
collective body of available studies. Also, given the large number of
endpoints and studies addressing PM2.5-related effects, the
assessment only included the more severe and better understood (in
terms of health consequences) health effects. As noted above, in
contrast to the prior risk assessment, the concentration-response
functions used in this assessment for each urban area are
[[Page 2639]]
based on results of studies for that specific area or from a multi-city
study that included that specific area.
The concentration-response relationships used in the assessment
were based on findings from human epidemiological studies that have
relied on fixed-site, population-oriented, ambient monitors as a
surrogate for actual ambient PM2.5 exposures. The risk
assessment addresses risks attributable to anthropogenic sources and
activities (i.e., risk associated with concentrations above policy-
relevant background \23\ or above various selected higher cutpoints
intended as surrogates for alternative assumed population thresholds).
This approach of estimating risks in excess of background was judged to
be more relevant to policy decisions regarding ambient air quality
standards than risk estimates that include effects potentially
attributable to uncontrollable background PM concentrations. For the
base case analyses, an estimate of the annual average background level
was used, rather than a maximum 24-hour value, since estimated risks
were aggregated for each day throughout the year.
---------------------------------------------------------------------------
\23\ Background PM concentrations used in the PM risk assessment
were defined in Chapter 2 of the Staff Paper as the PM
concentrations that would be observed in the U.S. in the absence of
anthropogenic emissions of PM and its precursors in the U.S.,
Canada, and Mexico. For the initial base case risk estimates, the
midpoints of the appropriate ranges of annual average estimates for
PM2.5 background presented in the Staff Paper were used
(i.e., eastern values were used for eastern study locations and
western values were used for western study locations). Estimated
policy-relevant background concentrations are 3.5 [mu]g/m\3\ in
eastern cities, and 2.5 [mu]g/m\3\ in western cities.
---------------------------------------------------------------------------
In order to estimate the incidence of a particular health effect
associated with recent conditions in a specific county or set of
counties attributable to ambient PM2.5 exposures in excess
of background or various alternative cutpoints, as well as the change
in incidence corresponding to a given change in PM2.5 levels
resulting from just meeting a specified set of alternative
PM2.5 standards, three elements are required. These elements
are: (1) Air quality information (including recent air quality data for
PM2.5 from ambient monitors for the selected location,
estimates of background PM2.5 concentrations appropriate for
that location, and a method for adjusting the recent data to reflect
patterns of air quality estimated to occur when the area just meets a
given set of PM2.5 standards); (2) relative risk-based
concentration-response functions that provide an estimate of the
relationship between the health endpoints of interest and ambient PM
concentrations; and (3) annual or seasonal baseline health effects
incidence rates and population data, which are needed to provide an
estimate of the annual or seasonal baseline incidence of health effects
in an area before any changes in PM air quality.
The risk assessment for PM2.5 included a series of base
case analyses that characterized the uncertainty associated with the
form of the concentration-response relationship drawn from the studies
used in the assessment--this uncertainty had by far the greatest impact
on estimated risks. Other uncertainties addressed in various
sensitivity analyses (e.g., the use of single-versus multi-pollutant
models, single-versus multi-city models, use of a distributed lag
model, alternative assumptions about the relevant air quality for long-
term exposure mortality, and alternative constant or varying background
levels) all have a more moderate and often variable impact on the risk
estimates in some or all of the cities.
In estimating health risks remaining upon just meeting the current
and alternative PM2.5 standards, the assessment includes a
series of base cases, while noting that the confidence ranges in the
estimates do not reflect all the identified uncertainties. As discussed
above in section II.A.3, additional uncertainty for short-term exposure
mortality is related to the use of alternative statistical models and
methods to control for time-varying effects, such as weather or season,
and to address alternative lag structures. To provide a consistent
basis for comparison across studies and locations, the risk assessment
used concentration-response functions based on the most common type of
analysis (``generalized additive methods'') and on lag structures
judged to be most appropriate for each specific health endpoint, as
discussed in the Staff Paper (EPA, 2005a, p. 4-24). The risk assessment
included a sensitivity analysis for one location where a wide array of
statistical models and lags was reported in the health study for that
location (Los Angeles, as reported in Moolgavkar, 2003). EPA recognizes
that there is additional uncertainty associated with choices about
appropriate modeling strategy (EPA, 2004, 8.4.2) and that this
uncertainty is not included in the confidence ranges presented for the
risk estimates.
As noted earlier, EPA recognizes that while there are likely
biological thresholds in individuals for specific health endpoints, the
available epidemiologic studies do not support or refute the existence
of thresholds at the population level for either long-term or short-
term PM2.5 exposures within the range of air quality
observed in the studies (EPA, 2004, 9.2.2.5). Thus, base case risks
were estimated using not only the linear or log-linear concentration-
response functions reported in the studies, but also using a series of
modified linear functions, as discussed below, as surrogates for
assumed non-linear functions that would reflect the possibility that
thresholds may exist in the reported associations within the range of
air quality observed in the studies.
For short-term exposure mortality and morbidity outcomes associated
with PM2.5, the initial base case includes linear or log-
linear concentration-response models reported in the epidemiology
studies which are applied down to the estimated policy-relevant
background concentration level. Generally, the lowest measured
concentrations in the short-term exposure studies were relatively near
or below the estimated policy-relevant background levels such that
little or no extrapolation was required beyond the range of data in the
studies. In the case of the long-term exposure mortality studies for
PM2.5 that have been included in the risk assessment, the
lowest measured levels were in the range 7.5 to 11 [mu]g/m\3\. For the
initial base case scenario for this endpoint, the reported linear
models were applied down to 7.5 [mu]g/m\3\, which is the lowest
measured level reported in the long-term studies. Going down to an
estimated policy-relevant background level for short-term exposure
studies and to 7.5 [mu]g/m\3\ for long-term studies provides a
consistent framework which facilitates comparison of risk estimates
across urban locations within each group of studies and avoids
significant extrapolation beyond the range of concentrations included
in these studies.
Additional base case scenarios for both short- and long-term
exposure health endpoints involved the use of alternative
concentration-response functions that incorporated a modified linear
slope with an imposed cutpoint (i.e., an assumed threshold). For
mortality associated with short-term exposure, the base case analyses
included risk estimates associated with cutpoints of 10, 15, and 20
[mu]g/m. For mortality associated with long-term PM2.5
exposure, cutpoints of 10 and 12 [mu]g/m\3\ were included. For the base
case scenarios involving alternative cutpoints, the approach used to
develop alternative functions incorporates a modified linear slope with
an imposed cutpoint (i.e., an assumed population threshold) that is
intended to reflect a
[[Page 2640]]
hypothetical inflection point in a typical non-linear, ``hockey-stick''
shaped function, below which there is little or no population response.
More specifically, the slope of the concentration-response relationship
has been adjusted assuming that the upward-sloping portion of the
``hockey stick'' would be the slope estimated in the original
epidemiologic study adjusted by the inverse of the proportion of the
range of PM levels observed in the study that was above the cutpoint.
The Staff Paper concludes that this simple slope adjustment approach
represents a reasonable approach to illustrating the potential impact
of possible non-linear concentration-response relationships. In its
review of the Staff Paper and risk assessment, the CASAC PM Panel
commented that for the purpose of estimating public health impacts, it
``favored the primary use of an assumed threshold of 10 [mu]g/m\3\''
and that ``a major research need is for more work to determine the
existence and level of any thresholds that may exist or the shape of
nonlinear concentration-response curves at low levels of exposure that
may exist'' (Henderson, 2005a).
3. Risk Estimates and Key Observations
In focusing on the five study areas that do not meet the current
PM2.5 standards based on 2001-2003 air quality data
(Detroit, Los Angeles, Philadelphia, Pittsburgh, and St. Louis), the
total mortality risk estimates associated with simulating air quality
reductions to just meet the current PM2.5 standards (based
on associations with long-term PM2.5 exposure, and using the
lowest cutpoint of 7.5 [mu]g/m\3\) range from several hundred to over
1500 deaths per year, which translate into an incidence rate of
approximately 16 to 35 deaths per year per hundred thousand
population.\24\ These estimated risks associated with long-term
exposure represent approximately 2.6 to 3.2 percent of total mortality
in those areas. Estimated risks associated with long-term exposure
based on an assumed cutpoint of 10 [mu]g/m\3\ are roughly half as large
as the estimates based on a cutpoint of 7.5 [mu]g/m\3\. In the same
five areas, the estimates of mortality risk associated with short-term
PM2.5 exposure, based on a cutpoint equal to policy-relevant
background or 10 [mu]g/m, range from less than 20 percent to over 50
percent of the estimates associated with long-term exposure.\25\
---------------------------------------------------------------------------
\24\ The full range of quantitative risk estimates associated
with just meeting the current PM2.5 standards are presented in
Tables 4-9, 4-10, 4-12, and 4-13 in Chapter 4 of the Staff Paper.
\25\ In some areas, the 95 percent confidence ranges associated
with the risk estimates for short-term exposure (but not long-term
exposure) extend to below zero, reflecting appreciably more
uncertainty in estimates based on positive but not statistically
significant associations.
---------------------------------------------------------------------------
Reductions in risk associated with simulating air quality to just
meet a range of lower alternative annual and 24-hour PM2.5
standards were also estimated in this assessment. The estimated risk
reductions are depicted graphically in the Staff Paper (EPA, 2005a,
Figures 5-1 and 5-2 and Figures 5A-1 and 5A-2), showing patterns of
estimated risk reductions associated with alternative suites of
standards for all the various assumed cutpoints. As would be expected,
patterns of increasing estimated risk reductions are observed as either
the annual or 24-hour standard, or both, are reduced over the range
considered in this assessment, and the estimated percentage reductions
in risk are strongly influenced by the assumed cutpoint level.
The discussion below highlights additional observations and
insights from this PM2.5 risk assessment, together with
important caveats and limitations.
(1) With respect to short-term exposure mortality and morbidity,
this risk assessment provides the basis for greater confidence in the
results as compared to the prior assessment, given that studies are now
available using PM2.5 as the indicator in a much greater
number of locations, and the assessment is able to use city-specific
functions that are matched to the locations for which risks are
estimated. This contrasts with the use of pooled concentration-response
functions in the prior assessment which did not include studies for the
specific cities included in that assessment. However, EPA recognizes
that the confidence ranges, which only reflect uncertainty associated
with the precision of the study (related to the population size and
duration of the study), may be larger for the current risk estimates
due to the use of concentration-response functions from smaller, city-
specific studies now versus the use of concentration-response functions
from pooled sets of studies that have greater statistical precision.
Comparing the risk estimates for the only two specific locations that
were included in both the prior and current assessments, the magnitude
of the estimates associated with just meeting the current annual
standard, in terms of percentage of total incidence, is similar in one
of the locations (Philadelphia) and the current estimate is lower in
the other location (Los Angeles).
(2) With respect to long-term exposure mortality risk estimates,
the prior risk assessment focused on the estimates based on the
original ACS study (Pope et al., 1995). Since that time additional
cohort analyses have been published and evaluated in the Criteria
Document. EPA has greater confidence in the current risk estimates for
long-term exposure mortality, given the extensive review of these
studies and the extension of the ACS study to additional years of data,
as well as improvements in the statistical approach. However, ACS-based
risk estimates remain sensitive to plausible changes in statistical
model specifications. The choice of studies and concentration-response
functions to use for the base case risk estimates is discussed in the
Staff Paper (EPA, 2005a, p. 4-25) and risk assessment report (Abt
Associates, 2005, pp.49-50) and is consistent with the advice provided
by both the National Academy of Sciences and the Science Advisory board
Clean Air Act Compliance Council (see footnote 22). At the same time,
EPA recognizes that alternative statistical models were examined in the
reanalysis of the ACS and Six-Cities studies, and that the uncertainty
associated with model selection (such as multipollutant models and
different effect estimates associated with different educational
levels) is not reflected in the confidence ranges presented in this
assessment. Thus, for long-term exposure mortality risk estimates there
are additional unquantified uncertainties associated with a lack of
understanding as to which statistical model best represents the actual
concentration-response function. The relative risk estimates used in
the current risk assessment from the ACS extended study are only
slightly smaller (1.06 with 95 percent confidence interval of 1.02-
1.11) compared to the original ACS study (1.07 with 95 percent
confidence interval 1.04-1.10) used in the prior assessment. In terms
of the magnitude of the risk estimates, the estimates in terms of
percentage of total incidence are very similar for the two specific
locations included in both the prior and current assessments.
(3) A fairly wide range of risk estimates are observed for
PM2.5-related morbidity and mortality risk associated with
recent air quality across the urban areas analyzed. The impact of
adding additional co-pollutants to the models was variable; sometimes
there was relatively little difference, while in other cases there were
larger differences. The wide variability in risk estimates associated
with a recent year of air
[[Page 2641]]
quality is to be expected given the wide range of PM2.5
levels across the urban areas analyzed and the variation observed in
the concentration-response relationships obtained from the original
epidemiologic studies. Among other factors, this variability may
reflect differences in the mixture of components or sources of fine
particles, populations, exposure considerations (e.g., degree of air
conditioning use), differences in co-pollutants and/or other stressors,
differences in study design, and differences related to exposure and
monitor measurement error.
(4) The single most important factor influencing the quantitative
estimates of risk is which of the alternative concentration-response
functions included in this assessment are considered to best represent
the unknown ``true'' concentration-response relationships. In
comparison, the following uncertainties have only a moderate impact on
the risk estimates in some or all of the cities: choice of an
alternative estimated constant background level, use of a distributed
lag model, and alternative assumptions about the relevant air quality
for estimating exposure levels for long-term exposure mortality. Use of
a distribution of daily background concentrations had very little
impact on the risk estimates.
The overall pattern of risk associated with short-term
PM2.5 exposures across the distribution of PM2.5
air quality, as typically observed in urban areas, is similar to that
observed in the last review. That is, on an annual basis, the very
highest days (which pose the greatest risk in terms of deaths per day)
contribute less to the total annual health risk associated with short-
term exposures than the middle of the distribution, due to the much
greater number of days that occur in this part of the air quality
distribution.
(5) Risk estimates associated with just meeting the current suite
of PM2.5 standards in five urban areas that do not meet the
current PM2.5 standards showed a wide range of
PM2.5-related risk estimates for short-term exposure
mortality and morbidity. This is likely due, in large part, to
differences in concentration-response relationships among single-
location short-term exposure studies, differences in baseline incidence
rates, and varying population sizes. Results of a sensitivity analysis
which applied one multi-city concentration-response function to all
five urban areas analyzed narrowed considerably the range of risk
estimates when a risk metric was used that normalized for different
population sizes. However, it is still unknown whether the wider range
of estimates observed using single-city concentration-response
functions reflect methodological differences between studies and/or
real city-to-city differences related to exposure, population,
composition of the particles, or other factors.
(6) For the risk estimates associated with just meeting the current
suite of PM2.5 standards and alternative suites of
standards, the single most important factor influencing the short- and
long-term exposure mortality and morbidity estimates is again which of
the alternative concentration-response functions included in this
assessment are considered to best represent the unknown ``true''
concentration-response relationships. Several additional sources of
uncertainty are introduced into this portion of the risk assessment,
including: (1) Uncertainty in the degree to which the pattern of air
quality concentration reductions estimated for the risk assessment
cities represents the distribution of actual PM concentration changes
that would be observed in a given area (``rollback uncertainty'') and
(2) uncertainty concerning the degree to which current PM risk
coefficients may reflect contributions from other pollutants, or
uncertainty concerning whether all of the constituents of
PM2.5 would be reduced in similar proportion to the
reduction in PM2.5 as a whole, and, if not, what impact this
would have on estimated reductions in risk. For areas where the current
annual standard is the controlling standard, one alternative approach
to rolling back the distribution of daily PM2.5
concentrations, in which the upper end of the distributions of
concentrations was reduced by a greater amount than the rest of the
distribution, had little impact on the risk estimates. This approach or
alternative approaches to rolling back the distribution of daily
concentrations may have a greater impact on the risk estimates in areas
where the daily standard is the controlling standard.
(7) For the risk estimates associated with just meeting the current
or alternative suites of PM2.5 standards, there is a
significant decrease in the mortality risk estimates based on short-
term PM2.5 exposure remaining as one considers alternative
higher cutpoints. There also is a significant increase observed in the
percent reduction in estimated risk upon just meeting alternative
standards with higher alternative cutpoints. These findings are even
more pronounced for the mortality risk estimates associated with long-
term PM2.5 exposure as higher alternative cutpoint levels
are considered.
C. Need for Revision of the Current Primary PM2.5 Standards
The initial issue to be addressed in the current review of the
primary PM2.5 standards is whether, in view of the advances
in scientific knowledge reflected in the Criteria Document and Staff
Paper, the existing standards should be revised. Based on the
information and conclusions presented in the Criteria Document,
summarized above in section II.A., the Staff Paper concludes that the
newly available information generally reinforces the associations
between PM2.5 and mortality and morbidity effects observed
in the last review. While important uncertainties and research
questions remain, much progress has been made in reducing some key
uncertainties since the last review. The examination of specific
components, properties, and sources of fine particles that are linked
with health effects remains an important research need. Other important
research needs include better characterizing the shape of
concentration-response functions, including identification of potential
threshold levels, and methodological issues such as those associated
with selecting appropriate statistical models in time-series studies to
address time-varying factors (such as weather) and other factors (such
as other pollution variables), and better characterizing population
exposures. Nonetheless, important progress has been made in advancing
our understanding of potential mechanisms by which ambient
PM2.5, alone and in combination with other pollutants, is
causally linked with cardiovascular, respiratory, and lung cancer
associations observed in epidemiologic studies. In addition, health
effects associations reported in epidemiologic studies have been found
to be generally robust to confounding by co-pollutants, there is now
greater confidence in the results of long-term exposure studies due to
reanalyses and extensions of the critical studies, and there is an
increased understanding of susceptible populations. Based on these
considerations, the Staff Paper finds clear support in the available
evidence for fine particle standards that are at least as protective as
the current PM2.5 standards (EPA, 2005a, p. 5-6).
Having reached this initial conclusion, the Staff Paper addresses
the question of whether the available evidence supports consideration
of standards that are more protective than the current PM2.5
standards. In so doing, the Staff Paper considers whether there is now
evidence (1) that statistically significant health effects associations
[[Page 2642]]
with short-term exposures to fine particles occur in areas that would
likely meet the current PM2.5 standards or (2) that such
associations with long-term exposures to fine particles extend down to
lower air quality levels than had previously been observed.\26\ This
takes into consideration the bases for the decisions made in 1997 in
setting the current PM2.5 standards. In generally
considering what areas would likely meet the current PM2.5
standards, the focus is principally on comparing the long-term average
PM2.5 level in an area with the level of the current annual
PM2.5 standard, since in 1997 that standard was set to be
the ``generally controlling'' standard to provide protection against
health effects related to both short- and long-term exposures to fine
particles. In conjunction with such an annual standard, the current 24-
hour standard was set to provide only supplemental protection against
days with high peak PM2.5 concentrations, localized
``hotspots,'' or risks arising from seasonal emissions that might not
be well controlled by a national annual standard.
---------------------------------------------------------------------------
\26\ In addressing this question, the Staff Paper first
recognizes, as discussed above in section II.A.3, that although
there are likely biologic threshold levels in individuals for
specific health responses, the available epidemiologic evidence
neither supports nor refutes the existence of thresholds at the
population level for the effects of PM2.5 on mortality
across the range of concentrations in the studies, for either long-
term or short-term PM2.5 exposures (EPA, 2004, section 9.2.2.5).
---------------------------------------------------------------------------
In first considering the available epidemiologic evidence related
to short-term exposures, the Staff Paper focuses on specific
epidemiologic studies that show statistically significant associations
between PM2.5 and health effects for which the Criteria
Document judges associations with PM2.5 to be likely causal
(EPA, 2005a, section 5.3.1.1). Many more U.S. and Canadian studies are
now available that provide evidence of associations between short-term
exposure to PM2.5 and serious health effects in areas with
air quality at and above the level of the current annual
PM2.5 standard (15 [mu]g/m3). Moreover, a few newly
available short-term exposure mortality studies provide evidence of
statistically significant associations with PM2.5 in areas
with air quality levels below the levels of the current
PM2.5 standards. In considering these studies, the Staff
Paper focuses on those that include adequate gravimetric
PM2.5 mass measurements, and where the associations are
generally robust to alternative model specification and to the
inclusion of potentially confounding co-pollutants. Three such studies
conducted in Phoenix (Mar et al., 2003), Santa Clara County, CA
(Fairley, 2003) and eight Canadian cities (Burnett and Goldberg, 2003)
report statistically significant associations between short-term
PM2.5 exposure and total and cardiovascular mortality in
areas in which long-term average PM2.5 concentrations ranged
between 13 and 14 [mu]g/m3 and 98th percentile
concentrations ranged between 32 and 59 [mu]g/m3.\27\
---------------------------------------------------------------------------
\27\ As noted in the Staff Paper, these studies were reanalyzed
to address questions about the application of the statistical
software used in the original analyses, and the study results from
Phoenix and Santa Clara County were little changed in alternative
models (Mar et al., 2003; Fairley, 2003), although Burnett and
Goldberg (2003) reported that their results were sensitive to using
different temporal smoothing methods. Two of these studies also
reported significant associations with gaseous pollutants (Mar et
al., 2003; Fairley, 2003), and the other study included multi-
pollutant model results in reanalyses, reporting that associations
with PM2.5 remained significant with gaseous pollutants
(Fairley, 2003).
---------------------------------------------------------------------------
In also considering the new epidemiologic evidence available from
U.S. and Canadian studies of long-term exposure to fine particles, the
Criteria Document notes that new studies have built upon studies
available in the last review and concludes that these studies have
confirmed and strengthened the evidence of associations for both
mortality and respiratory morbidity (EPA, 2004, section 9.2.3). For
mortality, the Criteria Document places greatest weight on the
reanalyses and extensions of the Six Cities and ACS studies, finding
that these studies provide strong evidence for associations with fine
particles (EPA, 2004, p. 9-34), notwithstanding the lack of consistent
results in other long-term exposure studies. For morbidity, the
Criteria Document finds that new studies of a cohort of children in
Southern California have built upon earlier limited evidence to provide
fairly strong evidence that long-term exposure to fine particles is
associated with development of chronic respiratory disease and reduced
lung function growth (EPA, 2004, pp. 9-33 to 9-34). In addition to
strengthening the evidence of association, the new extended ACS
mortality study observed statistically significant associations with
cardiorespiratory mortality (including lung cancer mortality) across a
range of long-term mean PM2.5 concentrations that was lower
than was reported in the original ACS study available in the last review.
Beyond the epidemiologic studies using PM2.5 as an
indicator of fine particles, a large body of newly available evidence
from studies that used PM10, as well as other indicators or
components of fine particles (e.g., sulfates, combustion-related
components), provides additional support for the conclusions reached in
the last review as to the likely causal role of ambient PM, and the
likely importance of fine particles in contributing to observed health
effects. Such studies notably include new multi-city studies,
intervention studies (that relate reductions in ambient PM to observed
improvements in respiratory or cardiovascular health), and source-
oriented studies (e.g., suggesting associations with combustion- and
vehicle-related sources of fine particles). The Criteria Document also
notes that new epidemiologic studies of asthma-related increased
physicians visits and symptoms, as well as new studies of cardiac-
related risk factors, suggest likely much larger public health impacts
due to ambient fine particles than just those indexed by the mortality
and morbidity effects considered in the last review (EPA, 2004, p. 9-94).
In reviewing this information, the Staff Paper recognizes that
important limitations and uncertainties associated with this expanded
body of evidence for PM2.5 and other indicators or
components of fine particles, noted above in section II.A.2, need to be
carefully considered in determining the weight to be placed on the body
of studies available in this review. For example, the Criteria Document
notes that while PM-effects associations continue to be observed across
most new studies, the newer findings do not fully resolve the extent to
which the associations are properly attributed to PM acting alone or in
combination with other gaseous co-pollutants, particularly
SO2, or to the gaseous co-pollutants themselves. The
Criteria Document concludes, however, that overall the various
approaches that have now been used to evaluate this issue substantiate
that associations for various PM indicators with mortality and
morbidity are generally robust to confounding by co-pollutants (EPA,
2004, p. 9-37).
While the limitations and uncertainties in the available evidence
suggest caution in interpreting the epidemiologic studies at the lower
levels of air quality observed in the studies, the Staff Paper
concludes that the evidence now available provides strong support for
considering fine particle standards that would provide increased
protection beyond that afforded by the current PM2.5
standards. The Staff Paper notes that a more protective suite of
PM2.5 standards would reflect the generally stronger and
broader body of evidence of associations with mortality and morbidity
now available in this review, both at levels
[[Page 2643]]
below the current standards and extending to lower levels of air
quality than in earlier studies, as well as increased understanding of
possible underlying mechanisms.
In addition to this evidence-based evaluation, the Staff Paper also
considers the extent to which health risks estimated to occur upon
attainment of the current PM2.5 standards may be judged to
be important from a public health perspective, taking into account key
uncertainties associated with the quantitative health risk estimates.
In so doing, the Staff Paper first notes that the risk assessment
addresses a number of key uncertainties through various base case
analyses, as well as through several sensitivity analyses, as discussed
above in section II.B. In considering the health risks estimated to
occur upon attainment of the current PM2.5 standards, the
Staff Paper focuses in particular on a series of base case risk
estimates, while recognizing that the confidence ranges in the selected
base case estimates do not reflect all the identified uncertainties.
These risks were estimated using not only the linear or log-linear
concentration-response functions reported in the studies,\28\ but also
using alternative modified linear functions as surrogates for assumed
non-linear functions that would reflect the possibility that thresholds
may exist in the reported associations within the range of air quality
observed in the studies. Regardless of the relative weight placed on
the risk estimates associated with the concentration-response functions
reported in the studies or with the modified functions favored by
CASAC,\29\ the risk assessment indicates the possibility that thousands
of premature deaths per year would occur in urban areas across the U.S.
upon attainment of the current PM2.5 standards.\30\ Beyond
the estimated incidences of premature mortality, the Staff Paper also
recognizes that similarly substantial numbers of incidences of hospital
admissions, emergency room visits, aggravation of asthma and other
respiratory symptoms, and increased cardiac-related risk are also
likely in many urban areas, based on risk assessment results (EPA,
2005a, Chapter 4) and on the discussion related to this pyramid of
effects in the Criteria Document (EPA, 2004, section 9.2.5). Based on
these considerations, the Staff Paper concludes that the estimates of
risks likely to remain upon attainment of the current PM2.5
standards are indicative of risks that can reasonably be judged to be
important from a public health perspective.
---------------------------------------------------------------------------
\28\ As discussed above in section II.B.2, the reported linear
or log-linear concentration-response functions were applied down to
7.5 [mu]g/m3 in estimating risk associated with long-term
exposure (i.e., the lowest measured level in the extended ACS
study), and down to the estimated policy-relevant background level
in estimating risk associated with short-term exposure (i.e., 3.5
[mu]g/m\3\ for eastern urban areas and 2.5 [mu]g/m\3\ for western
urban areas).
\29\ The CASAC PM Panel generally favored the primary use of an
assumed threshold of 10 [mu]g/m\3\ for the various concentration-
response functions used in the risk assessment (Henderson, 2005a).
\30\ The Staff Paper recognizes how highly dependent any
specific risk estimates are on the assumed shape of the underlying
concentration-response functions, noting nonetheless that mortality
risks are not completely eliminated when current PM2.5
standards are met in a number of example urban areas even using the
highest assumed cutpoint levels considered in the risk assessment
(EPA, 2005a, p. 5-15).
---------------------------------------------------------------------------
In considering available evidence, risk estimates, and related
limitations and uncertainties, the Staff Paper concludes that the
available information clearly calls into question the adequacy of the
current suite of PM2.5 standards and provides strong support
for revising the current PM2.5 standards to provide
increased public health protection. Also taking into account these
considerations, the CASAC advised the Administrator that a majority of
CASAC Panel members were in agreement that the primary 24-hour and
annual PM2.5 standards ``should be modified to provide
increase public health protection'' (Henderson, 2005a). The CASAC
further advised that changes to either the annual standard or the 24-
hour standard, or both, could be recommended, and expressed reasons
that formed the basis for the consensus among the Panel members for
placing more emphasis on lowering the 24-hour standard (Henderson,
2005a).\31\
---------------------------------------------------------------------------
\31\ Of the individual Panel members who submitted written
comments expressing views on appropriate levels of the
PM2.5 standards, only one did not suppport changes to
either the 24-hour or annual standard to provide additional public
health protection (Henderson, 2005a). In written comments, the
health scientists on the CASAC Panel did not agree on whether the
annual standard should be lowered.
---------------------------------------------------------------------------
In considering whether the suite of primary PM2.5
standards should be revised to provide requisite public health
protection, the Administrator has carefully considered the rationale
and recommendations contained in the Staff Paper, the advice and
recommendations from CASAC, and public comments to date on this issue.
In so doing, the Administrator places primary consideration on the
evidence obtained from the studies, and provisionally finds the
evidence of serious health effects reported in short-term exposure
studies conducted in areas that would attain the current standards to
be compelling, especially in light of the extent to which such studies
are part of an overall pattern of positive and frequently statistically
significant associations across a broad range of studies that
collectively represent a strong and robust body of evidence. As
discussed in the Criteria Document and Staff Paper, the Administrator
recognizes that much progress has been made since the last review in
addressing some of the key uncertainties that were important
considerations in establishing the current PM2.5 standards.
In considering the risk assessment presented in the Staff Paper, the
Administrator notes that the assessment contained a sensitivity
analysis but not a formal uncertainty analysis, making it difficult to
use the risk assessment to form a judgment of the probability of
various risk estimates. Instead, the Administrator views the risk
assessment in light of his evaluation of the underlying studies. Seen
in this light, the risk assessment informs the determination of the
public health significance of risks to the extent that the evidence is
judged to support an effect at a particular level of air quality. Based
on these considerations, the Administrator provisionally concludes that
the current primary PM2.5 standards, taken together, are not
requisite to protect public health with an adequate margin of safety
and that revision is needed to provide increased public health
protection.
D. Indicator of Fine Particles
In 1997, EPA established PM2.5 as the indicator for fine
particles. In reaching this decision, the Agency first considered
whether the indicator should be based on the mass of a size-
differentiated sample of fine particles or on one or more components
within the mix of fine particles. Secondly, in establishing a size-
based indicator, a size cut needed to be selected that would
appropriately distinguish fine particles from particles in the coarse mode.
In addressing the first question in the last review, EPA determined
that it was appropriate to control fine particles as a group, as
opposed to singling out any particular component or class of fine
particles. Community health studies had found significant associations
between various indicators of fine particles (including
PM2.5 or PM10 in areas dominated by fine
particles) and health effects in a large number of areas that had
significant mass contributions of differing components or sources of
fine particles, including sulfates, wood smoke, nitrates, secondary
organic compounds and acid sulfate aerosols. In addition, a number of
animal
[[Page 2644]]
toxicologic and controlled human exposure studies had reported health
effects associations with high concentrations of numerous fine particle
components (e.g., sulfates, nitrates, transition metals, organic
compounds), although such associations were not consistently observed.
It also was not possible to rule out any component within the mix of
fine particles as not contributing to the fine particle effects found
in epidemiologic studies. For these reasons, EPA concluded that total
mass of fine particles was the most appropriate indicator for fine
particle standards rather than an indicator based on PM composition (62
FR 38667, July 18, 1997).
Having selected a size-based indicator for fine particles, the
Agency then based its selection of a specific size cut on a number of
considerations. In focusing on a size cut within the size range of 1 to
3 [mu]m (i.e., the intermodal range between fine and coarse mode
particles), the Agency noted that the available epidemiologic studies
of fine particles were based largely on PM2.5; only very
limited use of PM1 monitors had been made. While it was
recognized that using PM1 as an indicator of fine particles
would exclude the tail of the coarse mode in some locations, in other
locations it would miss a portion of the fine PM, especially under high
humidity conditions, which would result in falsely low fine PM
measurements on days with some of the highest fine PM concentrations.
The selection of a 2.5 [mu]m size cut reflected the regulatory
importance that was placed on defining an indicator for fine particle
standards that would more completely capture fine particles under all
conditions likely to be encountered across the U.S., especially when
fine particle concentrations are likely to be high, while recognizing
that some small coarse particles would also be captured by
PM2.5 monitoring. Thus, EPA's selection of 2.5 [mu]m as the
size cut for the fine particle indicator was based on considerations of
consistency with the epidemiologic studies, the regulatory importance
of more completely capturing fine particles under all conditions, and
the potential for limited intrusion of coarse particles in some areas;
it also took into account the general availability of monitoring
technology (62 FR 38668).
In this current review, the same considerations continue to apply
for selection of an appropriate indicator for fine particles. As an
initial matter, the available epidemiologic studies linking mortality
and morbidity effects with short- and long-term exposures to fine
particles continue to be largely indexed by PM2.5. Some
epidemiologic studies also have continued to implicate various
components within the mix of fine particles that have been more
commonly studied (e.g., sulfates, nitrates, carbon, organic compounds,
and metals) as being associated with adverse effects (EPA, 2004, p. 9-
31, Table 9-3). In addition, several recent studies have used
PM2.5 speciation data to evaluate the association between
mortality and particles from different sources (Schwartz, 2003a; Mar et
al., 2003; Tsai et al., 2000; EPA, 2004, section 8.2.2.5). Schwartz
(2003a) reported statistically significant associations for mortality
with factors representing fine particles from traffic and residual oil
combustion that were little changed in reanalysis to address
statistical modeling issues, and also an association between mortality
and coal combustion-related particles that was reduced in size and lost
statistical significance in reanalysis. In Phoenix, significant
associations were reported between mortality and fine particles from
traffic emissions, vegetative burning, and regional sulfate sources
that remained unchanged in reanalysis models (Mar et al., 2003).
Finally, a small study in three New Jersey cities reported significant
associations between mortality and fine particles from industrial, oil
burning, motor vehicle and sulfate aerosol sources, though the results
were somewhat inconsistent between cities (Tsai et al., 2000).\32\ No
significant increase in mortality was reported with a source factor
representing crustal material in fine particles (CD, p. 8-85).
Recognizing that these three studies represent a very preliminary
effort to distinguish effects of fine particles from different sources,
and that the results are not always consistent across the cities, the
Criteria Document found that these studies indicate that exposure to
fine particles from combustion sources, but not crustal material, is
associated with mortality (EPA, 2004, p. 8-77). Animal toxicologic and
controlled human exposure studies have continued to link a variety of
PM components or particle types (e.g., sulfates, notably primary metal
sulfate emissions from residual oil burning, metals, organic
constituents, bioaerosols, diesel particles) with health effects,
though often at high concentrations (EPA, 2004, section 7.10.2). In
addition, some recent studies have suggested that the ultrafine subset
of fine particles (generally including particles with a nominal mean
aerodynamic diameter less than 0.1 [mu]m) may also be associated with
adverse effects (EPA, 2004, pp. 8-67 to 68).
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\32\ More specifically, statistically significant associations
were reported with factors representing fine particles from oil
burning, industrial and sulfate aerosol sources in Newark and with
particles from oil burning and motor vehicle sources in Camden, and
no statistically significant associations were reported in Elizabeth.
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The Criteria Document recognizes that, for a given health response,
some fine particle components are likely to be more closely linked with
that response than others. The presumption that different PM
constituents may have differing biological responses is toxicologically
plausible and an important source of uncertainty in interpreting such
epidemiologic evidence. For specific effects there may be stronger
correlation with individual PM components than with aggregate particle
mass. In addition, particles or particle-bound water can act as
carriers to deliver other toxic agents into the respiratory tract,
suggesting that exposure to particles may elicit effects that are
linked with a mixture of components more than with any individual PM
component (EPA, 2004, section 9.2.3.1.3).
Thus, epidemiologic and toxicologic studies have provided evidence
for effects associated with various fine particle components or size-
differentiated subsets of fine particles. The Criteria Document
concludes: ``These studies suggest that many different chemical
components of fine particles and a variety of different types of source
categories are all associated with, and probably contribute to,
mortality, either independently or in combinations'' (EPA, 2004, p. 9-
31). Conversely, the Criteria Document provides no basis to conclude
that any individual fine particle component cannot be associated with
adverse health effects (EPA, 2005a, p. 5-17). In short, there is not
sufficient evidence that would lead toward the selection of one or more
PM components as being primarily responsible for effects associated
with fine particles, nor is there sufficient evidence to suggest that
any component should be eliminated from the indicator for fine
particles. The Staff Paper continues to recognize the importance of an
indicator that not only captures all of the most harmful components of
fine particles (i.e., an effective indicator), but also emphasizes
control of those constituents or fractions, including sulfates,
transition metals, and organics that have been associated with health
effects in epidemiologic and/or toxicologic studies, and is thus most
likely to result in the largest risk reduction (i.e., an efficient
indicator). Taking into account the above considerations, the Staff
Paper concludes that it remains appropriate to
[[Page 2645]]
control fine particles as a group; i.e., that total mass of fine
particles is the most appropriate indicator for fine particle standards
(EPA, 2005a, p. 5-17).
With regard to an appropriate size cut for a size-based indicator
of total fine particle mass, the Criteria Document concludes that
advances in our understanding of the characteristics of fine particles
continue to support the use of particle size as an appropriate basis
for distinguishing between these subclasses, and that a nominal size
cut of 2.5 [mu]m remains appropriate (EPA, 2004, p. 9-22). This
conclusion follows from a recognition that within the intermodal range
of 1 to 3 [mu]m there is no unambiguous definition of an appropriate
size cut for the separation of the overlapping fine and coarse particle
modes. Within this range, the Staff Paper considered size cuts of both
1 [mu]m and 2.5 [mu]m. Consideration of these two size cuts took into
account that there is generally very little mass in this intermodal
range, although in some circumstances (e.g., windy, dusty areas) the
coarse mode can extend down to and below 1 [mu]m, whereas in other
circumstances (e.g., high humidity conditions, usually associated with
very high fine particle concentrations) the fine mode can extend up to
and above 2.5 [mu]m. The same considerations that led to the selection
of a 2.5 [mu]m size cut in the last review--that the epidemiologic
evidence was largely based on PM2.5 and that it was more
important from a regulatory perspective to capture fine particles more
completely under all conditions likely to be encountered across the
U.S. (especially when fine particle concentrations are likely to be
high) than to avoid some coarse-mode intrusion into the fine fraction
in some areas--led to the same recommendation by the Staff Paper (EPA,
2005a, p. 5-18) and CASAC (Henderson, 2005a) in this review. In
addition, the Staff Paper recognizes that particles can act as carriers
of water, oxidative compounds, and other components into the
respiratory system, which adds to the importance of ensuring that
larger accumulation-mode particles are included in the fine particle
size cut (EPA, 2005a, p. 5-18).
Consistent with the Staff Paper and CASAC recommendations, the
Administrator proposes to retain PM2.5 as the indicator for
fine particles. Further, the Administrator provisionally concludes that
currently available studies do not provide a sufficient basis for
supplementing mass-based fine particle standards with standards for any
specific fine particle component or subset of fine particles, or for
eliminating any individual component or subset of components from fine
particle mass standards. Addressing the current uncertainties in the
evidence of effects associated with various fine particle components
and types of source categories is an important element in EPA's ongoing
PM research program.
The Administrator notes that some commenters have expressed views
about the importance of evaluating health effect associations with
various fine particle components and types of source categories as a
basis for focusing ongoing and future research to reduce uncertainties
in this area and for considering whether alternative indicator(s) are
now or may be appropriate for standards intended to protect against the
array of health effects that have been associated with fine particles
as indexed by PM2.5.\33\ Information from such studies could
also help inform the development of strategies that emphasize control
of specific types of emission sources so as to address particles of
greatest concern to public health. While recognizing that the studies
evaluated in the Criteria Document provide some limited evidence of
such associations that is helping to focus research activities, the
Administrator solicits broad public comment on issues related to
studies of fine particle components and types of source categories and
their usefulness as a basis for consideration of alternative
indicator(s) for fine particle standards. In general, comment is
solicited on relevant new published research, recommendations for
studies that would be appropriate for inclusion in future research
activities, and approaches to assessing the available and future
research results to determine whether alternative indicators for fine
particles are warranted to provide effective protection of public
health from effects associated with long- and short-term exposure to
ambient fine particles.
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\33\ Such comments have focused in part on newer studies that
have become available since the close of the Criteria Document,
which EPA intends to include in its assessment of potentially
significant new studies discussed above in section I.D.
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More specifically, comment is also solicited on a number of related
issues. One such issue is the extent to which reducing particular types
of PM (differentiated by either size or chemistry) might alter the size
and toxicity of remaining particles, and on the extent to which fine
particles in urban and rural areas can be differentiated by size or
chemistry. Another issue deals with assessment of human exposure and
its relationship with pollution measurements at monitors (EPA, 2004,
chapter 5); comment is solicited on the extent to which the latest
scientific information can be used to improve our understanding of the
relationship of monitored pollution levels to human exposure. Comment
is also solicited on studies using concentrated ambient particles
(CAPs) and their use in examining the toxicity of specific mixtures of
pollutants or of particular source categories.
E. Averaging Time of Primary PM2.5 Standards
In the last review, EPA established two PM2.5 standards,
based on annual and 24-hour averaging times, respectively (62 FR at
38668-70). This decision was based in part on evidence of health
effects related to both short-term (from less than 1 day to up to
several days) and long-term (from a year to several years) measures of
PM. EPA noted that the large majority of community epidemiologic
studies reported associations based on 24-hour averaging times or on
multiple-day averages. Further, EPA noted that a 24-hour standard could
also effectively protect against episodes lasting several days, as well
as providing some degree of protection from potential effects
associated with shorter duration exposures. EPA also recognized that an
annual standard would provide effective protection against both annual
and multi-year, cumulative exposures that had been associated with an
array of health effects, and that a much longer averaging time would
complicate and unnecessarily delay control strategies and attainment
decisions. EPA considered the possibility of seasonal effects, although
the very limited available evidence of such effects and the seasonal
variability of sources of fine particle emissions across the country did
not provide an adequate basis for establishing a seasonal averaging time.
In considering whether the information available in this review
supports consideration of different averaging times for
PM2.5 standards, the Staff Paper concludes that the
available information is generally consistent with and supportive of
the conclusions reached in the last review to set PM2.5
standards with both annual and 24-hour averaging times. In considering
the new information, the Staff Paper makes the following observations
(EPA, 2005a, section 5.3.3):
(1) There is a growing body of studies that provide additional
evidence of effects associated with exposure periods shorter than 24-
hours (e.g., one to several hours) (EPA, 2004, section 3.5.5.1). While
the Staff Paper concludes
[[Page 2646]]
that this information remains too limited to serve as a basis for
establishing a shorter-than-24-hour fine particle primary standard at
this time, it also noted that this information gives added weight to
the importance of a standard with a 24-hour averaging time.
(2) Some recent PM10 studies have used a distributed lag
over several days to weeks preceding the health event, although this
modeling approach has not been extended to studies of fine particles
(EPA, 2004, section 3.5.5). While such studies continue to suggest
consideration of a multiple day averaging time, the Staff Paper notes
that limiting 24-hour concentrations of fine particles will also
protect against effects found to be associated with PM averaged over
many days in health studies. Consistent with the conclusion reached in
the last review, the Staff Paper concludes that a multiple-day
averaging time would add complexity without providing more effective
protection than a 24-hour average.
(3) While some newer studies have investigated seasonal effects
(EPA, 2004, section 3.5.5.3), the Staff Paper concludes that currently
available evidence of such effects is still too limited to serve as a
basis for considering seasonal standards.
Based on the above considerations, the Staff Paper and CASAC
(Henderson, 2005a) recommend retaining the current annual and 24-hour
averaging times for PM2.5 primary standards. The
Administrator concurs with the staff and CASAC recommendations and
proposes that averaging times for PM2.5 standards should
continue to include annual and 24-hour averages to protect against
health effects associated with short-term (hours to days) and long-term
(seasons to years) exposure periods.
F. Form of Primary PM2.5 Standards
1. 24-Hour PM2.5 Standard
In 1997 EPA established the form of the 24-hour PM2.5
standard as the 98th percentile of the annual 24-hour concentrations at
each population-oriented monitor within an area, averaged over three
years (62 FR at 38671-74). EPA selected such a concentration-based form
because of its advantages over the previously used expected-exceedance
form.\34\ A concentration-based form is more reflective of the health
risk posed by elevated PM2.5 concentrations because it gives
proportionally greater weight to days when concentrations are well
above the level of the standard than to days when the concentrations
are just above the standard. Further, a concentration-based form better
compensates for missing data and less-than-every-day monitoring; and,
when averaged over 3 years, it has greater stability and, thus,
facilitates the development of more stable implementation programs.\35\
After considering a range of concentration percentiles from the 95th to
the 99th, EPA selected the 98th percentile as an appropriate balance
between adequately limiting the occurrence of peak concentrations and
providing increased stability and robustness. Further, by basing the
form of the standard on concentrations measured at population-oriented
monitoring sites (as specified in 40 CFR part 58), EPA intended to
provide protection for people residing in or near localized areas of
elevated concentrations.
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\34\ The form of the 1987 24-hour PM10 standard is
based on the expected number of days per year (averaged over 3
years) on which the level of the standard is exceeded; thus,
attainment of the one-expected exceedance form is determined by
comparing the fourth-highest concentration in 3 years with the level
of the standard.
\35\ See American Trucking Associations v. EPA, 283 F. 3d at
374-75 (legitimate for EPA to consider promotion of overall
effectiveness of NAAQS implementation programs, including their
overall stability, in setting a standard that is requisite to
protect the public health).
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In this review, the Staff Paper concludes that it is appropriate to
retain a concentration-based form that is defined in terms of a
specific percentile of the distribution of 24-hour PM2.5
concentrations at each population-oriented monitor within an area,
averaged over 3 years. This staff recommendation is based on the same
reasons that were the basis for EPA's selection of this type of form in
the last review. As to the specific percentile value to be considered,
the Staff Paper took into consideration (1) the relative risk reduction
afforded by alternative forms at the same standard level, (2) the
relative year-to-year stability of the air quality statistic to be used
as the basis for the form of a standard, and (3) the implications from
a public health communication perspective of the extent to which either
form allows different numbers of days in a year to be above the level
of the standard in areas that attain the standard. Based on these
considerations, the Staff Paper recommends either retaining the 98th
percentile form or revising it to be based on the 99th percentile form,
and notes that primary consideration should be given to the combination
of form and level, as compared to looking at the form in isolation
(EPA, 2005a, p. 5-44).
In considering the information provided in the Staff Paper, most
CASAC Panel members favored continued use of the 98th percentile form
because it is more robust than the 99th percentile form, such that it
would provide more stability to prevent areas from bouncing in and out
of attainment from year to year (Henderson 2005a). In recommending
retention of the 98th percentile form, the CASAC Panel recognized that
it is the link between the form and level of a standard that determines
the degree of public health protection afforded by a standard.
In considering the available information and the Staff Paper and
CASAC recommendations, the Administrator proposes that the form of the
24-hour standard should be based on the 98th percentile form. In so
doing, the Administrator has focused on the relative stability of the
98th and 99th percentile forms as a basis for selecting the 98th
percentile form, while recognizing that the degree of public health
protection likely to be afforded by a standard is a result of the
combination of the form and the level of the standard.
2. Annual PM2.5 Standard
In 1997 EPA established the form of the annual PM2.5
standard as an annual arithmetic mean, averaged over 3 years, from
single or multiple community-oriented monitors. This form of the annual
standard was intended to represent a relatively stable measure of air
quality and to characterize area-wide PM2.5 concentrations
in conjunction with a 24-hour standard designed to provide adequate
protection against localized peak or seasonal PM2.5 levels.
The current annual PM2.5 standard level is to be compared to
measurements made at the community-oriented monitoring site recording
the highest level, or, if specific constraints are met, measurements
from multiple community-oriented monitoring sites may be averaged (Part
50 App. N section 2.1(a) and (b) and Part 58 App. D at 2.8.1.6.1; 62 FR
38,672, July 18, 1997). Community-oriented monitoring sites were
specified to be consistent with the intent that a spatially averaged
annual standard protect those in smaller communities, as well as those
in larger population centers. The constraints on allowing the use of
spatially averaged measurements were intended to limit averaging across
poorly correlated or widely disparate air quality values.\36\ This
approach was judged to be consistent with the epidemiologic studies on
which the PM2.5 standard
[[Page 2647]]
was primarily based, in which air quality data were generally averaged
across multiple monitors in an area or were taken from a single monitor
that was selected to represent community-wide exposures, not localized
``hot spots'' (62 FR 38672). These criteria and constraints were
intended to ensure that spatial averaging would not result in inequities
in the level of protection afforded by the PM2.5 standards (Id.).
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\36\ The current constraints include the criteria that the
correlation coefficient between monitor pairs to be averaged be at
least 0.6, and that differences in mean air quality values between
monitors to be averaged not exceed 20 percent (Part 58 App. D at
2.8.1.6.1).
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In this review, there now exist much more PM2.5 air
quality data than were available in the last review. Consideration in
the Staff Paper of the spatial variability across urban areas that is
revealed by this new database has raised questions as to whether an
annual standard that allows for spatial averaging, within currently
specified or alternative constraints, would provide appropriate public
health protection. Analyses in the Staff Paper to assess these
questions, as discussed below, have taken into account both aggregate
population risk across an entire urban area and the potential for
disproportionate impacts on potentially vulnerable subpopulations
within an area.
The effect of allowing the use of spatial averaging on aggregate
population risk was considered in sensitivity analyses included in the
health risk assessment (EPA, 2005a). In particular, analyses were done
in several urban areas that compared estimated mortality risks based on
calculating compliance with alternative standards (1) using air quality
values from the highest community-oriented monitor in an area and (2)
using air quality values averaged across all such monitors within the
constraints allowed by the current standard.\37\ As expected, estimated
risks associated with long-term exposures remaining upon just meeting
the current annual standard are greater when spatial averaging is used
than when the highest monitor is used (i.e., the estimated reductions
in risk associated with just attaining the current or alternative
annual standards are less when spatial averaging is used), as the use
of the highest monitor leads to greater modeled reductions in ambient
PM2.5 concentrations.\38\
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\37\ As discussed in the Staff Paper, section 4.2.2, the
monitored air quality values were used to determine the design value
for the annual standard in each area, as applied to a ``composite''
monitor to reflect area-wide exposures. Changing the basis of the
annual standard design value from the concentration at the highest
monitor to the average concentration across all monitors changes the
ambient PM2.5 levels that are needed to just meet the
current or alternative annual standards. With averaging, less
overall reduction in ambient PM2.5 is needed to just meet
the standards.
\38\ For example, based on analyses conducted in three example
urban areas, estimated mortality incidence associated with long-term
exposure based on the use of spatial averaging is about 10 to over
40 percent higher than estimated incidence based on the use of the
highest monitor (EPA, 2005a, p. 5-41).
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In considering the potential for disproportionate impacts on
potentially vulnerable subpopulations, analyses were done to assess
whether any such groups are more likely to live in census tracts in
which the monitors recording the highest air quality values in an area
are located. Data were obtained for demographic parameters measured at
the census tract level, including education level, income level, and
percent minority population. Data from the census tract in each area in
which the highest air quality value was monitored were compared to the
area-wide average value (consistent with the constraints on spatial
averaging provided by the current standard) in each area. (Schmidt et
al., 2005). Recognizing the limitations of such cross-sectional
analyses, the Staff Paper observes that the results suggest that the
highest concentrations in an area tend to be measured at monitors
located in areas where the surrounding population is more likely to
have lower education and income levels, and higher percentage minority
levels (EPA, 2005a, p. 5-41).\39\ Noting the intended purposes of the
form of the annual standard, as discussed above, the Staff Paper
concludes that the existing constraints on spatial averaging may not be
adequate to avoid substantially greater exposures in some areas,
potentially resulting in disproportionate impacts on potentially
vulnerable subpopulations.
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\39\ As summarized in section II.A.4 above, the Criteria
Document notes that some epidemiologic study results, most notably
the associations between mortality and long-term PM2.5
exposure in the ACS cohort, have shown larger effect estimates in
the cohort subgroup with lower education levels (EPA, 2004, p. 8-
103). The Criteria Document also notes that lower education level
can be a marker for lower socioeconomic status that may be related
to increased vulnerability to the effects of fine particle
exposures, for example, as a result of greater exposure to sources
such as roadways. Lower education level may be associated with other
potential risk factors, such as poorer health status or access to
health care, that may also result in increased susceptibility to the
effects of air pollution exposure (EPA, 2004, section 9.2.4.5)
---------------------------------------------------------------------------
In considering whether more stringent constraints on the use of
spatial averaging may be appropriate, the Staff Paper presents results
of an analysis of recent air quality data on the correlations and
differences between monitor pairs in metropolitan areas across the
country (Schmidt et al., 2005). For all pairs of PM2.5
monitors, the median correlation coefficient based on annual air
quality data is approximately 0.9, which is substantially higher than
the current criterion for correlation of at least 0.6, which was met by
nearly all monitor pairs. Similarly, the current criterion that
differences in mean air quality values between monitors not exceed 20
percent was met for most monitor pairs, while the annual median and
mean differences for all monitor pairs are 5 percent and 8 percent,
respectively. This analysis also shows that in some areas with highly
seasonal air quality patterns (e.g., due to seasonal wood smoke
emissions), substantially lower seasonal correlations and larger
seasonal differences can occur relative to those observed on an annual
basis. This analysis provides some perspective on the constraints on
spatial averaging that were put in place in the last review, before
data were widely available on spatial distributions of PM2.5
air quality levels, based on the extensive air quality data and related
analyses that have become available since the last review.
In considering the results of the analyses discussed above, the
Staff Paper concludes that it is appropriate to consider either
eliminating the provision that allows for spatial averaging from the
form of an annual PM2.5 standard or revising the allowance
for spatial averaging to be based on more restrictive criteria. More
specifically, based on the analyses discussed above, the Staff Paper
recommends consideration of revised criteria such that the correlation
coefficient between monitor pairs to be averaged be at least 0.9,
determined on a seasonal basis, with differences between monitor values
not to exceed 10 percent (EPA, 2005a, p. 5-42).
In considering the Staff Paper recommendations based on the results
of the analyses discussed above, and focusing on a desire to be
consistent with the epidemiologic studies on which the PM2.5
health effects are based and concern over the evidence of potential
disproportionate impact on potentially vulnerable subpopulations, the
Administrator proposes to revise the form of the annual
PM2.5 standard consistent with the Staff Paper
recommendation to change the criteria for use of spatial averaging such
that the correlation coefficient between monitor pairs must be at least
0.9, determined on a seasonal basis, with differences between monitor
values not to exceed 10 percent. The Administrator also solicits
comment on the other Staff Paper-recommended alternative of revising
the form of the annual PM2.5
[[Page 2648]]
standard to one based on the highest community-oriented monitor in an
area, with no allowance for spatial averaging.
G. Level of Primary PM2.5 Standards
In the last review, having concluded that both 24-hour and annual
PM2.5 standards were appropriate, EPA selected a level for
each standard that was appropriate for the function to be served by
such standard (62 FR 38652). As discussed above, EPA concluded at that
time that the suite of PM2.5 standards could most
effectively and efficiently protect public health by treating the
annual standard as the generally controlling standard for lowering both
short- and long-term PM2.5 concentrations.\40\ In
conjunction with such an annual standard, the 24-hour standard was
intended to provide protection against days with high peak
PM2.5 concentrations, localized ``hotspots,'' and risks
arising from seasonal emissions that would not be well controlled by an
annual standard.\41\
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\40\ In so doing, EPA noted that an annual standard would focus
control programs on annual average PM2.5 concentrations,
which would generally control the overall distribution of 24-hour
exposure levels, as well as long-term exposure levels, and would
also result in fewer and lower 24-hour peak concentrations.
Alternatively, a 24-hour standard that focused controls on peak
concentrations could also result in lower annual average
concentrations. Thus, EPA recognized that either standard could
provide some degree of protection from both short- and long-term
exposures, with the other standard serving to address situations
where the daily peaks and annual averages are not consistently
correlated (62 FR 38669).
\41\ See also American Trucking Associations v. EPA, 283 F.3d at
373 (endorsing this reasoning).
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In selecting the level for the annual standard in the last review,
EPA used an evidence-based approach that considered the evidence from
both short- and long-term exposure studies. The risk assessment
conducted in the last review, while providing qualitative insights
about the distribution of risks, was considered to be too limited to
serve as a quantitative basis for decisions on the standard levels. In
accordance with Staff Paper and CASAC views on the relative strengths
of the short- and long-term exposure studies, greater emphasis was
placed on the short-term exposure studies. In so doing, EPA first
determined a level for the annual standard based on the short-term
exposure studies, and then considered whether the long-term exposure
studies suggested the need for a lower level. While recognizing that
health effects could occur over the full range of concentrations
observed in the studies, EPA concluded that the strongest evidence for
short-term PM2.5 effects occurs at concentrations near the
long-term (e.g., annual) average in those studies reporting
statistically significant health effects. Thus, in the last review, EPA
selected a level for the annual standard that was below the lowest
long-term average PM2.5 concentration in a short-term
exposure study that reported statistically significant health effects.
Further consideration of the average PM2.5 concentrations
across the cities in the key long-term exposure studies available at
that time did not provide a basis for establishing a lower annual
standard level.
In this review, the approach used in the Staff Paper as a basis for
staff recommendations on standard levels builds upon and broadens the
general approach used by EPA in the last review. This broader approach
reflects the more extensive and stronger body of evidence now available
on health effects related to both short- and long-term exposure to
PM2.5, together with the availability of much more extensive
PM2.5 air quality data. This newly available information has
been used to conduct a more comprehensive risk assessment for
PM2.5. As a consequence, the broader approach used in the
Staff Paper discusses ways to take into account both evidence-based and
quantitative risk-based considerations and places relatively greater
emphasis on evidence from long-term exposure studies than was done in
the last review.
Given the extensive body of new evidence based specifically on
PM2.5 that is now available, and the resulting broader
approach presented in the Staff Paper, the Administrator considers it
appropriate to use a different approach from that used in the last
review to select appropriate standard levels. More specifically, the
Administrator's proposal relies on an evidence-based approach that
considers the much expanded body of evidence from short-term exposure
PM2.5 studies as the principal basis for selecting the level
of the 24-hour standard and the stronger and more robust body of
evidence from the long-term exposure PM2.5 studies as the
principal basis for selecting the level of the annual standard. In the
Administrator's view, the very large number of health effect studies
that are now available provide the most reliable basis for standard
setting. With respect to the quantitative risk assessment, the
Administrator recognizes that it rests on a more extensive body of data
and is more comprehensive in scope than the assessment conducted in the
last review, but is mindful that significant uncertainties continue to
underlie the resulting risk estimates. Such uncertainties generally
relate to a lack of clear understanding of a number of important
factors, including for example: The shape of concentration-response
functions, particularly when, as here, effect thresholds can neither be
discerned nor determined not to exist; issues related to selection of
appropriate statistical models for the analysis of the epidemiologic
data; the role of potentially confounding and modifying factors in the
concentration-response relationships; issues related to simulating how
PM2.5 air quality distributions will likely change in any
given area upon attaining a particular standard, since strategies to
reduce emissions are not yet defined; and whether there would be
differential reductions in the many components within PM2.5
and if so whether this would result in differential reductions in risk.
In the case of fine particles, the Administrator recognizes that such
uncertainties are likely to be unusually large due to the complexity in
the composition of the mix of fine particles generally present in the
ambient air. Further, in the Administrator's view, a risk assessment
based on studies that do not resolve the issue of a threshold is
inherently limited as a basis for standard setting, since it will
necessarily predict that ever lower standards result in ever lower
risks, which has the effect of masking the increasing uncertainty
inherent as lower levels are considered. As a result, while the
Administrator views the risk assessment as providing supporting
evidence for the conclusion that there is a need to revise the current
suite of PM2.5 standards, he judges that it does not provide
a reliable basis to determine what specific quantitative revisions are
appropriate.
1. 24-Hour PM2.5 Standard
Based on the approach discussed above, the Administrator has relied
upon evidence from the short-term exposure PM2.5 studies as
the principal basis for selecting the level of the 24-hour standard. In
considering these studies as a basis for the level of a 24-hour
standard, and having selected a 98th percentile form for the standard,
the Administrator agrees with the focus in the Staff Paper of looking
at the 98th percentile values in these studies. In so doing, the
Administrator recognizes that these studies provide no evidence of
clear effect thresholds or lowest-observed-effects levels. Thus, in
focusing on 98th percentile values in these studies, the Administrator
is seeking to establish a standard level that will require improvements
in air quality generally in areas in which short-term exposure to
PM2.5 can reasonably be expected to be associated with serious
[[Page 2649]]
health effects. While strategies that may be employed in the future to
bring about such improvements in air quality in any particular area are
not yet defined, most such strategies are likely to move the broad
distribution of PM2.5 air quality values in an area lower,
resulting in reductions in risk associated with exposures to
PM2.5 levels across a wide range of concentrations.
Based on the information in the Staff Paper and a supporting staff
memo,\42\ the Administrator observes an overall pattern of
statistically significant associations reported in studies of short-
term exposure to PM2.5 across a wide range of 98th
percentile values. More specifically, there is a strong predominance of
studies with 98th percentile values down to about 39 [mu]g/m\3\ (in
Burnett and Goldberg, 2003) reporting statistically significant
associations with mortality, hospital admissions, and respiratory
symptoms. For example, within this range of air quality, statistically
significant associations were reported for mortality in the combined
Six City study (and three of the individual cities within that study)
(Klemm and Mason, 2003), the Canadian 8-City Study (Burnett and
Goldberg, 2003), and in studies in Santa Clara County, CA (Fairley,
2003) and Philadelphia (Lipfert, 2000); for hospital admissions and
emergency department visits in Seattle (Sheppard et al., 2003), Toronto
(Burnett et al., 1997; Thurston et al., 1994), Detroit (Ito, 2003, for
ischemic heart disease and pneumonia, but not for other causes), and
Montreal (Delfino et al., 1998, 1997, for some but not all age groups
and years); for respiratory symptoms in panel studies in a combined Six
City study (Schwartz et al., 1994) and in two Pennsylvania cities
(Uniontown in Neas et al., 1995; State College in Neas et al., 1996);
and for lung function in Philadelphia (Neas et al., 1999).\43\ Studies
in this air quality range that reported positive but not statistically
significant associations with mortality include studies in Detroit
(Ito, 2003), Pittsburgh (Chock et al., 2000), and Montreal (Goldberg
and Burnett, 2003).
---------------------------------------------------------------------------
\42\ As discussed in the Staff Paper (EPA, 2005a, p. 5-30) and
supporting staff memo (Ross and Langstaff, 2005), staff focused on
U.S. and Canadian short-term exposure PM2.5 studies that
had been reanalyzed as appropriate to address statistical modeling
issues and considered the extent to which the reported associations
are robust to co-pollutant confounding and alternative modeling
approaches and the extent to which the studies used relatively
reliable air quality data.
\43\ Of the studies within this group that evaluated
multipollutant associations, as discussed above in section II.A.3,
the results reported in Fairley (2003), Sheppard et al. (2003), and
Ito (2003) were generally robust to inclusion of gaseous co-
pollutants, whereas the effect estimate in Thurston et al. (1994)
was substantially reduced with the inclusion of O3.
---------------------------------------------------------------------------
Within the range of 98th percentile PM2.5 concentrations
of about 35 to 30 [mu]g/m\3\, this strong predominance of statistically
significant results is no longer observed. Rather, within this range,
some studies report statistically significant results (Mar et al.,
2003; Ostro et al., 2003), other studies report mixed results in which
some associations reported in the study are statistically significant
and others are not (Delfino et al., 1997; Peters et al., 2000),\44\ and
another study reports associations in two of six cities that are not
statistically significant (Klemm and Mason, 2003). Further, the very
limited number of studies in which the 98th percentile values are below
this range do not provide a basis for reaching conclusions about
associations at such levels (Stieb et al., 2000; Peters et al., 2001).
Thus, in the Administrator's view, this body of evidence provides
confidence that statistically significant associations are occurring
down close to this range, and it provides a clear basis for concluding
that this range represents a range of reasonable values and thus for
selecting a 24-hour standard level from within this range. The
Administrator further notes that focusing on the range of 35 to 30
[mu]g/m\3\ is consistent with the interpretation of the evidence held
by most CASAC Panel members as reflected in their recommendation to
select a 24-hour PM2.5 standard level within this range
(Henderson, 2005a). The Administrator recognizes, however, the separate
point that most CASAC Panel members favored the range of 35 to 30
[mu]g/m\3\ for the 24-hour PM2.5 standard in concert with an
annual standard set in the range of 14 to 13 [mu]g/m3 (Henderson,
2005a), as discussed in section II.G.2 below.
---------------------------------------------------------------------------
\44\ For example, Delfino et al. (1997) report statistically
significant associations between PM2.5 and respiratory
emergency department visits for elderly people (>64 years old), but
not children (< 2 years old) in one part of the study period (summer
1993) but not the other (summer 1992). Peters et al. (2000) report
new findings of associations between fine particles and cardiac
arrhythmia, but the Criteria Document observes that the strongest
associations were reported for a small subset of the study
population that had experienced 10 or more defibrillator discharges
(EPA, 2004, p. 8-164).
---------------------------------------------------------------------------
In considering what 24-hour standard is requisite to protect public
health with an adequate margin of safety, the Administrator is mindful
that this choice requires judgment based on an interpretation of the
evidence that neither overstates nor understates the strength and
limitations of the evidence or the appropriate inferences to be drawn
from the evidence. In the absence of evidence of any clear effect
thresholds, the Administrator may select a specific standard level from
within a range of reasonable values. In making this judgment, the
Administrator notes that the general uncertainties related to the shape
of the concentration-response functions and the selection of
appropriate statistical models affect the likelihood that observed
associations are causal down to the lowest concentrations in the
studies. Further, and more specifically, the variation in results found
in the short-term exposure studies in which the 98th percentile values
were below 35 [mu]g/m\3\ indicates an increase in uncertainty as to
whether likely causal associations extend down below this level.
In considering the extent to which the quantitative risk assessment
inform his selection of a 24-hour PM2.5 standard, the
Administrator recognizes that risk estimates based on simulating the
attainment of standards set at lower levels within this range will
inevitably suggest some additional reductions in risk at each lower
standard level considered. However, these quantitative risk estimates
largely depend upon assumptions made about the lowest level at which
reported associations will likely persist and remain causal in nature.
Thus, the Administrator is hesitant to use such risk estimates as a
basis for proposing a standard level below 35 [mu]g/m\3\, and instead
prefers to rely on inferences that are based directly on the evidence
in the studies themselves.
Taking the above considerations into account, the Administrator
proposes to set the level of the primary 24-hour PM2.5
standard at 35 [mu]g/m3. In the Administrator's judgment,
based on the currently available evidence, a standard set at this level
would protect public health with an adequate margin of safety from
serious health effects including premature mortality and hospital
admissions for cardiorespiratory causes that are likely causally
associated with short-term exposure to PM2.5. This judgment
by the Administrator appropriately considers the requirement for a
standard that is neither more nor less stringent than necessary for
this purpose and recognizes that the CAA does not require that primary
standards be set at a zero-risk level, but rather at a level that
reduces risk sufficiently so as to protect public health with an
adequate margin of safety. Being mindful that the available evidence
does not provide a basis for identifying a bright line within the range
of 35 to 30 [mu]g/m3 that clearly provides the appropriate
degree of public health protection, the Administrator also
[[Page 2650]]
solicits comment on selecting a lower level within this range.
Having reached this decision to propose a level of 35 [mu]g/
m3 for the 24-hour PM2.5 standard based on the
approach to interpreting the available evidence described above, the
Administrator recognizes that other approaches to selecting a standard
level have been presented to the Agency. These other approaches reflect
alternative views, principally expressed in public comments to date, as
to the appropriate interpretation of the scientific evidence and the
appropriate policy response in light of that interpretation. One such
view focuses very strongly on the uncertainties inherent in the
epidemiologic and toxicologic studies and the quantitative risk
assessment as the basis for concluding that no change to the current
24-hour PM2.5 standard of 65 [mu]g/m3 is
warranted. Such commenters prefer greater weight, for example, on
issues related to the sensitivity in the magnitude and statistical
significance of relative risks reported in studies using different
statistical models, noting that further research is needed to inform
modeling strategies that will appropriately adjust for temporal trends
and weather variables in time-series studies. Additional uncertainties
arise from the potential confounding by co-pollutants, and the
potential differential toxicity of components within the mix of fine
particles. These commenters suggest that the magnitude of risks
associated with fine particle exposures have decreased since the last
review. Some such commenters also focus on considerations such as the
absence of clear evidence from toxicologic studies and from studies
focused on elucidating specific physiologic mechanisms by which
PM2.5 may be causing the observed effects. Such commenters
recognize a need for a 24-hour PM2.5 standard, but consider
the evidence to be too uncertain overall to warrant any tightening of
the standard and instead believe the appropriate policy response in
light of this uncertainty is to retain the current level of the 24-hour
standard.
Other commenters who also focus strongly on the uncertainties
inherent in the epidemiologic and toxicologic studies and the
quantitative risk assessment reach a somewhat different conclusion as
to the appropriate policy response in light of these uncertainties.
This group of commenters sees a basis for lowering the level of the 24-
hour PM2.5 standard, but does not believe that a level as
low as 35 [mu]g/m3 is warranted. Such commenters note that
while many of the studies within the range of air quality from
approximately 39 [mu]g/m3 up to the level of the current
standard of 65 [mu]g/m3 report statistically significant
results, only a few such studies independently evaluated confounding by
co-pollutants. This lack of a broader assessment of co-pollutants,
together with other types of uncertainties as noted above, leads such
commenters to conclude that a standard level selected from below this
range is not warranted, and that the appropriate policy response is to
select a standard level from within the range of about 40 to 65 [mu]g/m3.
In sharp contrast, others view the epidemiologic evidence and other
health studies as strong and robust, and generally place much weight on
the results of the quantitative risk assessment as a basis for
concluding that a much stronger policy response is warranted, generally
consistent with a standard level at or below 25 [mu]g/m3.
While recognizing that important uncertainties are inherently present
in both the evidence and estimated risks, these commenters generally
support a view that such uncertainties warrant a highly precautionary
policy response, particularly in view of the serious nature of the
health effects at issue, and should be addressed by selecting a
standard level that incorporates a large margin of safety.
The Administrator recognizes that these sharply divergent views on
the appropriate level of the standard are based on very different
interpretations of the science itself including its relative strengths
and limitations and on very different judgments as to how such
scientific evidence should be used in making policy decisions on
proposed standards. Consistent with the goal of soliciting comments on
a wide array of views, the Administrator also solicits broad public
comment on these and other alternative approaches and on the related
standard levels, such as levels from 35 [mu]g/m3 up to 65
[mu]g/m3 or from 30 [mu]g/m3 down to 25 [mu]g/
m3, that commenters may believe are appropriate, along with
the rationale supporting such approaches and levels. In addition, the
Administrator solicits comments on issues related to the interpretation
of relevant epidemiologic and toxicologic studies, including approaches
to addressing uncertainties related to the sensitivity of results to
alternative statistical modeling approaches, co-pollutant confounding,
and the lack of a discernable threshold of effects, as well as
approaches to more fully characterize uncertainties in quantitative
risk assessments based on epidemiologic studies.
2. Annual PM2.5 Standard
Based on the approach discussed at the beginning of this section,
the Administrator has relied upon evidence from the long-term exposure
PM2.5 studies as the principal basis for selecting the level
of the annual standard. In considering these studies as a basis for the
level of an annual standard, the Administrator agrees with the focus in
the Staff Paper of looking at the long-term mean PM2.5
concentrations across the cities included in such studies. In so doing,
the Administrator recognizes that these studies, like the short-term
exposure studies, provide no evidence of clear effect thresholds or
lowest-observed-effects levels. Thus, in focusing on the cross-city
long-term mean concentrations in these studies, the Administrator is
seeking to establish a standard level that will require improvements in
air quality in areas in which long-term exposure to PM2.5
can reasonably be expected to be associated with serious health effects.
Based on the characterization and assessment of the long-term
exposure PM2.5 studies presented in the Criteria Document
and Staff Paper, the Administrator recognizes the importance of the
validation efforts and reanalysis that have been done since the last
review of the original Six Cities and ACS mortality studies. These new
assessments provide evidence of generally robust associations and
provide a basis for greater confidence in the reported associations
than in the last review, for example, in the extent to which they have
made progress in understanding the importance of issues related to co-
pollutant confounding and the specification of statistical models.
Consistent with the information available in the last review, these two
key long-term exposure mortality studies reported long-term mean
PM2.5 concentrations across all the cities included in the
studies of 18 and 21 [mu]g/m3, respectively. The
Administrator also particularly recognizes the importance of the
extended ACS mortality study, published since the last review, which
provides new evidence of mortality related to lung cancer and further
substantiates the statistically significant associations with
cardiorespiratory-related mortality observed in the original studies.
The Administrator notes that the statistically significant associations
reported in the extended ACS study, in a large number of cities across
the U.S., provide evidence of effects at a lower long-term mean
PM2.5 concentration (17.7 [mu]g/m3) than had been
observed in the original study,
[[Page 2651]]
although the relative risk estimates are somewhat smaller in magnitude
than those reported in the original study. The assessment in the
Criteria Document of these mortality studies, taking into account study
design, the strength of the study (in terms of statistical significance
and precision of result), and the consistency and robustness of
results, concludes that it would be appropriate to give the greatest
weight to the reanalyses of the Six Cities and ACS studies, and in
particular to the results of the extended ACS study (EPA, 2004, p. 9-
33) in weighing the evidence of mortality effects associated with long-
term exposure to PM2.5. Consistent with that assessment, the
Administrator places greatest weight on these studies as a basis for
selecting the level of the annual PM2.5 standard.
In addition to these mortality studies, the Administrator also
recognizes the availability of relevant morbidity studies providing
evidence of respiratory morbidity, including decreased lung function
growth, in children with long-term exposure to PM2.5.
Studies conducted in the U.S. and Canada include the 24-city study
considered in the last review and new studies of cohorts of children in
southern California, in which the long-term mean PM2.5
concentrations in all the cities included in the studies are
approximately 14.5 and 15 [mu]g/m3, respectively. As
discussed in section II.A. above, in the 24-city study, statistically
significant associations were reported between long-term fine particle
exposures and lung function measures at a single point in time, whereas
positive but not statistically significant associations were reported
with prevalence of several respiratory conditions. As interpreted in
the last review, the results from the 24-city study are uncertain as to
the extent to which the association extends below a long-term mean
PM2.5 concentration of approximately 15 [mu]g/m3.
The new southern California children's cohort study provides evidence
of important respiratory morbidity effects in children, including
evidence for a new measure of morbidity, decreased growth in lung
function. Reports from this study suggest that long-term
PM2.5 exposure is associated with decreases in lung function
growth, as measured over a four-year follow-up period, although
statistically significant associations are not consistently reported.
The Administrator recognizes that these are important new findings,
indicating that long-term PM2.5 exposure may be associated
with respiratory morbidity in children. However, the Administrator also
observes this is the only study reporting decreased lung function
growth, conducted in just one area of the country, such that further
study of this health endpoint in other areas of the country would be
needed to increase confidence in the reported associations. Thus, at
this time, the Administrator provisionally concludes that this study
provides an uncertain basis for establishing the level of a national
standard.
As discussed in the Staff Paper (EPA, 2005a, p. 5-22), the
Administrator generally agrees that it is appropriate to consider a
level for an annual PM2.5 standard that is below the
averages of the long-term PM2.5 concentrations across the
cities in the key long-term exposure mortality studies, recognizing
that the evidence of an association in any such study is strongest at
and around the long-term average where the data in the study are most
concentrated. The Administrator is mindful that considering what
standard is requisite to protect public health with an adequate margin
of safety requires policy judgments that neither overstate nor
understate the strength and limitations of the evidence or the
appropriate inferences to be drawn from the evidence. The Administrator
provisionally concludes that these key mortality studies, together with
the morbidity studies, provide a basis for considering a standard level
no higher than 15 [mu]g/m3. This level is somewhat below the
long-term mean concentrations in the key mortality studies and
consistent with the interpretation of the evidence from the morbidity
studies discussed above. Further, in the Administrator's view, these
studies do not provide a clear basis for selecting a level lower than
the current standard of 15 [mu]g/m3.
In considering the extent to which the quantitative risk assessment
can help to inform these judgments with regard to the annual
PM2.5 standard, the Administrator again recognizes that risk
estimates based on simulating the attainment of standards set at lower
levels, as expected, continue to suggest some additional reductions in
risk at the lower standard level considered in the assessment, and that
these estimates largely depend upon assumptions made about the lowest
level at which reported associations will likely persist and remain
causal in nature. Thus, the Administrator is again hesitant to use such
risk estimates as a basis for proposing a lower annual standard level
than 15 [mu]g/m3, the level that is based directly on the
evidence in the studies themselves, as discussed above.
Taking the above considerations into account, the Administrator
proposes to retain the level of the primary annual PM2.5
standard at 15 [mu]g/m3. In the Administrator's judgment,
based on the currently available evidence, a standard set at this level
would be requisite to protect public health with an adequate margin of
safety from serious health effects including premature mortality and
respiratory morbidity that are likely causally associated with long-
term exposure to PM2.5. This judgment by the Administrator
appropriately considers the requirement for a standard that is neither
more nor less stringent than necessary for this purpose and recognizes
that the CAA does not require that primary standards be set at a zero-
risk level, but rather at a level that reduces risk sufficiently so as
to protect public health with an adequate margin of safety.
In so doing, the Administrator recognizes that the CASAC Panel did
not endorse retaining the annual standard at the current level of 15
[mu]g/m3 (Henderson, 2005a, p. 7). In weighing the
recommendation of the CASAC Panel, the Administrator has carefully
considered the stated reasons for it. In discussing its recommendation
(Henderson, 2005a), the CASAC Panel first noted that changes to either
the annual or 24-hour PM2.5 standard, or both, could be
recommended. Three reasons were then given for placing more emphasis on
lowering the 24-hour standard than the annual standard: (1) The vast
majority of studies indicating effects of short-term PM2.5
exposure were carried out in settings in which PM2.5
concentrations were largely below the current 24-hour standard level of
65 [mu]g/m3; (2) the amount of evidence on short-term
exposure effects, at least as reflected by the number of reported
studies, is greater than for long-term exposure effects; and (3)
toxicologic findings are largely related to the effects of short-term,
rather than long-term, exposures. In not endorsing the option of
retaining the level of the current annual standard in conjunction with
lowering the 24-hour standard, the CASAC Panel observed that some
cities have relatively high annual PM2.5 concentrations
without much day-to-day variation and that such cities would only
rarely exceed a 24-hour standard, even if it were set at a level below
the current standard. In such a city, attaining a 24-hour standard
would likely have minimal if any effect on the long-term mean PM2.5
concentration and consequently would be less likely to reduce health
effects associated with long-term exposures. These observations
[[Page 2652]]
were taken as an indication of the desirability of lowering the level
of the annual PM2.5 standard as well as that of the 24-hour
standard. Based on these considerations and taking into account the
results of the risk assessment, most CASAC Panel members favored
setting an annual standard in the range of 14 to 13 [mu]g/
m3, along with lowering the 24-hour standard (Henderson, 2005a).
In considering these views, the Administrator notes that the
appropriateness of setting an annual standard that would lower annual
PM2.5 concentrations in cities across the country depends
upon a policy judgment as to what annual level is required to protect
public health with an adequate margin of safety from long-term
exposures to PM2.5 in light of the available evidence. In
considering the evidence of effects associated with long-term
PM2.5 exposure as a basis for selecting an adequately health
protective annual standard, as discussed above, the Administrator
provisionally concludes that the evidence does not provide a basis for
requiring annual levels below 15 [mu]g/m3. Thus, the
Administrator agrees conceptually with the CASAC Panel that any
particular 24-hour standard may not result in reductions in the level
of long-term exposures to PM2.5 in all areas with relatively
higher than typical annual PM2.5 concentrations and lower
than typical ratios of peak-to-mean values. Further, the Administrator
agrees that this general advice supports relying on the annual
standard, and not the 24-hour standard, to achieve the appropriate
level of protection from long-term exposures to PM2.5.
However, the Administrator does not believe that this advice
necessarily translates into a reason for setting the annual
PM2.5 standard at a level below the current level of 15
[mu]g/m3. As discussed above, the Administrator believes the
principal basis for selecting the appropriate level of an annual
standard should be the evidence provided by the long-term studies, in
conjunction with judgments concerning whether and over what range of
concentrations reported associations are likely causal, and this
evidence reasonably supports retaining the current level of the annual
standard.
The Administrator places great importance on the advice of CASAC,
and therefore solicits broad public comment on the range of 15 down to
13 [mu]g/m3, the low end of the range recommended by CASAC,
for the level of the annual PM2.5 standard as well as on the
reasoning that formed the basis for that recommendation. A decision to
select a standard from within this range would place greater weight on
the strength of the associations reported in the key epidemiologic
mortality and morbidity long-term exposure studies down to the lower
part of the range of PM2.5 concentrations observed across
all the cities included in these studies. Such a standard could also
reflect greater reliance on the results of the quantitative risk
assessment that suggested increased reductions in risk associated with
meeting an annual standard at such lower levels.
The Administrator recognizes that an even stronger view of the
appropriate policy response to the currently available evidence has
been expressed by some public commenters. These commenters have focused
principally on the strength of the long-term exposure studies,
including the new children's cohort study conducted in southern
California, as well as on those results from the quantitative risk
assessment that are based on the assumption that there is no threshold
of effects down to the lowest levels observed in those studies. Such
considerations generally have led these commenters to express views
that support a highly precautionary policy response and the selection
of a standard level that incorporates a large margin of safety,
consistent with an annual PM2.5 standard level of 12 [mu]g/
m3. The Administrator recognizes that this view is based on
a different interpretation of the science itself including its relative
strengths and limitations and on different judgments as to how such
scientific evidence should be used in making policy decisions on
proposed standards. Consistent with the goal of soliciting comments on
a wide array of views, the Administrator also solicits broad public
comment on this alternative approach and on the related standard level
of 12 [mu]g/m3.
The Administrator also recognizes a contrasting view as to the
interpretation of and weight to be accorded to the results from the
ACS-based studies (Pope et al., 1995; Krewski et al., 2000; Pope et
al., 2002). In this view, the ACS-based studies are not sufficiently
robust to support a policy response that would tighten the annual
PM2.5 standard based on the evidence. This view emphasizes
the sensitivity of the results of these studies to plausible changes in
model specification with regard to accounting for the geographical
proximity of cities and the correlation of air pollutant concentrations
within a region, effect modification by education level, and inclusion
of SO2 in the model. In this view, these sensitivities
suggest potential confounding or effect modification that has not been
taken into account. For example, concern has been raised about the
sensitivity of results in the reanalysis of data from the ACS cohort
study (Krewski et al., 2000) to inclusion of SO2 in the
models. As discussed in section II.A.2.b above, the reanalysis found
that PM2.5, sulfates, and SO2 were each
associated with mortality in single-pollutant models. However, in two-
pollutant models with SO2 and PM2.5, the relative
risk for PM2.5 was substantially smaller and no longer
statistically significant, whereas the effect estimates for
SO2 were not sensitive to inclusion of PM2.5 or
sulfates in two-pollutant models. In this view, the ACS-based risk
estimates are more robust for SO2 than for PM2.5
or sulfates. In further extended analyses, Pope et al. (2002) reported
that effect estimates were not highly sensitive to spatial smoothing
approaches intended to address spatial autocorrelation, while findings
of effect modification by education level were reaffirmed. Results of
multi-pollutant models were not reported by Pope et al. (2002). Because
the correlation coefficient between PM2.5 and SO2
was 0.50 in the ACS data, in this view it is plausible to believe that
the independent effects of the two pollutants could be disentangled
with additional study.
In this view, there is a separate but related concern that
tightening the annual standard now, without a clear understanding of
which specific PM-related pollutants are most toxic, will have very
uncertain public health payoffs. In response to the advice of the
National Research Council (NRC) and other scientists, the Agency is
undertaking, as one of its higher priorities, a substantial research
program to clarify which aspects of PM-related pollution are
responsible for elevated risks of mortality and morbidity. For example,
the Health Effects Institute has issued a request for applications to
analyze the largest database on specific components of PM that has ever
been assembled for public health and medical researchers. The time line
for this multi-million dollar research program is well designed to
inform the Agency's next periodic reevaluation of the primary ambient
air quality standard for PM2.5. In light of the degree of
sensitivity of the ACS-based relative risk estimates to model
specifications and the significant research underway, in this view, it
would be wiser to consider modification of the annual standard with a
fuller body of information in hand rather than initiate a change in the
annual standard at this time.
The Administrator solicits comment on this view and on the issues
raised in interpreting the results of the ACS-based
[[Page 2653]]
studies. For example, comment is solicited on the extent to which the
associations reported in the ACS-based studies suggest that
SO2 should be considered as a surrogate for fine particles
and/or the broader mix of air pollutants or as an independent pollutant
exhibiting separate effects. Comment is also solicited on relevant
research that would improve our understanding of issues related to
model specification and alternative analytic approaches that would
better inform judgments based on such epidemiologic studies in the future.
H. Proposed Decisions on Primary PM2.5 Standards
For the reasons discussed above, and taking into account the
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of CASAC, and public
comments to date, the Administrator proposes to revise the current
primary PM2.5 standards. Specifically, the Administrator
proposes to revise (1) the level of the 24-hour PM2.5
standard to 35 [mu]g/m3, and (2) the form of the annual
PM2.5 standard by changing the constraints on the use of
spatial averaging to include the criterion that the minimum correlation
coefficient between monitor pairs to be averaged be 0.9 or greater,
determined on a seasonal basis, and the criterion that differences
between monitor values not exceed 10 percent. Data handling conventions
are specified in proposed revisions to Appendix N, as discussed in
Section V below, and the reference method for monitoring PM as
PM2.5 is specified in proposed minor revisions to Appendix
L, as discussed in Section VI below.
In recognition of alternative views of the science and the
appropriate policy response based on the currently available
information, the Administrator also solicits comments on (1)
alternative levels of the 24-hour PM2.5 standard within the
range of 35 to 30 [mu]g/m3, and alternative approaches for
selecting the level of the 24-hour PM2.5 standard, and
related levels (such as approaches that suggest retaining the current
level of 65 [mu]g/m3, setting a level no higher than 25
[mu]g/m3, or setting a level within the range of 65 down to
35 [mu]g/m3); (2) alternative levels of the annual
PM2.5 standard below 15 [mu]g/m3 down to12 [mu]g/
m3; (3) issues related to consideration of alternative
indicators of fine particle components; and (4) an alternative form of
the annual PM2.5 standard based on the highest community-
oriented monitor in an area. Based on the comments received and the
accompanying rationales, the Administrator may adopt other standards
within the range of the alternatives identified above in lieu of the
standards he is proposing today.
The Administrator solicits comment on all aspects of this proposed
decision. Comment is specifically invited on the methodology for
evaluating the uncertainty and significance of risks to public health.
The Administrator believes that it is important to further develop ways
of addressing uncertainty when estimating such risk, recognizing the
wide variety of information available in the underlying health effects
and other studies. The Agency seeks comment on methods and approaches
for conducting a more formalized uncertainty analysis. In addition, the
Agency seeks comment on how to evaluate the results from a formalized
uncertainty analysis or from the Staff Paper's risk assessment, which
addresses multiple health effects across multiple populations, in the
context of judging the public health importance of such risks and
determining the requisite level of public health protection for the PM
standards.
To address issues related to the transition from the current
PM2.5 standards to revised PM2.5 standards, the
Administrator intends to seek public comment on EPA's implementation
plans for the revised PM2.5 standards, including its plans
for assuring an effective transition, as part of an advance notice of
proposed rulemaking (ANPR) on NAAQS implementation that will be
published in an early in 2006. In this ANPR, EPA will be discussing
issues related to the timing and regulatory implications of this
transition. The EPA intends to present and take comment on the need and
potential approaches for revocation of the current 24-hour
PM2.5 standard, and on issues related to the establishment
of no-backsliding requirements, such as those adopted by the Agency in
1997 with respect to the ozone NAAQS. The EPA also expects to address a
variety of implementation issues concerning revised PM2.5
standards in the ANPR. The ANPR will explain the designation process
and its timing, and the timing of SIP submittals for both attainment
and nonattainment areas. The EPA also expects to address issues
regarding the attainment dates for areas designated nonattainment. The
EPA will also discuss new source permitting requirements for both
attainment and nonattainment areas, i.e., the PSD and Part D NSR
programs. If the Administrator promulgates a revised PM2.5
standard, EPA will determine the final implementation approach for that
standard.
III. Rationale for Proposed Decisions on Primary PM10 Standards
This action presents the Administrator's proposed decisions on
revision to the primary NAAQS for PM10. The rationale for
the proposed revisions of the primary PM10 NAAQS includes
consideration of: (1) Evidence of health effects related to short- and
long-term exposures to thoracic coarse particles; (2) insights gained
from a quantitative risk assessment prepared by EPA; and (3) specific
conclusions regarding the need for revisions to the current standards
and the elements of PM10 standards (i.e., indicator,
averaging time, form, and level) that, taken together, would be
requisite to protect public health with an adequate margin of safety.
In developing this rationale, EPA has taken into account the
information available from a growing, but still limited, body of
evidence on health effects associated with thoracic coarse particles
from studies that use PM10-2.5 as a measure of thoracic
coarse particles. The EPA has drawn upon an integrative synthesis of
the body of evidence on associations between exposure to ambient
thoracic coarse particles and a range of health endpoints (EPA, 2004,
Chapter 9), focusing on those health endpoints for which the Criteria
Document concludes that the associations are suggestive of possible
causal relationships. In its policy assessment of the evidence judged
to be most relevant to making decisions on elements of the standards,
EPA has placed greater weight on U.S. and Canadian epidemiological
studies using thoracic coarse particles measurements, since studies
conducted in other countries may well reflect different demographic and
air pollution characteristics.
While there is little question that particles in the thoracic
coarse particle size range can present a risk of adverse effects to the
most sensitive regions of the respiratory tract, the characterization
of health effects attributable to various levels of exposure to ambient
thoracic coarse particles is subject to uncertainties that are markedly
greater than is the case for fine particles. As discussed below,
however, there is a growing body of evidence available since the last
review of the PM NAAQS, with important new information coming from
epidemiologic, toxicologic, and dosimetric studies. Moreover, the newly
available research studies have undergone intensive scrutiny through
multiple layers of peer review and extended opportunities for public
review and comment. While
[[Page 2654]]
important uncertainties remain, the review of the health effects
information has been extensive and deliberate. In the judgment of the
Administrator, this intensive evaluation of the scientific evidence has
provided an adequate basis for proposing regulatory decisions at this
time. This review also provides important input to EPA's research plan
for improving our future understanding of the relationships between
exposures to ambient thoracic coarse particles and health effects.
A. Evidence of Health Effects Related to Thoracic Coarse Particle Exposure
The first PM NAAQS (36 FR 8186) used an indicator based solely on a
preexisting monitor for total suspended particles (TSP) that was not
designed to focus on particles of greatest risk to health. In preparing
for the initial review of those standards, EPA placed a major emphasis
on developing a new indicator that considered the significant amount of
evidence on particle size, composition, and relative risk of effects
from penetration and deposition to the major regions of the respiratory
tract (Miller et al., 1979). The development and assessment of these
lines of evidence in the PM Criteria Document and PM Staff Paper
published between 1979 and 1986 culminated in revised standards for PM
that used PM10 as the indicator (52 FR 24634). The major
conclusion from that review, which remained unchanged in the 1997
review, was that ambient particles smaller than or equal to 10 [mu]m in
aerodynamic diameter are capable of penetrating to the deeper
``thoracic'' \45\ regions of the respiratory tract and present the
greatest concern to health (61 FR 65648). While considerable advances
have been made, the available evidence in this review continues to
support the basic conclusions reached in the 1987 and 1997 reviews
regarding penetration and deposition of fine and thoracic coarse
particles. As discussed in the Criteria Document, both fine and
thoracic coarse particles penetrate to and deposit in the alveolar and
tracheobronchial regions. For a range of typical ambient size
distributions, the total deposition of thoracic coarse particles to the
alveolar region can be comparable to or even larger than that for fine
particles. For areas with appreciable coarse particle concentrations,
thoracic coarse particles would tend to dominate particle deposition to
the tracheobronchial region for mouth breathers (EPA, 2004, p. 6-16).
Deposition of particles to the tracheobronchial region is of particular
concern with respect to aggravation of asthma.
---------------------------------------------------------------------------
\45\ The `thoracic' regions of the respiratory tract are located
in the chest (thorax) and are comprised of the tracheo-bronchial
region with connecting airways and the alveolar, or gas-exchange
region of the lung. For ease of communications, `thoracic' particles
penetrating to these regions are often called `inhalable' particles.
---------------------------------------------------------------------------
In the last review, little new toxicologic evidence was available
on potential effects of thoracic coarse particles and there were few
epidemiologic studies that had included direct measurements of thoracic
coarse particles. Evidence of associations between health outcomes and
PM10 that were conducted in areas where PM10 was
predominantly composed of thoracic coarse particles was an important
part of the basis for reaching conclusions about the requisite level of
protection provided against coarse particles for the final standards.
The new studies available in this review include a number of
epidemiologic studies that have reported associations with health
effects using direct measurements of PM10-2.5, as well as a
number of new toxicologic studies.
This section outlines key information contained in the Criteria
Document (Chapters 6-9 and the Staff Paper (Chapter 3) on known or
potential effects associated with exposure to thoracic coarse particles
and their major constituents. The information highlighted here
summarizes: (1) New information available on potential mechanisms for
health effects associated with exposure to thoracic coarse particles or
their constituents; (2) the nature of the effects that have been
associated with ambient thoracic coarse particles or their
constituents; (3) an integrative assessment of the evidence on health
effects related to thoracic coarse particles; (4) subpopulations that
appear to be sensitive to effects of exposure to thoracic coarse
particles; and (5) the public health impact of exposure to ambient
thoracic coarse particles.
1. Mechanisms
As summarized above, the first review of the PM NAAQS found a
strong basis for concluding that thoracic coarse particles could be
plausibly linked to health effects. This was based on an integrated
assessment of the physical and chemical characteristics of ambient
coarse particles, the evidence regarding health effects that could be
associated with deposition of coarse particulate substances in the
different regions of the respiratory tract, and the relative potential
for penetration and deposition of ambient distributions of coarse
particles in the human respiratory tract (52 FR 24634). In the 1987
review, EPA found that occupational and toxicologic studies provided
ample cause for concern related to higher levels of thoracic coarse
particles. Such findings indicated that elevated levels of thoracic
coarse particles were linked with effects such as aggravation of asthma
and increases in upper respiratory illness, which was consistent with
dosimetric evidence of enhanced deposition of thoracic coarse particles
in the respiratory tract (61 FR 65649).
Toxicologic and controlled human exposure studies available in
previous reviews have generally used particle exposures at levels
higher than ambient levels, relying on various particle components or
surrogates. Such studies reported some effects on the respiratory
tract, indicative of inflammatory or irritant effects for particles in
both the fine and thoracic coarse particle size range (EPA, 1982,
chapters 12 and 13; EPA, 1996, chapters 10 and 11). As discussed above
in section II.A, the results of numerous new toxicologic and controlled
human exposure studies have implicated a number of potential mechanisms
or pathways for effects associated with PM. Many of these studies have
used particle exposures that are generally more relevant to studying
the effects of fine particles than those of thoracic coarse particles.
However, several studies, discussed more fully below, have suggested
mechanisms or pathways for thoracic coarse particles to cause
inflammatory and other effects on the respiratory system. This evidence
generally supports previous conclusions that thoracic coarse particles
can affect the respiratory system.
Some limited evidence is available from recent toxicologic studies
on effects of exposure to thoracic coarse particles, specifically using
PM10-2.5, for either acute or chronic exposures (EPA, 2004,
p. 9-55). This toxicologic evidence includes results from studies where
respiratory cell cultures were exposed to ambient particles, thus
providing insight into potential mechanisms for respiratory effects of
thoracic coarse particles. The types of effects reported include
inflammatory and allergic effects. For example, two recent studies
report inflammatory responses in cells exposed to extracts of water-
soluble and water-insoluble materials from thoracic coarse particles
and fine particles collected in Chapel Hill, NC (Monn and Becker, 1999;
Soukup and Becker, 2001). One study focused on water-soluble materials,
and reported significant immune system effects with water-soluble
extracts of ambient PM10-2.5, in contrast to the lack of
effects observed with extracts from
[[Page 2655]]
ambient PM2.5 as well as indoor-collected
PM10-2.5 and PM2.5. The authors report that
different components of PM10-2.5 appeared to have different
effects, with endotoxin implicated in inflammatory effects, while
coarse particulate metals appeared to have a role in cytotoxicity
effects (Monn and Becker, 1999). A followup study in the same
laboratory (Soukup and Becker, 2001) reports that the insoluble
materials from thoracic coarse particles resulted in several effects on
immune system cells.\46\ In this extract of thoracic coarse particles,
endotoxin appeared to be the most pro-inflammatory component, but
components other than endotoxin or metals appeared to contribute to
other effects. Using particles collected in two urban areas in the
Netherlands, Becker et al. (2003) reported that thoracic coarse
particles, but not fine or ultrafine particles, resulted in effects
related to inflammation and decreased pulmonary defenses. This small
group of studies thus suggests that exposure to thoracic coarse
particles may cause pro-inflammatory effects, as well as cytotoxicity
and oxidant generation (EPA, 2004, section 7.4.2). While still limited,
these emerging new studies provide additional insight into potential
mechanisms for respiratory effects of thoracic coarse particles. The
results also indicate that different health responses may be linked
with different components of thoracic coarse particles.
---------------------------------------------------------------------------
\46\ Examples of such effects include cytokine production,
decreased phagocytic ability and oxidant generation.
---------------------------------------------------------------------------
In contrast, one recent study exposed human red blood cell cultures
to ambient coarse particles collected in Italy and found only limited
effects on blood cells (Diociaiuti et al., 2001). The addition of
thoracic coarse particles that were collected in Italy to human
respiratory tract cell cultures produced only limited evidence of
carcinogenic effects; some response was seen with thoracic coarse
particles but greater response was reported with fine particle
exposures (Hornberg et al., 1998). These latter results are consistent
with the evidence from epidemiologic studies, which provide no direct
evidence for carcinogenicity of thoracic coarse particles.
As noted in past reviews (EPA, 1981b, 1996b), deposition of a
variety of particle types in the tracheobronchial region, including
resuspended urban dust and coarse-fraction organic materials, has the
potential to affect lung function and aggravate symptoms, particularly
in asthmatics. Of particular note are limited toxicologic studies that
found urban road dust can produce cellular and immunological effects
(e.g., Kleinman et al., 1995; Steerenberg et al., 2003). Road dust is a
major source of thoracic coarse particles in urban areas and is
therefore representative of the components expected to be found in
resuspended thoracic coarse particles. In the 1996 Staff Paper, results
from the study by Kleinman and colleagues (1995) were highlighted in
which effects were observed in rats with inhalation exposure to road
dust. These effects included changes in the structure of the rat
airways as well as effects on immune cells. Higher concentrations of
road dust were needed to cause effects, compared with exposures to fine
particle components (e.g., sulfates, nitrates), in part because of the
limited penetration of coarse-sized particles past the nose of the rats
studied (EPA, 1996b, p. V-70).\47\ Another study used a standard
toxicologic approach to studying allergic responses, and the authors
concluded that exposure to road tunnel dust particles resulted in
greater allergy-related effects than did exposure to several other
particle samples, including residual oil fly ash and diesel exhaust
particles (Steerenberg et al., 2003).\48\ In this study, the particles
were collected in a road tunnel and placed directly in the animal
respiratory tract, so differences in inhalability of larger particles
in rodents was not an issue. In contrast, a number of studies have
reported that Mt. St. Helens volcanic ash, which is generally in the
size range of thoracic coarse particles, has very little toxicity in
animal or in vitro toxicologic studies (EPA, 2004, p. 7-216).
---------------------------------------------------------------------------
\47\ The particles used in this study were collected by vacuum
sweeping of freeway surfaces in California, and were generally 5
[mu]m in diameter or lower (Kleinman et al., 1995).
\48\ This approach, using ovalbumin-sensitized mice, is commonly
used for comparing allergic potency of air pollutants. The authors
also tested responses in an additional toxicologic model, based on
pollen-sensitized rats, and reported responses only with diesel
exhaust particles (Steerenberg et al., 2003, p. 1436).
---------------------------------------------------------------------------
The Criteria Document finds that the limited number of recent
toxicologic studies using PM10-2.5 provide some evidence
that coarse fraction particle exposures can result in effects primarily
linked to the respiratory system, related to inflammation or
aggravation of allergic effects. Toxicologic studies have suggested
potential pathways for effects from a few sources or components of
thoracic coarse particles, such as road dust particles, metals or
organic constituents. The need to better understand the relationship
between different components or sources of thoracic coarse particles
remains a key area of uncertainty with regard to the effects of
thoracic coarse particles.
2. Nature of Effects
In the last review, EPA considered a substantial number of
epidemiological studies using PM10, which contains both fine
and coarse particles, as a measure of exposure to PM. In many such
studies in which fine and coarse particles occur at similar levels, it
is difficult or impossible to determine whether fine and coarse
particles both played major roles in the associations. Accordingly,
considerable emphasis was placed on the more limited body of evidence
from PM10 studies in locations where coarse particles were a
much greater fraction of PM10 than were fine particles.
These findings indicated that short-term exposure to thoracic coarse
particles in such areas was linked with respiratory morbidity effects,
such as aggravation of asthma, increases in respiratory symptoms and
respiratory infections (62 FR 38677). The single available short-term
exposure study that compared associations between mortality and fine
and coarse particles reported a significant association between short-
term exposure to PM10-2.5 and mortality in one of six cities
(Steubenville, OH). In this location, an unusually high correlation
between high levels of fine and thoracic coarse particles suggested a
common industrial source, and a clear conclusion about the relative
contribution was not possible. The study found no association with
thoracic coarse particles in a combined multi-city analysis (Schwartz
et al., 1996; CD, p. 8-40 to 8-41).\49\ No studies in the past review
provided clear epidemiologic evidence of mortality or morbidity effects
related to long-term exposure to PM10-2.5. EPA observed that
toxicologic studies offered some qualitative evidence suggesting the
potential for effects on the respiratory system with long-term exposure
to coarse particles or coarse particle constituents (62 FR 38678).
---------------------------------------------------------------------------
\49\ Note that in more recent reanalyses of this study to
investigate statistical modeling issues, the association for
Steubenville was not statistically significant in most models
reported in the two reanalyses (Klemm and Mason, 2003; Schwartz, 2003a).
---------------------------------------------------------------------------
In this review, epidemiologic studies have continued to support a
relationship between short-term exposure to thoracic coarse particles
and respiratory morbidity, with effects ranging from increased
respiratory symptoms to hospitalization for respiratory diseases. As
discussed below, the new studies also suggest associations with effects
on the cardiovascular system and possibly with
[[Page 2656]]
mortality. Figure 2 summarizes results from both multi-city and single-
city epidemiologic studies using short-term exposures to
PM10-2.5, including all U.S. and Canadian studies that used
direct measurements of PM10-2.5\50\ and for which effect
estimates and confidence intervals were reported. Consistent with the
presentation of fine particle study results in Figure 1, the central
effect estimate is indicated by a diamond for each study result, with
the vertical bar representing the 95 percent confidence interval around
the estimate. The results of these epidemiologic studies are discussed
below.
---------------------------------------------------------------------------
\50\ All epidemiologic studies discussed below included
measurements of thoracic coarse particles either through monitors
that collected thoracic coarse particles separately (e.g.,
dichotomous monitors) or using data from side-by-side (co-located)
monitors for fine particles and PM10. Investigators have
sometimes also used prediction models to ``fill'' or estimate PM
concentrations where measurements are not available (most often
where data are collected less frequently than daily). In one
particular study in Coachella Valley, measurements were made of fine
and thoracic coarse particle concentrations for two and a half
years. The investigators predicted PM10-2.5
concentrations for a longer time series, based on a ten-year data
set for PM10 for use in the health study (Ostro et al., 2003).
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a. Effects Associated With Short-Term Exposure to Thoracic Coarse
Particles
The discussion below focuses first on evidence related to
respiratory morbidity effects, since information available in the
previous review provided plausible evidence that short-term exposure to
thoracic coarse particles was associated with such effects. This is
followed by a discussion of new findings on potential cardiovascular
effects of thoracic coarse particles, as well as new evidence from
studies of mortality.
i. Morbidity
(a) Effects on the Respiratory System
Evidence available in the last review suggested that aggravation of
asthma
[[Page 2657]]
and respiratory infections and symptoms were associated with
PM10 in areas where thoracic coarse particles were a much
greater fraction of PM10 than were fine particles, such as
Anchorage, AK, and southeast Washington (62 FR 38679). Only one
epidemiologic study had used PM10-2.5 data; it reported a
positive, but not statistically significant, association between
respiratory hospital admissions and PM10-2.5 in Toronto
(Thurston et al., 1994).
Several new studies of respiratory symptoms and lung function have
included both PM10-2.5 and PM2.5 data, and these
results suggest a role for thoracic coarse particles as well as for
fine particles in associations with respiratory symptoms (EPA, 2004, p.
8-311). In the Six Cities study, a statistically significant increase
in cough for children was found with PM10-2.5 but not with
PM2.5, while the reverse was true for lower respiratory
symptoms. When both PM10-2.5 and PM2.5 were
included in models, the effect estimates were reduced for each, but
PM10-2.5 retained significance in the association with cough
and PM2.5 retained significance in the association with
lower respiratory symptoms (Schwartz and Neas, 2000).\51\ Changes in
lung function were evaluated in three cities in Pennsylvania, and in
all three, short-term exposure to thoracic coarse particles was not
significantly associated with peak flow rate, although some
statistically significant associations were found with exposure to fine
particles (EPA, 2004, p. 8-312).
---------------------------------------------------------------------------
\51\ The authors conclude that for acute asthma-related
responses as well as daily mortality, fine particles are a stronger
predictor of health response that are thoracic coarse particles
(Schwartz and Neas, 2000, p. 8).
---------------------------------------------------------------------------
Three new U.S. and Canadian epidemiologic studies have reported
associations between short-term exposure to PM10-2.5 with
hospital admissions for respiratory diseases, including asthma,
pneumonia and COPD (Burnett et al., 1997; Ito, 2003; Sheppard et al.,
2003). As shown in Figure 2, the effect estimates for these
associations are positive and some are statistically significant. In
these associations with respiratory hospitalization, the risk estimates
tend to fall in the range of 5 to 15 percent per 25 [mu]g/m3
PM10-2.5 (EPA, 2004, p. 8-193).
Because fine particles and ozone, as well as other gaseous air
pollutants, are known to cause respiratory effects, a key consideration
for assessing this body of studies is assessment of potential
confounding by these co-pollutants, as discussed in detail in Section
8.4.3 of the Criteria Document. The associations reported between
respiratory hospital admissions and short-term exposure to
PM10-2.5 were largely unchanged in most cases when gaseous
co-pollutants were included in the models (EPA, 2004, Figure 8-18;
Burnett et al., 1997; Ito, 2003).\52\ Few investigators have evaluated
potential confounding of PM10-2.5 effects with adjustment
for PM2.5 in multi-pollutant models. Only the study
conducted in Detroit included such multi-pollutant models for
respiratory hospitalization and was reanalyzed to address potential
statistical modeling questions. In this study, the simultaneous
consideration of PM10-2.5 and PM2.5 resulted in
reduction in the size of the effect estimate, as well as loss of
statistical significance, for both pollutants. The authors report that
the correlation between the two pollutants was ``modest'' (correlation
coefficient of 0.42) (Lippmann et al., 2000, p. 33). The results in
this study vary by health outcome; for example, for pneumonia
hospitalization, effect estimates for PM2.5 were little
changed but those for PM10-2.5 decreased substantially in
magnitude in two-pollutant models. In contrast, effect estimates for
PM2.5 with COPD hospitalization decreased dramatically,
whereas those for PM10-2.5 were only slightly decreased in
size in two-pollutant models (Ito, 2003, pp. 152, 153).
---------------------------------------------------------------------------
\52\ More specifically, the effect estimates for associations
between PM10-2.5 and hospitalization for COPD and
pneumonia in Detroit are largely unchanged with the addition of
gaseous co-pollutants to the models, except in one case where the
PM10-2.5 effect estimate for COPD hospitalization is
substantially reduced in size with the inclusion of O3 in
the model (Ito, 2003). Results for the study in Toronto also show
relatively consistent effect estimate size for associations between
PM10-2.5 and respiratory hospitalization, except for the
models including NO2 and all four gaseous pollutants
(Burnett et al., 1997).
---------------------------------------------------------------------------
Additional insight into the respiratory effects of coarse particles
is provided by studies using PM10 in locations where
thoracic coarse particles were a much greater fraction of
PM10 than were fine particles. This review includes new
PM10 studies in such relatively high coarse-fraction areas,
such as Reno, NV and Anchorage, AK.\53\ In these areas, statistically
significant associations have been reported between PM10 and
hospitalization for respiratory diseases (Chen et al., 2000) and
outpatient medical visits for asthma (Choudhury et al., 1997). These
findings support the evidence from the limited group of studies
discussed above that have reported associations between measured
PM10-2.5 and respiratory morbidity.
---------------------------------------------------------------------------
\53\ For example, Anchorage, AK and Reno, NV do not currently
attain the PM10 24-hour standard which is set at 150
[mu]g/m3. Based on 2002-2004 data, the 98th percentile
PM2.5 concentrations in these areas were 21 and 25 [mu]g/
m3, respectively. As noted in the fine particle
discussion above, no short-term exposure studies to date have shown
statistically significant associations between fine particles and
effects with 98th percentile values this low. This suggests that
coarse particles either caused or contributed to the observed
PM10 associations.
---------------------------------------------------------------------------
Considering evidence from across a range of respiratory morbidity
health outcomes, the Criteria Document concludes that the epidemiologic
evidence indicates that both fine and thoracic coarse particles impact
respiratory health (EPA, 2004, p. 8-311).
(b) Effects on the Cardiovascular System
Two new studies conducted in the U.S. and Canada have also reported
associations between short-term exposure to PM10-2.5 and
hospital admissions for various cardiovascular diseases. The results of
these studies are included in Figure 2, where it can be seen that the
associations are generally positive and the results of the larger
studies with more statistical power are statistically significant
(Burnett et al., 1997, cardiovascular disease hospitalization; Ito,
2003, ischemic heart disease hospitalization). The excess risks for
hospital admissions for cardiovascular diseases range from about 1 to
10 percent per 25 [mu]g/m3 PM10-2.5, as seen in
the Detroit study (EPA, 2004, p. 8-310). In addition, a statistically
significant association was reported between PM10 and
increased hospitalization for cardiovascular diseases in Tucson, AZ, an
urban area where thoracic coarse particles are a much greater fraction
of PM10 than are fine particles (Schwartz, 1997).\54\ The
Criteria Document finds that associations between cardiovascular
hospitalization and short-term PM10-2.5 exposure were
relatively unchanged when gaseous co-pollutants were included in the
models (EPA, 2004, Figure 8-17; Burnett et al., 1997; Ito, 2003).\55\
In assessing potential confounding between PM2.5 and
PM10-2.5, one new study in Detroit reported that
simultaneous consideration of PM10-2.5 and PM2.5
resulted in a reduction in effect estimate
[[Page 2658]]
size and a lack of statistical significance for both PM indicators
(Ito, 2003). In the reanalysis for this study, for example, a
significant association was reported between PM10-2.5 and
hospitalization for ischemic heart disease in a single-pollutant model,
and in a two-pollutant model the effect estimates for PM2.5
and PM10-2.5 were both reduced in magnitude and neither
remained statistically significant (Ito, 2003, pp. 152, 153).
---------------------------------------------------------------------------
\54\ Tucson currently attains the PM10 standard, and
the 98th percentile 24-hour average concentrations reported for
PM2.5 are 15 and 17[mu]g/m3 at two monitoring sites in the area.
\55\ The effect estimates for associations between
PM10-2.5 and hospitalization for ischemic heart disease
and heart failure in Detroit are largely unchanged with the addition
of gaseous co-pollutants to the models (Ito, 2003). Results
presented for the study in Toronto also show relatively consistent
effect estimate size for associations between PM10-2.5
and cardiovascular hospitalization, except for the models including
NO2 and all four gaseous pollutants (Burnett et al., 1997).
---------------------------------------------------------------------------
Epidemiologic studies have also reported associations between
short-term exposures to ambient PM (generally using PM10 or
PM2.5) and more subtle cardiovascular health outcomes (e.g.,
changes in heart rhythm or cardiovascular biomarkers) (EPA, 2004, p. 8-
169). Only one of this new set of epidemiologic studies included
PM10-2.5, and no significant associations were reported
between onset of myocardial infarction and short-term
PM10-2.5 exposures (EPA, 2005a, p. 8-165; Peters et al., 2001).
ii. Mortality
In the few epidemiologic studies available for the last review,
only the Six City study summarized above evaluated the relationship
between short-term exposure to PM10-2.5 and mortality. That
study provided a suggestion of a potential effect of thoracic coarse
particles only in the city with the highest coarse and fine particle
concentrations, but it was not possible to separate fine and thoracic
coarse particle contributions.
As shown in Figure 2 for U.S. and Canadian studies, effect
estimates for associations between mortality and short-term exposure to
PM10-2.5 are generally positive and similar in magnitude to
those for PM2.5 and PM10 though most are not
statistically significant. In general, the confidence intervals
(indicating uncertainty) are greater for associations between mortality
and PM10-2.5 than for associations with PM2.5, as
is apparent when directly comparing results from numerous studies as
shown in Figure 8-5 of the Criteria Document (EPA, 2004, p. 8-61). In
the same comparison, it can be seen that the size of the effect
estimates for the associations are in the same range. In general,
effect estimates are somewhat larger for respiratory and cardiovascular
mortality than for total mortality. Two of the five effect estimates
for cardiovascular mortality with short-term PM10-2.5
exposure are positive and statistically significant (Mar et al., 2003;
Ostro et al., 2003) while none of the effect estimates for total
mortality reach statistical significance. The new studies include a
multi-city study that uses data from the eight largest Canadian cities
and reported associations between total mortality and
PM10-2.5 as well as PM2.5 and PM10.
The effect estimates were of similar magnitude for each PM indicator
(Burnett and Goldberg, 2003), but the association with
PM10-2.5 did not reach statistical significance. The
magnitude of the effect estimates for PM10-2.5 are similar
to those for PM2.5, generally falling in the range of 3 to 8
percent for cardiovascular mortality per 25 [mu]g/m3 PM10-2.5.
Potential confounding by co-pollutant gases has been assessed in
some of these mortality studies. As shown in Figures 8-16 through 8-18
of the Criteria Document, the associations reported with
PM10-2.5 are generally unchanged in effect size when co-
pollutant gases are included in multi-pollutant models. The evidence
available on potential confounding between PM2.5 and
PM10-2.5 is limited, but the Criteria Document includes
results from two studies that showed effects of the two PM indicators
to be relatively independent in multi-pollutant models, however, these
particular analyses were not included in reanalyses to address
statistical modeling questions.\56\
---------------------------------------------------------------------------
\56\ One study was the Canadian 8-city study, in which multi-
pollutant models included PM2.5 and PM10-2.5
and gaseous co-pollutants, with moderate reductions in the effect
estimate size for both PM indicators (Burnett et al., 2000).
Moolgavkar (2000) presented results of two-pollutant models for
PM2.5 and PM10-2.5 with COPD hospitalization
in Los Angeles, and again, effect estimates for both pollutants were
generally reduced somewhat in size. The author also reports that
associations with PM10-2.5 were generally reduced in size
and lost statistical significance in two-pollutant models including
CO. These two studies were reanalyzed to address potential issues
with statistical model specification, but these multi-pollutant
model results were not included in the reanalysis reports.
---------------------------------------------------------------------------
iii. Effects of Thoracic Coarse Particle Components or Sources in
Epidemiologic Studies
In considering the epidemiologic evidence on morbidity or mortality
associations with short-term exposure to thoracic coarse particles, EPA
recognizes that the issue of the relative toxicity of different PM
components, discussed above in section II.A.1 for fine particles, is an
important uncertainty for thoracic coarse particles as well. Several
toxicologic studies, discussed above in section III.A.1, have reported
evidence of effects with different components or sources of thoracic
coarse particles. However, the available epidemiologic studies that
have used PM10-2.5 did not evaluate associations with
specific components of thoracic coarse particles (EPA, 2004, section
8.2.2.5.2). As discussed in section II.A, several studies have reported
that PM2.5 from combustion-related sources is more strongly
linked with mortality than PM2.5 of crustal origin. However,
these findings are not directly relevant to findings related to
thoracic coarse particles. Combustion sources are a major contributor
to PM2.5 emissions, but not to emissions of PM10-2.5, while crustal
material is an important component of PM10-2.5 but only a small portion
of PM2.5 (EPA, 2005a, Table 2-2).
One study that does have relevance to considering the effects of
PM10-2.5 from different sources assessed the contribution of
dust storms to PM10-related mortality. The authors focused
on days when dust storms or high wind events occurred in Spokane,
during which thoracic coarse particles from surrounding rural soils are
the dominant fraction of PM10. No evidence was reported of
increased mortality on days with high PM10 levels related to
these dust storms (average PM10 level was 221 [mu]g/
m3 higher on dust storm days than on other study days)
(Schwartz, et al., 1999), suggesting that PM10-2.5 from
wind-blown rural dust is also not likely associated with mortality.\57\
EPA has also observed that the available epidemiologic studies using
PM10-2.5 have been conducted in urban areas, such as
Phoenix, Detroit and Seattle. Coarse particles are generally not
distributed over broad areas, but rather reflect contributions from
more localized sources, thus it is more difficult than for fine
particles to generalize the results of these studies to areas with
other types of sources.
---------------------------------------------------------------------------
\57\ In addition, studies conducted in several areas in the
western U.S. have reported that associations between PM10
and mortality or morbidity remained unchanged or became larger and
more precise when days indicative of wind-blown dust or high
PM10 concentration days were excluded from the analyses
(Pope et al., 1999; Schwartz, 1997; Chen et al., 2000; Hefflin et
al., 1994). This group of studies does not provide conclusive
evidence of any effects or lack of effects associated with wind-
blown dust or high concentration days, nor were the studies designed
specifically for that purpose. The results do, however, indicate
that associations between PM10 and health outcomes in
these western areas are not overly influenced or ``driven by'' such days.
---------------------------------------------------------------------------
The Criteria Document finds that the new epidemiologic studies
support the conclusions drawn in the previous review, and indicate that
short-term exposure to thoracic coarse particles is likely associated
with respiratory morbidity. The epidemiologic studies report
statistically significant associations between short-term
PM10-2.5 exposure and outcomes ranging from respiratory
symptoms to hospitalization for respiratory diseases (EPA, 2004, p. 8-
312). A limited body of new
[[Page 2659]]
epidemiologic evidence suggests that short-term exposure to thoracic
coarse particles is associated with effects on the cardiovascular
system. Finally, the Criteria Document finds that evidence from health
studies on associations between short-term exposure to
PM10-2.5 and mortality is ``limited and clearly not as
strong'' as evidence for associations with PM2.5 or
PM10 but nonetheless is suggestive of associations with
mortality (EPA, 2004, p. 9-28, 9-32). As discussed briefly above, some
epidemiologic evidence suggests that there are components of thoracic
coarse particles (e.g., crustal material in non-urban areas) that are
less likely to have adverse effects, at least at lower concentrations,
than other components. Based on the epidemiologic evidence, the
Criteria Document concluded that the limited body of evidence provided
suggestive evidence for associations between throacic coarse particles
and various mortality and morbidity effects ``in some locations'' (EPA,
2004, p. 8-338).
b. Effects Related to Long-Term Exposure to Thoracic Coarse Particles
In the last review, the available prospective cohort study results
had shown no evidence of associations between long-term exposure to
thoracic coarse particles and either mortality (Dockery et al., 1993;
Pope et al., 1995) or morbidity (Dockery et al., 1996; Raizenne et al.,
1996). As discussed above for PM2.5, new studies available
in this review include the reanalyses and extended analyses for the Six
Cities and ACS cohort studies of mortality, and new analyses from the
southern California children's cohorts of morbidity effects.
In both the reanalyses and extended analyses of the ACS cohort
study, long-term exposure to PM10-2.5 was not significantly
associated with mortality (CD, p. 8-105; Krewski et al., 2000; Pope et
al., 2002). Based on evidence from reanalyses and extended analyses
using ACS cohort data, the Criteria Document concludes that the long-
term exposure studies find no associations between long-term exposure
to thoracic coarse particles and mortality (EPA, 2004, p. 8-307).
In the previous review, results from the Harvard 24-city study had
shown associations between respiratory illness prevalence and decreased
lung function in children with fine particles or fine particle
indicators, but not with the larger size fractions (Dockery et al.,
1996; Raizenne et al., 1996). Further EPA staff evaluation of the data
from this study that suggested that lung function decrements were not
associated with long-term exposure to thoracic coarse particles (EPA,
1996b, p. V-67a) . In this group of cities, mean thoracic coarse
particle concentrations ranged from approximately 4 to 15 [mu]g/
m3. Several new studies have used data from the Southern
California children's cohorts, one of which included
PM10-2.5 data; in these cities, mean thoracic coarse
particle concentrations ranged from 6 to 39 [mu]g/m3. In
this study, decreases in several measures of lung function growth were
associated with long-term exposure to PM10-2.5 (as well as
PM10 and PM2.5) though not all associations
reached statistical significance (Gauderman et al., 2000). Further, in
analyses for a second cohort of children, no statistically significant
associations were reported between lung function growth and long-term
PM10-2.5 exposure (Gauderman et al., 2002, p. 81). The
correlation reported between PM10-2.5 and PM2.5
in this area was unusually high (r=0.76); in two-pollutant models, the
authors observe that the effects reported with both pollutants were
reduced in magnitude, and did not remain statistically significant,
with somewhat larger reductions for PM10-2.5 associations
than for PM2.5 (Gauderman et al., 2000, p. 1387). Thus,
results from one children's cohort study provide no evidence of
associations between long-term to exposure to PM10-2.5 and
respiratory morbidity, while findings from a more recent cohort study
provide only very limited evidence for such effects. Overall, EPA finds
that the available evidence provides little support to link long-term
exposures to thoracic coarse particles with respiratory morbidity (EPA,
2004, p. 9-34).
3. Integration and Interpretation of the Health Evidence
As discussed in section II.A.3, the Criteria Document and Staff
Paper focused on well-recognized criteria in evaluating the
epidemiologic evidence, including the strength of associations;
robustness of reported associations to the use of alternative model
specifications, potential confounding by co-pollutants, and exposure
misclassification related to measurement error; consistency of findings
in multiple studies of adequate power, and in different persons,
places, circumstances and times; and the nature of concentration-
response relationships. These evaluations addressed key methodological
issues that are relevant to interpretation of evidence from
epidemiologic studies. Further, findings from epidemiologic studies
were integrated with available experimental evidence (e.g., dosimetric
and toxicologic), in considering the extent of coherence and biological
plausibility of effects observed in epidemiologic studies. This
integrative assessment formed the basis for the Criteria Document and
Staff Paper to draw judgments about the extent to which causal
inferences can be made about observed associations between health
endpoints and thoracic coarse particles combination with other
pollutants. The key elements of these evaluations are summarized below.
Many of these issues are discussed in section II.A.3 above for fine
particles, and are thus only briefly summarized here with regard to
implications for thoracic coarse particles.
(1) Effect estimates from associations between short-term exposures
to thoracic coarse particles and various health outcomes are generally
small in size. The Criteria Document observes that the associations are
similar in size to those reported for PM2.5, but with less
precision as the measurement error for PM10-2.5 is greater
than that for PM2.5. Thus, the Criteria Document concludes
that the magnitude of PM10-2.5 associations is similar to
those for fine particles, but the lesser precision of the associations
reduces the strength of the evidence for thoracic coarse particles
(EPA, 2004, p. 9-41).
(2) EPA has evaluated the robustness of epidemiologic associations
in part by considering the effect of differences in statistical model
specification, exposure error on PM-health associations, and potential
confounding by co-pollutants.
Sensitivity to model specification was discussed above for fine
particles, and, in general, similar conclusions apply to studies using
PM10-2.5. Section 8.4.2 of the Criteria Document discusses a
series of reanalyses that address issues related to a specific type of
statistical model (``generalized additive methods'') used in some
recent epidemiologic studies. The results of the reanalyses showed
little change in effect estimates for some studies; in others the
effect estimates were reduced in size though it was observed that the
reductions were often not substantial (EPA, 2004, p. 9-35). Overall,
the Criteria Document concludes that associations between short-term
exposure to PM and various health outcomes are generally robust to the
use of alternative modeling strategies, recognizing that further
evaluation of alternative modeling strategies is warranted. It was also
observed that the results of reanalyses indicated that effect estimates
were more sensitive to the modeling approach used to account for
temporal effects and weather variables than to the specific model
specifications, and thus
[[Page 2660]]
recommended further exploration of alternative modeling approaches for
time-series analyses (EPA, 2004, pp. 8-236 to 8-237).
Recent epidemiologic studies have also evaluated the influence of
exposure error on PM-health associations. This includes both
consideration of error in measurements of PM, and the degree to which
measurements from an individual monitor reflect exposures to the
surrounding community. As discussed in section 8.4.5 of the Criteria
Document, several studies have shown that fairly extreme conditions
(e.g., very high correlation between pollutants and no measurement
error in the ``false'' pollutant) are needed for complete ``transfer of
causality'' of effects from one pollutant to another (EPA, 2004, p. 9-
38). Exposure error is likely to be more important for associations
with PM10-2.5 than with PM2.5, since there is
generally greater error in PM10-2.5 measurements,
PM10-2.5 concentrations are less evenly distributed across a
community, and thoracic coarse particles are less likely to penetrate
into buildings (EPA, 2004, p. 9-38). Thus, factors related to exposure
error likely result in reduced precision for epidemiologic associations
with PM10-2.5.
There are two key implications of this uncertainty for this review.
First, for an individual epidemiologic association, the increased
uncertainty in measurements would tend to increase the standard error
about the effect estimate, possibly reducing statistical significance
of the findings. This would mean that a set of positive but generally
not statistically significant associations between PM10-2.5
and a health outcome could be reflecting a true association that is
measured with error (EPA, 2004, p. 5-126). Second, this uncertainty
about measurements is an important consideration in evaluating the air
quality concentrations with which a statistical association is
reported. The air quality levels reported in these studies, as measured
by ambient concentrations at monitoring sites within the study areas,
are not necessarily good surrogates for the population exposures that
are likely associated with the observed effects in the study areas or
that would likely be associated with effects in other urban areas
across the country. The concentrations measured at one particular site
may over-or under-estimate air quality levels in other parts of the
area. In evaluating the air quality data from the locations in which
epidemiologic associations were reported, as discussed in the Staff
Paper and below in section III.G, examples of both cases are seen. For
example, in Coachella Valley, mortality was statistically significantly
associated with PM10-2.5 measurements made at one site
(Ostro et al., 2003), but these air quality measurements appear to
represent concentrations on the high end of PM10-2.5 levels
for Coachella Valley communities. In contrast, statistically
significant associations were reported with PM10-2.5
measurements in Detroit (Ito, 2003), and in this case the data appear
to represent concentrations on the low end of PM10-2.5
levels for the Detroit area (EPA, 2005a, p. 5-65, 5-66).
Finally, some investigators have assessed the robustness of
associations between health outcomes and short-term exposures to
PM10-2.5 in multi-pollutant models to potential confounding
by the gaseous co-pollutants or fine particles. A high degree of
correlation between the concentrations of thoracic coarse particles and
other pollutants (either gaseous co-pollutants or fine particles) can
make interpretation of the study results difficult. Multi-pollutant
models including PM10-2.5 and gaseous co-pollutants are
included in Figures 8-16 through 8-18 of the Criteria Document, where
it can be seen that associations with PM10-2.5 are largely
unchanged when gaseous co-pollutants are added to the models (EPA,
2004, section 8.4.3). Further, in the available epidemiologic studies,
it can be seen that correlations between the gaseous co-pollutants (CO,
NO2, O3, SO2) and PM10-2.5
concentrations are often lower than correlations between the gases and
fine particles.\58\ While recognizing that disentangling the effects
attributable to various pollutants within an air pollution mixture is
challenging, the Criteria Document concludes that effect estimates for
associations between PM, including PM10-2.5, and health
endpoints are generally robust to confounding by gaseous co-pollutants
(EPA, 2004, p. 9-37).
---------------------------------------------------------------------------
\58\ For example, from the studies included in Figures 8-16
through 8-18, correlation coefficients reported in Detroit between
PM10-2.5 and the four gaseous co-pollutants ranged from
0.13 to 0.32, whereas the correlation coefficients between PM2.5 and the
gaseous co-pollutants range from 0.38-0.49 (Ito, 2003).
---------------------------------------------------------------------------
Less information is available from studies that specifically
assessed potential confounding between fine and thoracic coarse
particles, as noted above. The reported correlation coefficients
between PM10-2.5 and PM2.5 are in the low to
moderate range for most such studies, i.e., generally in a range of
below 0.3 to 0.5, with some notably higher correlation coefficients
reported in Phoenix (0.59) and Steubenville (0.69). As observed
previously, one study in Detroit evaluated the effects of both
PM2.5 and PM10-2.5 simultaneously where the
correlation between the two pollutants was ``modest'' (correlation
coefficient of 0.42). The authors report a reduction in coefficients
for both PM10-2.5 and PM2.5 in associations with
mortality and hospital admissions for respiratory or cardiovascular
diseases (Ito, 2003, pp. 152-153); the degree of reduction in size
varied for different health outcomes. Similarly, Schwartz and Neas
(2000) report some reduction in effect estimate size for both
PM10-2.5 and PM2.5 associations across six cities
in two-pollutant models, but the association reported between
PM10-2.5 and cough remains statistically significant.\59\
Two studies reported associations between PM10-2.5 and
mortality (Ostro et al., 2003, Coachella Valley; Mar et al., 2003,
Phoenix); stronger associations were reported with PM10-2.5
than PM2.5 by Ostro et al., although the authors note the
reduced sample size for PM2.5 may have influenced the
statistical power (Ostro et al., 2003). Both areas have relatively low
fine particle concentrations, with 98th percentile PM2.5
concentrations of about 32 [mu]g/m3 in Phoenix and 34 [mu]g/
m3 in Coachella Valley, while the correlation coefficient
reported between PM2.5 and PM10-2.5 was low in
Coachella Valley (0.28) and fairly high in Phoenix (0.59). This limited
body of evidence suggests that PM10-2.5 and PM2.5
have associations with health outcomes that are likely independent of
one another, but further work is needed to help distinguish the
contributions of thoracic coarse particles on health outcomes from
those of fine particles.
---------------------------------------------------------------------------
\59\ The correlation coefficients between PM10-2.5
and PM2.5 range from 0.23 to 0.45 in five of the six
cities (Boston, Knoxville, Portage, Topeka, and St. Louis), with a
correlation coefficient of 0.69 in Steubenville.
---------------------------------------------------------------------------
Overall, the Criteria Document concludes that associations reported
between health outcomes and short-term exposure to PM10-2.5
are generally robust to the use of alternative modeling strategies, to
adjustment for the potential confounding effects of gaseous co-
pollutants, and in terms of exposure error (EPA, 2004, p. 9-46).
However, the remaining uncertainties are larger in assessing the degree
to which effects observed with thoracic coarse particle exposures are
independent from effects of fine particles. In addition, in
interpreting the results of epidemiologic studies, it is difficult to
determine how well PM10-2.5 concentrations measured at
ambient monitoring stations
[[Page 2661]]
characterize the magnitude of population exposures to thoracic coarse
particles.
(3) In assessing consistency in effect estimates, the epidemiologic
study results suggest that effect estimates may differ from one
location to another, but the extent of variation is not clear. For
example, in one multi-city study, some limited evidence was reported in
the reanalysis to address model specification issues that suggested
some heterogeneity among the 8 largest Canadian cities for associations
with PM10-2.5, although there had been no evidence of
heterogeneity in initial study findings (Burnett and Goldberg, 2003;
EPA, 2004, p. 9-39). As was observed for fine particles, there are a
number of factors that would be likely to cause variation in PM-health
outcomes in different populations and geographic areas. The Criteria
Document discusses such factors, including the mix of PM sources and
composition, the mix of other gaseous pollutants, geographic features
that would affect the spatial distribution of ambient PM, and
population characteristics that affect susceptibility or exposure
levels (EPA, 2004, p. 9-41). In addition, the use of data collected on
a 1-in-6 or 1-in-3 day schedule results in reduced statistical power,
resulting in less precision for estimated effect estimates for the
individual cities and increased potential variability in results (EPA,
2004, p. 9-40). Overall, the Criteria Document concludes that there is
some consistency in effect estimates for hospitalization for
respiratory and cardiovascular causes with short-term exposure to
thoracic coarse particles, though fewer studies are available on which
to make such an assessment than are available for fine particles (EPA,
2004, p. 9-47).
(4) Of the group of new epidemiologic studies that have evaluated
the shape of concentration-response functions, many (generally using
PM10) have been unable to detect threshold levels in the
relationship between short-term PM exposure and mortality. One single-
city study used PM10-2.5 and PM2.5 measurements
in Phoenix and reported that there was no indication of a threshold in
the association between PM10-2.5 and mortality (Smith et
al., 2000; EPA, 2004, p. 8-322). However, a few analyses have provided
suggestions of some potential threshold levels, generally at fairly low
ambient concentrations. Thus, the Criteria Document concludes that the
evidence did not support selecting any particular population threshold
for PM10-2.5, recognizing that there may be thresholds for
specific health responses in individuals, and that it is possible that
such thresholds exist toward the lower end of the range of air quality
measurements in the health studies, but cannot be detected due to
variability in susceptibility across a population. Even in those few
studies with suggestive evidence of such thresholds, the potential
thresholds are at fairly low concentrations (EPA, 2004, sections 8.4.7
and 9.2.2.5).
(5) Several issues related to exposure time periods were assessed
in the Criteria Document, as summarized in section 3.6.5 of the Staff
Paper. One key issue is the lag period between thoracic coarse particle
exposure and health outcome in short-term exposure studies. In many
epidemiologic studies, the authors have reported a pattern of positive
associations across several consecutive lag periods for thoracic coarse
particles, such that an effect estimate for any individual lag day for
thoracic coarse particles likely underestimates the magnitude of the
PM-health response. A number of recent studies that have investigated
associations with distributed lags provide effect estimates for health
responses that persist over a period of time (days to weeks) after the
exposure period and the effect estimates are often, but not always,
larger in size that those for single-day lag periods; however,
available studies have generally not included PM10-2.5 (EPA,
2004, p. 8-281). As reported for fine particles, the Criteria Document
concludes that it is likely that the most appropriate lag period for a
study will vary, depending on the health outcome and the specific
pollutant under study. (EPA, 2004, p. 8-279).
(6) In integrating evidence from across scientific disciplines, the
Criteria Document and Staff Paper observed that the body of
epidemiologic evidence on thoracic coarse particles is smaller than
that for fine particles and the evidence available from toxicologic
studies is also more limited. The clearest case for a causal
relationship for coarse particles is for effects on the respiratory
system. The epidemiologic results showing respiratory effects is
consistent with the assessment of regional particle penetration and
deposition, as well the observations from more limited toxicologic
studies. The fractional deposition of elevated coarse particle
concentrations is significant in the tracheobronchial region, which is
particularly sensitive in asthmatic individuals. From the limited
number of toxicologic studies using PM10-2.5, as noted above
in section III.A.1, there is some evidence that exposure to thoracic
coarse particles results in respiratory-related effects such as
inflammation or oxidative stress. In addition, allergic adjuvant
effects were linked with road dust exposures. These findings are
generally consistent with epidemiologic evidence linking
PM10-2.5 with respiratory morbidity, such as increased
respiratory symptoms and hospitalization for respiratory diseases such
as asthma or COPD.
The evidence is less coherent for effects on the cardiovascular
system. Some epidemiologic studies have reported significant
associations with hospital admissions for cardiovascular diseases, and
associations reported with cardiovascular mortality are positive and
some are statistically significant (see Figure 2). However, the very
limited available evidence from toxicologic studies or epidemiologic
studies on more subtle cardiovascular effects has not provided evidence
that demonstrates plausible mechanisms or pathways for these effects.
Based on an integrative assessment of the evidence, the Criteria
Document concludes that this growing but still limited body of health
evidence is suggestive of causality in associations between short-term
(but not long-term) exposures to thoracic coarse particles and health
effects, particularly for associations with respiratory morbidity.
(7) In summary, based on the available evidence and the evaluation
of that evidence in the Criteria Document and Staff Paper, the Criteria
Document concludes that the body of evidence on effects related to
exposure to thoracic coarse particles is less strong than that for fine
particles, but provides suggestive evidence of causality for short-term
exposure to PM10-2.5 and morbidity, including
hospitalization for respiratory diseases, increased respiratory
symptoms and decreased lung function, and possibly mortality (EPA,
2004, pp. 9-79, 9-80). The Staff Paper recognizes, however, that the
substantial uncertainties associated with this limited body of evidence
suggest that it should be interpreted with a high degree of caution
(EPA, 2005a, p. 5-70).
4. Sensitive Subgroups for Effects of Thoracic Coarse Particle Exposure
As described in section II.A.4, there are several population groups
that may be susceptible or vulnerable to PM-related effects. These
groups include those with preexisting lung diseases, such as asthma,
and children and older adults. Emerging evidence indicates that people
from lower socioeconomic strata or who have particularly elevated
exposures may be more vulnerable to PM-related effects. However, the
available evidence does not generally
[[Page 2662]]
allow distinctions to be drawn between the PM indicators, in terms of
which groups might have greater susceptibility or vulnerability to
PM2.5 or PM10-2.5 (EPA, 2005a pp. 3-35 to 36).
5. Impacts on Public Health From Thoracic Coarse Particle Exposure
While recognizing that the health evidence regarding effects of
thoracic coarse particles is more limited, the Criteria Document has
concluded that the evidence suggests causal associations between short-
term exposure to thoracic coarse particles and morbidity effects, such
as respiratory symptoms or hospital admissions for respiratory
diseases, and possibly mortality. As observed above, the potentially
susceptible populations for such effects include people with
preexisting respiratory diseases, including asthma, and children and
older adults. In focusing on respiratory effects likely associated with
PM10-2.5, it can be observed that population groups with
respiratory diseases such as asthma or COPD include tens of millions of
people (EPA, 2004; Tables 9-4 and 9-5). Considering the magnitude of
these subpopulations and risks identified in health studies, the
Criteria Document concludes that exposure to thoracic coarse particles
can have an important public health impact.
B. Quantitative Risk Assessment
The general overview and discussion of key components of the risk
assessment used to develop risk estimates for PM2.5
presented in section II.B above is also applicable to the assessment
done for PM10-2.5 in this review. However, the scope of the
risk assessment for PM10-2.5 is much more limited than that
for PM2.5, reflecting the much more limited body of
epidemiologic evidence and air quality information available for
PM10-2.5. As discussed in chapter 4 of the Staff Paper, the
PM10-2.5 risk assessment includes risk estimates for just
three urban areas for two categories of health endpoints related to
short-term exposure to PM10-2.5: hospital admissions for
cardiovascular and respiratory causes and respiratory symptoms.
Consistent with the approach used in the PM2.5 risk
assessment, discussed above in section II.B, PM10-2.5-
related health risks attributable to anthropogenic sources and
activities (i.e., risk associated with concentrations above background
or above various selected higher cutpoints intended as surrogates for
alternative assumed population thresholds) were estimated by using the
reported linear or log-linear concentration-response functions from
epidemiologic studies and available air quality data from the locations
in which the studies had been conducted. A series of base case analyses
were conducted, using the same assumed cutpoints as were used in the
assessment of short-term exposures to PM2.5.
Estimates of hospital admissions attributable to short-term
exposure to PM10-2.5 have been developed for Detroit
(cardiovascular and respiratory admissions) and Seattle (respiratory
admissions), and estimates of respiratory symptoms have been developed
for St. Louis.\60\ Base case estimates of respiratory-related hospital
admissions under recent air quality levels in Detroit are on the order
of several hundred admissions per year across the range of assumed
cutpoints considered in this assessment. The Detroit estimates are
roughly one to two orders of magnitude greater than the range of
estimated asthma-related admissions in Seattle, which can be attributed
in part to differences in baseline risks related to respiratory-related
health endpoints as well as to differences in PM10-2.5 air
quality levels in these two areas. More specifically, recent (e.g.,
2001-2003) PM10-2.5 concentrations are substantially higher
in Detroit, where the current 24-hour PM10 standard is not
met, than they are in Seattle (where the 24-hour PM10 design
value is well below the level of the current PM10 standard).
In considering risk estimates for respiratory symptoms in St. Louis,
the number of days of cough in children living in St. Louis associated
with recent PM10-2.5 levels range from approximately 27,000
days per year \61\ at the lowest assumed cutpoint to almost 3,000 days
per year at the highest assumed cutpoint. For the same time period,
PM10-2.5 air quality levels in St. Louis are high, where,
like Detroit, the current 24-hour PM10 standard is not met.
---------------------------------------------------------------------------
\60\ Quantitative risk estimates associated with recent air
quality levels for these three cities are presented in Figures 4-11
and 4-12 in Chapter 4 of the Staff Paper.
\61\ This represents roughly 1100 days of cough per 100,000
people in the general population, of which approximately 12 percent
are children.
---------------------------------------------------------------------------
While one of the goals of the PM10-2.5 risk assessment
was to provide estimates of the risk reductions associated with just
meeting alternative PM10-2.5 standards, the nature and
magnitude of the uncertainties and concerns associated with this
portion of the risk assessment weigh against use of these risk
estimates as a basis for recommending specific standard levels (EPA,
2005a, p. 5-69). These uncertainties and concerns include, but are not
limited to the following:
(1) As noted above in section II.A and discussed more fully below
in section III.G, the PM10-2.5 levels measured at ambient
monitoring sites in recent years may be quite different from the levels
used to characterize exposure in the original epidemiologic studies
based on monitoring sites in different location, thus possibly over- or
underestimating population risk levels.
(2) There is greater uncertainty about the reasonableness of the
use of proportional rollback to simulate just meeting alternative
PM10-2.5 standards in any urban area relative to that for
PM2.5 due to the limited availability of historic
PM10-2.5 air quality data.
(3) The locations used in the PM10-2.5 risk assessment
are not representative of urban areas in the U.S. that experience the
most significant 24-hour peak PM10-2.5 concentrations, and
thus, observations about relative risk reductions associated with
alternative standards may not be relevant to the areas expected to have
the greatest health risks associated with elevated ambient
PM10-2.5 levels.
(4) The health effects database that supplies the concentration-
response relationships used in the PM10-2.5 risk assessment
is much smaller than that available for PM2.5, which limits
EPA's ability to evaluate the robustness of the risk estimates for the
same health endpoints across different locations.
C. Need for Revision of the Current Primary PM10 Standards
The initial issue to be addressed in the current review of the
primary PM10 standards is whether, in view of the advances
in scientific knowledge reflected in the Criteria Document and Staff
Paper, the existing standards should be revised. The Staff Paper
addresses this question by first considering the conclusions reached in
the last review, the subsequent litigation of that decision, and the
nature of the new information available in this review.
In 1997, in conjunction with establishing new PM2.5
standards, EPA concluded that continued protection against potential
effects associated with thoracic coarse particles in the size range of
2.5 to 10 [mu]m was warranted based on particle dosimetry, toxicologic
information, and limited epidemiologic evidence (62 FR 38,677). This
information indicated that thoracic coarse particles can deposit in the
sensitive regions of the lung of most concern (e.g., the
tracheobronchial and alveolar regions, which together make
[[Page 2663]]
up the thoracic region),\62\ and that they can be expected to aggravate
effects in individuals with asthma and contribute to increased upper
respiratory illness (62 FR 38,666-8).
---------------------------------------------------------------------------
\62\ EPA further concluded at that time that the risks of
adverse health effects associated with deposition of particles in
the thoracic region are ``markedly greater than for deposition in
the extrathoracic (head) region,'' and that risks from extrathoracic
deposition are ``sufficiently low that particles which deposit only
in that region can safely be excluded from the standard indicator''
(62 FR 38,666).
---------------------------------------------------------------------------
Further, EPA decided that the new function of PM10
standard(s) would be to provide such protection against effects
associated with particles in this narrower size range between 2.5 to 10
[mu]m. Although some consideration had been given to a more narrowly
defined indicator that did not include fine particles (e.g.,
PM10-2.5), EPA decided that it was more appropriate to
continue to use PM10 as the indicator for standards to
control thoracic coarse particles. This decision was based in part on
the recognition that the only studies of clear quantitative relevance
to health effects most likely associated with thoracic coarse particles
used PM10 in areas where the coarse fraction was the
dominant fraction of PM10, namely two studies conducted in
areas that substantially exceeded the 24-hour PM10 standard
(62 FR 38,679). The decision also reflected the fact that there were
only very limited ambient air quality data then available specifically
on thoracic coarse particles, in contrast to the extensive monitoring
network already in place for PM10. In essence, EPA concluded
at that time that it was appropriate to continue to control thoracic
coarse particles, but that the only information available upon which to
base such standards was indexed in terms of PM10.
In subsequent litigation regarding the 1997 PM NAAQS revisions,
however, the court held in part that PM10 is a ``poorly
matched indicator'' for thoracic coarse particles in the context of a
rule that also includes PM2.5 standards because
PM10 includes PM2.5. American Trucking
Associations v. EPA, 175 F.3d. at 1054. Although the court found
``ample support'' (id.) for EPA's decision to regulate thoracic coarse
particles, it vacated the 1997 revised PM10 standards for
that reason. The result of subsequent EPA actions, discussed above in
section I.C, is that the 1987 PM10 standards remain in place
(65 FR 80776, 80777, Dec. 22, 2000) and the present review is
consequently of those 1987 standards.
In this review, the Staff Paper focuses on the information now
available from a growing, but still limited, body of evidence on health
effects associated with thoracic coarse particles from studies that use
PM10-2.5 as the measure of thoracic coarse particles. In
addition, there is now much more information available to characterize
air quality in terms of PM10-2.5 than was available in the
last review.\63\ In considering this information, the Staff Paper finds
that the major considerations that formed the basis for EPA's 1997
decision to retain PM10 as the indicator for thoracic coarse
particles, rather than a more narrowly defined indicator that does not
include fine particles, no longer apply. More specifically, the
continued use of PM10 as an indicator for standards intended
to protect against health effects associated with thoracic coarse
particles is no longer appropriate since information is now available
that supports the use of a more directly relevant indicator,
PM10-2.5. Further, continuing to rely principally on health
effects evidence indexed by PM10 to determine the
appropriate averaging time, form, and level of a standard is no longer
necessary or appropriate since a number of more directly relevant
studies, indexed by PM10-2.5, are also now available. Thus,
separate from any legal considerations, the Staff Paper concludes it is
appropriate to revise the current PM10 standards in part by
revising the indicator for thoracic coarse particles, and by basing any
such revised standard principally on the currently available evidence
and air quality information indexed by PM10-2.5, but also
considering evidence from studies using PM10 in locations
where PM10-2.5 is the predominant fraction (EPA, 2005a,
section 5.4.1).
---------------------------------------------------------------------------
\63\ Coarse particle concentrations from EPA's monitoring
network are currently determined using a difference method in
locations with same-day data from co-located PM10 and
PM2.5 FRM monitors.
---------------------------------------------------------------------------
Recognizing that dosimetric evidence formed the principal basis for
the initial establishment of the PM10 indicator in 1987, and
supported the decision in 1997 to retain the PM10 indicator,
the Staff Paper also considers whether currently available dosimetric
evidence continues to support the basic conclusions reached in those
reviews of the standards. In particular, consideration is given to
available information about patterns of penetration and deposition of
thoracic coarse particles in the sensitive thoracic region of the lung
and to whether an aerodynamic size of 10 [mu]m remains a reasonable
separation point for particles that penetrate and potentially deposit
in the thoracic regions. The Staff Paper concludes that while
considerable advances have been made in understanding particle
dosimetry, the available evidence continues to support those basic
conclusions from past reviews. More specifically, both fine particles,
indexed by PM2.5, and thoracic coarse particles, indexed by
PM10-2.5, penetrate to and deposit in the thoracic regions.
Further, for a range of typical ambient size distributions, the total
deposition of thoracic coarse particles to the alveolar region can be
comparable to or even larger than that for fine particles (EPA, 2004,
p. 6-16).
Beyond the dosimetric evidence, as noted in past reviews (EPA,
1981b, 1996b), toxicologic studies show that the deposition of a
variety of particle types in the tracheobronchial region, including
resuspended urban dust and coarse-fraction organic materials, has the
potential to affect lung function and aggravate respiratory symptoms,
particularly in asthmatics. Of particular note are limited toxicologic
studies that found urban road dust can produce cellular and
immunological effects (e.g., Kleinman, et al., 1995; Steerenberg et
al., 2003).\64\ In addition, some very limited in vitro toxicologic
studies show some evidence that coarse particles may elicit pro-
inflammatory effects (EPA, 2004, section 7.4.4). Further, the Staff
Paper assessment of the physicochemical properties and occurrence of
ambient coarse particles suggests that both the chemical makeup and the
spatial distribution of coarse particles are likely to be more
heterogeneous than for fine particles (EPA, 2005a, chapter 2). In
particular, as discussed below in section III.D, coarse particles in
urban areas can contain all of the components found in more rural
areas, but be contaminated by a number of additional materials, from
motor vehicle-related emissions to metals and transition elements
associated with industrial operations. The Staff Paper concludes that
the weight of the dosimetric, limited toxicologic, and atmospheric
science evidence, taken together, lends support to the plausibility of
the PM10-2.5-related effects reported in urban epidemiologic
studies, and provides support for retaining some standard for thoracic
coarse particles so as to continue programs to protect public health
from such effects (EPA, 2005a, p. 5-49).
---------------------------------------------------------------------------
\64\ The Criteria Document notes that toxicologic studies, in
general, use exposure concentrations that are generally much higher
than ambient concentrations (EPA, 2004, p. 9-51).
---------------------------------------------------------------------------
The available epidemiologic evidence, discussed above in section
III.A, includes studies of associations between short-term exposure to
thoracic coarse particles, indexed by PM10-2.5, and
[[Page 2664]]
health endpoints, as well as evidence from PM10 studies
conducted in areas in which the coarse fraction is dominant. More
specifically, several U.S. and Canadian studies now provide evidence of
associations between short-term exposure to PM10-2.5 and
various morbidity endpoints. Three such studies conducted in Toronto
(Burnett et al., 1997), Seattle (Sheppard et al., 2003), and Detroit
(Ito, 2003) report statistically significant associations between
short-term PM10-2.5 exposure and respiratory- and cardiac-
related hospital admissions, and a fourth study (Schwartz and Neas,
2000) conducted in six U.S. cities including Boston, St. Louis,
Knoxville, Topeka, Portage, and Steubenville reports statistically
significant associations across these six areas with respiratory
symptoms in children. These studies were mostly done in areas in which
PM2.5, rather than PM10-2.5, is the larger
fraction of ambient PM10, and they are not representative of
areas with relatively high levels of thoracic coarse particles (EPA,
2005a, p. 5-49).
In evaluating the epidemiologic evidence from health studies on
associations between short-term exposure to PM10-2.5 and
mortality, the Criteria Document concluded that such evidence was
``limited and clearly not as strong'' as that for associations with
PM2.5 or PM10 but nonetheless was suggestive of
associations with mortality (EPA, 2004, p. 9-28, 9-32). Statistically
significant mortality associations were reported in short-term exposure
studies conducted in areas with relatively high PM10-2.5
concentrations, including Phoenix (Mar et al., 2003), Coachella Valley,
CA (Ostro et al., 2003), and in the initial analysis of data from
Steubenville (as part of the Six Cities study, Schwartz et al., 1996),
although in a reanalysis of this study, the results were generally not
statistically significant (Klemm and Mason, 2003). In areas with lower
PM10-2.5 concentrations, no statistically significant
associations were reported with mortality, though most were positive.
The Staff Paper also considers relevant epidemiologic studies
indexed by PM10 that were conducted in areas where the
coarse fraction of PM10 is typically much greater than the
fine fraction. Such studies include findings of associations between
short-term exposure to PM10 and hospitalization for
cardiovascular diseases in Tucson, AZ (Schwartz, 1997), hospitalization
for COPD in Reno/Sparks, NV (Chen et al., 2000), and medical visits for
asthma or respiratory diseases in Anchorage, AK (Gordian et al., 1996;
Choudhury et al., 1997). In addition, a number of epidemiologic studies
have reported significant associations with mortality, respiratory
hospital admissions and respiratory symptoms in the Utah Valley area
(e.g., Pope et al., 1989; 1991; 1992). This group of studies provides
additional supportive evidence for associations between short-term
exposure to thoracic coarse particles and health effects, particularly
morbidity effects, generally in areas not meeting the PM10
standards (EPA, 2005a, p. 5-50).\65\
---------------------------------------------------------------------------
\65\ Based on recent air quality data, as well as the summary
information provided for PM concentrations used in the studies, the
existing PM10 standards are not met in any of these study
cities except Tucson, AZ. Based on 2002-2004 air quality data, the
98th percentile PM2.5 concentrations in three of these
areas range from 15 to 25 [mu]g/m3, while in Utah Valley
the concentrations range from 37 to 54 [mu]g/m3.
---------------------------------------------------------------------------
In contrast to the findings from the short-term exposure studies
discussed above, available epidemiologic studies do not provide
evidence that long-term exposure to thoracic coarse particles is
associated with mortality or morbidity (EPA, 2005a, p. 3-25). More
specifically, no association is found between long-term exposure to
thoracic coarse particles and mortality in the reanalyses and extended
analysis of the ACS cohort (EPA, 2005a, p. 8-307). Further, little
evidence is available on potential respiratory and cardiovascular
morbidity effects of long-term exposure to thoracic coarse particles
(EPA, 2005a, p. 3-23-24).
Taken together, the Staff Paper concludes that the health evidence,
including dosimetric, toxicologic and epidemiologic study findings,
supports retaining some standard to protect against effects associated
with short-term exposure to thoracic coarse particles. However, the
substantial uncertainties associated with this limited body of
epidemiologic evidence on health effects related to exposure to
PM10-2.5, including the difficulty in separating the effects
of fine and thoracic coarse particles, suggest a high degree of caution
in interpreting this evidence, especially at the lower levels of
ambient particle concentrations in the morbidity studies discussed
above (EPA, 2004, p. 5-50).
Beyond this evidence-based evaluation, the Staff Paper also
considers the extent to which PM10-2.5-related health risks
estimated to occur at current levels of ambient air quality may be
judged to be important from a public health perspective, taking into
account key uncertainties associated with the estimated risks.
Consistent with the approach used to address this issue for
PM2.5-related health risks, discussed above in section II.B,
the Staff Paper considers the results of a series of base case analyses
that reflect in part the uncertainty associated with the form of the
concentration-response functions drawn from the studies used in the
assessment. In this assessment, which is much more limited than the
risk assessment conducted for PM2.5, health risks were
estimated for three urban areas by using the reported linear or log-
linear concentration-response functions as well as modified functions
that incorporate alternative assumed cutpoints as surrogates for
potential population thresholds (discussed above in section III.B). In
considering the risk estimates from this limited assessment, and
recognizing the very substantial uncertainties inherent in basing an
assessment on such limited information, the Staff Paper concludes that
the results for the two areas in the assessment that did not meet the
current PM10 standards are indicative of risks that can
reasonably be judged to be important from a public health perspective,
in contrast to the appreciably lower risks estimated for the area that
did meet the current standards (EPA, 2005a, p. 5-52).
The Staff Paper recognizes the substantial uncertainties associated
with the limited available epidemiologic evidence and the inherent
difficulties in interpreting the evidence for purposes of setting
appropriate standards for thoracic coarse particles. Nonetheless, in
considering the available evidence, the public health implications of
estimated risks associated with current levels of air quality, and the
related limitations and uncertainties, the Staff Paper concludes that
this information supports (1) revising the current PM10
standards in part by revising the indicator for thoracic coarse
particles, and (2) consideration of a standard that will continue to
provide public health protection from short-term exposure to thoracic
coarse particles of concern that have been associated with morbidity
effects and possibly mortality at current levels in some urban areas
(EPA, 2005a, p. 5-52).
In CASAC's review of these Staff Paper recommendations, there was
general concurrence among CASAC Panel members that there is a need to
revise the current PM10 standards and establish a primary
standard specifically targeted to address particles in the size range
of 2.5 to 10 [mu]m (Henderson, 2005b). In making this recommendation,
CASAC indicated its agreement with the summary of the scientific data
regarding the potential adverse health effects from exposures to
thoracic coarse particles in
[[Page 2665]]
section 5.4 of the Staff Paper upon which the EPA staff recommendations
were based.
In considering whether the primary PM10 standards should
be revised, the Administrator has carefully considered the rationale
and recommendations contained in the Staff Paper, the advice and
recommendations of CASAC, and public comments to date on this issue.
The Administrator provisionally concludes that the health evidence,
including dosimetric, toxicologic and epidemiologic study findings,
supports retaining a standard to protect against effects associated
with short-term exposure to thoracic coarse particles. Further, the
Administrator believes that the new evidence on health effects from
studies that use PM10-2.5 as a measure of thoracic coarse
particles, together with the much more extensive data now available to
characterize air quality in terms of PM10-2.5, provide an
appropriate basis for revising the current PM10 standards in
part by revising the indicator to focus more narrowly on particles
between 2.5 and 10 [mu]m. The Administrator also notes that the need
for a standard for thoracic coarse particles has already been upheld
based upon evidence of health effects considerably more limited than
now available. American Trucking Associations v. EPA, 175 F. 3d at
1054. Based on these considerations, the Administrator provisionally
concludes that the current suite of PM10 standards should be
revised, and that the revised standard(s) should provide more targeted
protection from short-term exposure to those thoracic coarse particles
that are of concern to public health.
D. Indicator of Thoracic Coarse Particles
In considering an appropriate indicator for a standard intended to
afford protection from health effects associated with exposure to
thoracic coarse particles of concern, the Staff Paper starts by making
the following observations:
(1) The most obvious choice for a thoracic coarse particle standard
is the size-differentiated, mass-based indicator used in the
epidemiologic studies that provide the most direct evidence of such
health effects, PM10-2.5.
(2) The upper size cut of a PM10-2.5 indicator is
consistent with dosimetric evidence that continues to reinforce the
finding from past reviews that an aerodynamic size of 10 [mu]m is a
reasonable separation point for particles that penetrate to and
potentially deposit in the thoracic regions of the respiratory tract.
(3) The lower size cut of such an indicator is consistent with the
choice of 2.5 [mu]m as a reasonable separation point between fine and
coarse fraction particles.
(4) Further, the limited available information is not sufficient to
define an indicator for thoracic coarse particles solely in terms of
metrics other than size-differentiated mass, such as specific chemical
components.
(5) The available epidemiologic evidence for effects of
PM10-2.5 exposure is quite limited and is inherently
characterized by large uncertainties, reflective in part of the more
heterogeneous nature of the spatial distribution and chemical
composition of thoracic coarse particles and the more limited and
generally uncertain measurement methods that have historically been
used to characterize their ambient concentrations.
In evaluating relevant information from atmospheric sciences,
toxicology, and epidemiology related to thoracic coarse particles, the
Staff Paper notes that there appears to be clear distinctions between
(1) the character of the ambient mix of particles generally found in
urban areas as compared to that found in nonurban and, more
specifically, rural areas, and (2) the nature of the evidence
concerning health effects associated with thoracic coarse particles
generally found in urban versus rural areas. Based on such information,
and on specific initial advice from CASAC (Henderson, 2005a), the Staff
Paper considers a more narrowly defined indicator for thoracic coarse
particles that focuses on the mix of such particles that is
characteristic of that generally found in urban areas where thoracic
coarse particles are strongly influenced by traffic-related or
industrial sources. In so doing, the Staff Paper focuses on comparing
the potential health effects associated with thoracic coarse particles
in urban and rural settings, as discussed below.
Atmospheric science and monitoring information indicates that
exposures to thoracic coarse particles tend to be higher in urban areas
than in nearby rural locations. Further, the mix of thoracic coarse
particles typically found in urban areas contains a number of
contaminants that are not commonly present to the same degree in the
mix of natural crustal particles that is typical of rural areas. The
elevation of PM10-2.5 levels in urban locations as compared
to those at nearby rural sites suggests that sources located within
urban areas are generally the cause of elevated urban concentrations;
conversely, PM10-2.5 concentrations in such urban areas are
not largely composed of particles blown in from more distant regions
(EPA, 2005a, sections 2.4.5 and 5.4.2.1). Important sources of thoracic
coarse particles in urban areas include dense traffic that suspends
significant quantities of dust from paved roads, as well as industrial
and combustion sources and construction activities that contribute to
ambient coarse particles both directly and through deposition to soils
and roads (EPA, 2005a, Table 2-2). It follows that the mix of thoracic
coarse particles in urban areas would differ in composition from that
in rural areas, being influenced to a relatively greater degree by
components from urban mobile and stationary source emissions.
While detailed composition data are more limited for
PM10-2.5 than for PM2.5, available measurements
from some areas as well as studies of road dust components do show a
significant influence of urban sources on both the composition and mass
of thoracic coarse particles generally found in urban areas. Although
crustal elements and natural biological materials represent a
significant fraction of thoracic coarse particles in urban areas, both
their relative quantity and character may be altered by urban sources.
For example, in industrial cities, primary particle emissions from
industrial sources and resuspended road dust can increase the relative
amount of iron in the mix of PM10-2.5, one of the metals
that has been noted as being of some interest in the studies of
mechanisms of toxicity for PM, as well as other industrial process-
related and potentially toxic materials such as nickel, cadmium, and
chromium (EPA, 2005a, p. 5-54). Traffic-related activities can also
grind and resuspend vegetative materials into forms not as common in
more natural areas (Rogge et al., 1993). Studies of urban road dusts
find that levels of a variety of components are increased from traffic
as well as from other anthropogenic urban sources, including products
of incomplete combustion (e.g. polycyclic aromatic hydrocarbons) from
motor vehicle emissions and other sources, brake and tire wear, rust,
salt and biological materials (EPA, 2004, p. 3D-3). Limited ambient
coarse fraction composition data from various comparisons find that
metals and sometimes elemental carbon contribute a greater proportion
of thoracic coarse particle mass in urban areas than in nearby rural
areas. In addition, while large uncertainties exist in emissions
inventory data, the Staff Paper observes that major sources of
PM10-2.5 emissions in the urban counties in which
epidemiologic studies have been conducted are paved roads and ``other''
[[Page 2666]]
sources (largely construction), and that such areas also have larger
contributions from industrial emissions, whereas unpaved roads and
agriculture are the main sources of PM10-2.5 emissions
outside of urban areas.
Toxicologic studies, although quite limited, support the view that
thoracic coarse particles from sources common in urban areas are of
greater concern than uncontaminated materials of geologic origin. One
major source of thoracic coarse particles in urban areas is paved road
dust; the Criteria Document discusses results from a recent toxicologic
study in which road tunnel dust particles had greater allergic adjuvant
activity than several other particle samples (Steerenberg et al., 2003;
EPA, 2004, pp. 7-136, 137). This study supports evidence available in
the last review regarding potential effects of road dust particles
(EPA, 1996b, p. V-70). In contrast, a number of studies have reported
that Mt. St. Helens volcanic ash, an example of natural crustal
material of geologic origin, has very little toxicity in animal or in
vitro toxicologic studies (EPA, 2004, p. 7-216).
A few toxicologic studies have used ambient thoracic coarse
particles from urban/suburban locations (PM10-2.5), and the
results suggest that effects can be linked with several components of
PM10-2.5. These in vitro toxicologic studies linked thoracic
coarse particles with effects including cytotoxicity, oxidant
formation, and inflammatory effects (EPA, 2005a, sections 3.2 and
5.4.1). These studies suggest that several components (e.g., metals,
endotoxin, other materials) may have roles in various health responses
but do not suggest a focus on any individual component.
Although largely focused on undifferentiated PM10, the
series of epidemiologic observations and toxicologic experiments
related to the Utah Valley suggest that directly emitted (fine and
coarse) and resuspended (coarse) urban industrial emissions are of
concern. Of particular interest are area studies spanning a 13-month
period when a major source of PM10 in the area, a steel
mill, was not operating. Observational studies found that respiratory
hospital admissions for children were lower when the plant was shut
down (Pope et al., 1989). More recently, a set of toxicologic and
controlled human exposure studies have used particles extracted from
filters from ambient PM10 monitors from periods when the
plant did and did not operate. In both human volunteers and animals,
greater lung inflammatory responses were reported with particles
collected when the source was operating, as compared to the period when
the plant was closed (EPA, 2004, p. 9-73). In addition, in some studies
it was suggested that the metal content of the particles was most
closely related to the effects reported (EPA, 2004, p. 9-74). While
peak days in the Utah Valley occur in conditions that enhance fine
particle concentrations, over the long run, over half of the
PM10 was in the coarse fraction. The aggregation of
particles collected on the filters during the study period reflect this
long-term composition and represent the kinds of industrial components
that would be incorporated in road dusts in the area.
Epidemiologic studies that have examined exposures to thoracic
coarse particles generally found in urban environments, together with
studies that have taken into account exposures to natural crustal
materials typical of rural areas, generally support the view that the
mix of thoracic coarse particles generally found in urban areas is of
concern to public health, in contrast to natural crustal dusts of
geologic origin. With respect to the urban results, several recent
studies have shown associations between PM10-2.5 and health
outcomes in a few sites across the U.S. and Canada. Associations have
been reported with morbidity in a few urban areas, some of which had
relatively low PM10-2.5 concentrations. For mortality,
statistically significant associations have been reported only for two
urban areas that have notably higher ambient PM10-2.5
concentrations. These associations are with short-term exposures to
aggregated PM10-2.5 mass, and no epidemiologic evidence is
available on associations with different components or sources of
PM10-2.5. However, these studies have all been conducted in
urban areas of the U.S., and thus reflect effects associated with the
ambient mix of thoracic coarse particles generally present in urban
environments.
In contrast, recent evidence from epidemiologic studies has
suggested that mortality and possibly other health effects are not
associated with thoracic coarse particles from dust storms or other
such wind-related events that result in suspension of natural crustal
materials of geologic origin. The clearest example is provided by a
study in Spokane, WA, which specifically assessed whether mortality was
increased on dust-storm days using case-control analysis methods. The
average PM10 level was more than 200 [mu]g/m3
higher on dust storm days than on control days, and the authors report
no evidence of increased mortality on these specific days (Schwartz et
al., 1999). One caveat of note is the possibility that people may
reduce their exposure to ambient particles on the most dusty days
(e.g., Gordian et al., 1996; Ostro et al., 2000). Nevertheless, these
studies provide no suggestion of significant health effects from
uncontaminated natural crustal materials that would typically form a
major fraction of coarse particles in non-urban or rural areas.
Beyond the urban and rural distinctions discussed above, the Staff
Paper also considers the extent to which there is evidence of effects
with exposure to the ambient thoracic coarse particles in communities
predominantly influenced by agricultural or mining sources.\66\ For
example, in the last review, EPA considered health evidence related to
long-term silica exposures from mining activities, but found that there
was a lack of evidence that such emissions contribute to effects linked
with ambient PM exposures (EPA, 1996b, p. V-28). Similarly in this
review, there is an absence of evidence related to such community
exposures. While crustal and organic dusts generated from agricultural
activity can include a variety of biological materials, and some
occupational studies discussed in the Criteria Document report effects
at occupational exposure levels (EPA, 2004, Table 7B-3, p. 7B-11), such
studies do not provide relevant evidence for effects at much lower
levels of community exposures. Further, it is unlikely that such
sources contribute to the effects that have been observed in the recent
urban epidemiologic studies.
---------------------------------------------------------------------------
\66\ Mining sources are intended to include all activities that
encompass extraction and/or mechanical handling of natural geologic
crustal materials.
---------------------------------------------------------------------------
The Criteria Document concludes its integrated assessment of the
effects of natural crustal materials as follows:
Certain classes of ambient particles appear to be distinctly
less toxic than others and are unlikely to exert human health
effects at typical ambient exposure concentrations (or perhaps only
under special circumstances). For example, particles of crustal
origin, which are predominately in the coarse fraction, are
relatively non-toxic under most circumstances, compared to
combustion-related particles (such as from coal and oil combustion,
wood burning, etc.) However, under some conditions, crustal
particles may become sufficiently toxic to cause human health
effects. (EPA, 2004, p. 8-344)
The Staff Paper assessment of the available evidence relevant to
the appropriate scope of an indicator for coarse particles can be
summarized as follows. Ambient concentrations of thoracic coarse
particles generally
[[Page 2667]]
reflect contributions from local sources, and the limited information
available from speciation of thoracic coarse particles and emissions
inventory data indicate that the sources of thoracic coarse particles
in urban areas generally differ from those found in nonurban areas. As
a result, the mix of thoracic coarse particles people are typically
exposed to in urban areas can be expected to differ appreciably from
the mix typically found in non-urban or rural areas. Ambient
PM10-2.5 exposure is associated with health effects in
studies conducted in urban areas, and the limited available health
evidence more strongly implicates the ambient mix of thoracic coarse
particles that is dominated by traffic-related and industrial sources
than that from uncontaminated soil or geologic sources. The limited
evidence does not support either the existence or the lack of causative
associations for community exposures to thoracic coarse particles from
agricultural or mining industries. Given the apparent differences in
composition and in the epidemiologic evidence, the Staff Paper
concludes that it is not appropriate to generalize the available
evidence of associations with health effects that have been related to
thoracic coarse particles generally found in urban areas and apply it
to the mix of particles typically found in nonurban or rural areas
(EPA, 2005a, p. 5-57).
Collectively, this evidence suggests that a more narrowly defined
indicator for thoracic coarse particles should be considered that would
protect public health against effects that have been linked with the
mix of thoracic coarse particles generally present in urban areas. Such
an indicator would be principally based on particle size, but also
reflect a focus on the mix of thoracic coarse particles that is
generally present in urban environments and the sources that
principally generate that mix. The Staff Paper recommends consideration
of thoracic coarse urban particulate matter \67\ as an indicator for a
thoracic coarse particle standard, referring to the mix of airborne
particles between 2.5 and 10 [mu]m in diameter that are generally
present in urban environments, which, as discussed above, are
principally comprised of resuspended road dust typical of high traffic-
density areas and emissions from industrial sources and construction
activities (EPA, 2005a, p. 5-54, 5-57-58). The Staff Paper concludes
that such an indicator would more likely be an effective indicator for
standards to protect against health effects that have been associated
with thoracic coarse particles than a more broadly focused
PM10-2.5 indicator. This indicator would also be consistent
with an appropriately cautious interpretation of the epidemiologic
evidence that does not potentially over-generalize the results of the
limited available studies.
---------------------------------------------------------------------------
\67\ The acronym ``UPM10-2.5'' is used in the Staff
Paper to refer to this indicator.
---------------------------------------------------------------------------
In conjunction with this recommendation of an indicator defined in
terms of the mix of thoracic coarse particles that are generally
present in urban areas, the Staff Paper also discusses the importance
of a monitoring network designed so as to be consistent with the intent
of such an indicator and that would facilitate implementation of such a
standard. EPA has historically used implementation policies to address
elevations in thoracic coarse particle levels that may occur in urban
areas as a result of dust storms or other such events for which this
staff-recommended indicator is not intended to apply. Both new criteria
for monitor network design and revised natural/exceptional events
policies should work in concert with a revised thoracic coarse particle
indicator to ensure the most effective application of a thoracic coarse
particle standard.
In its review of the Staff Paper recommendation for a thoracic
coarse particle indicator (Henderson, 2005b), the CASAC generally
agreed that ``thoracic coarse particles in urban areas can be expected
to differ in composition from those in rural areas;'' that ``coarse
particles in urban or industrial areas are likely to be enriched by
anthropogenic pollutants that tend to be inherently more toxic than the
windblown crustal material which typically dominates coarse particle
mass in arid rural areas;'' and that ``evidence of associations with
health effects related to urban coarse-mode particles would not
necessarily apply to non-urban or rural coarse particles.'' Further,
most CASAC Panel members concurred that ``the current scarcity of
information on the toxicity of rural dusts makes it necessary'' for EPA
to base its standard for thoracic coarse particles ``on the known
toxicity of urban-derived coarse particles.'' While most Panel members
concurred with the thoracic coarse particle indicator recommended in
the Staff Paper, a few members recommended specifying a
PM10-2.5 indicator in conjunction with monitoring network
design criteria and natural/exceptional events policies that would
emphasize urban influences. In either case, CASAC indicated that the
intent of any such indicator should be to ``provide protection against
those components of PM10-2.5 that arise from anthropogenic
activities occurring in or near urban and industrial areas.''
In considering an appropriate indicator for a standard intended to
afford protection from health effects associated with exposure to
thoracic coarse particles of concern, the Administrator has carefully
considered the rationale and recommendations contained in the Staff
Paper, the advice and recommendations from CASAC, and public comments
to date on this issue. In so doing, the Administrator believes, despite
the substantial limitations and uncertainties in the relevant
information available, that it is appropriate to propose a new
indicator for such particles at this time. Further, the Administrator
believes that any such indicator should be defined not only by particle
size, to generally include those particles between 2.5 and 10 [mu]m in
diameter, but also by qualifications that narrow the scope of the
indicator. In considering an indicator that is intended to focus on the
mix of thoracic coarse particles generally present in urban
environments and commonly derived from sources typically found in urban
environments, consistent with Staff Paper and CASAC recommendations,
the Administrator notes that identifying it as an ``urban'' thoracic
coarse particle indicator could be misconstrued as meaning that the
standard is limited to certain geographic locations and, thus, not a
national standard. To avoid this semantic problem, the Administrator
has sought to define the indicator in a way that more clearly focuses
on the nature of the mix of thoracic coarse particles intended to be
included and the sources that principally generate that mix, rather
than just where they are found, and that also explicitly focuses on
what would be excluded from such an indicator. In so doing, the
Administrator intends the proposed indicator to be equivalent to the
one recommended in the Staff Paper and endorsed by CASAC, but to do so
in a manner that will be more clearly understood and less likely to be
misinterpreted.
Taking into account the considerations discussed above, the
Administrator proposes to establish a new indicator for thoracic coarse
particles in terms of PM10-2.5, the definition of which
includes qualifications that identify both the mix of such particles
that are of concern to public health, and are thus included in the
indicator, and those for which currently available information is not
sufficient to infer a public health concern, and are thus excluded.
More specifically, the proposed PM10-2.5 indicator is
qualified so as to include any ambient mix of PM10-2.5 that is
[[Page 2668]]
dominated by resuspended dust from high-density traffic on paved roads
and PM generated by industrial sources and construction sources, and
excludes any ambient mix of PM10-2.5 that is dominated by
rural windblown dust and soils and PM generated by agricultural and
mining sources. In short, the indicator is not defined by nor limited
to any specific geographic area, but includes the mix of
PM10-2.5 in any location that is dominated by these sources.
With the indicator as defined above, each area in the country would
fall into one or the other of these two categories: (1) Either the
majority of the ambient mix of PM10-2.5 in an area is
resuspended dust from high-density traffic on paved roads and PM
generated by industrial sources and construction sources, or (2) the
majority of the ambient mix is rural windblown dust and soils and PM
generated by agricultural and mining sources. The indicator would apply
when PM10-2.5 generated by one or more of these named
sources in the first category constitutes a majority of the ambient mix
of PM10-2.5. The EPA recognizes that in many cases it will
be clear which of these two categories applies, while in other cases it
may be difficult to determine the appropriate category. As described in
more detail in the preamble to EPA's proposed monitor network design
rule, published elsewhere in today's Federal Register, the proposed
minimum monitor siting criteria would provide guidance on
distinguishing between areas where the mix of PM10-2.5 of
concern would likely be dominated by the named sources in the first
category and those areas where it would not. Consequently, all
PM10-2.5 captured by a monitor that is properly sited in
light of the indicator described above, as discussed in the proposed
monitoring rule, would be considered in applying the standard, since
the monitor would be capturing the mix of ambient PM10-2.5
covered by the proposed indicator. As such, the proposed indicator does
not present the type of over-inclusion or under-inclusion problems
noted by the court with respect to a PM10 indicator (see
American Trucking Associations v. EPA, 175 F.3d at 1054), since the
application of the proposed indicator would result in compliance being
based on measurement of the mix of ambient PM10-2.5 at which
the standard is directed.
The regulation for the proposed thoracic coarse particle indicator
states that ``[a]gricultural sources, mining sources, and other similar
sources of crustal material shall not be subject to control in meeting
this standard.'' This proposed language reflects that the information
supporting the proposed standard for thoracic coarse particles does not
support extending controls to thoracic coarse particles from
agricultural, mining sources, and other similar sources of crustal
material. This statement in the regulations therefore is designed to
make clear that there is no need nor basis to control these sources to
obtain the public health benefits intended by the proposed indicator.
Although the Administrator believes that an indicator qualified
through reference to these categories and named sources appropriately
identifies the ambient mixes that the epidemiologic studies indicate
are of concern to public health, he solicits comment as to whether
there may be other classes of sources which should also be included or
excluded from the indicator. More generally, comment is also solicited
on the approach of defining the indicator in terms of both particle
size and categories of named sources.
The Administrator recognizes that the proposed indicator, which
includes considerations beyond particle size in its definition,
represents a shift in the way in which PM indicators have been defined
historically, and thus poses new challenges in ensuring a common
understanding of how it can be appropriately and consistently
implemented in areas across the country. In the Administrator's view,
the application of this proposed indicator in conjunction with the
proposed monitoring network design criteria, published elsewhere in
today's Federal Register, and proposed rules for the treatment of air
quality data influenced by exceptional events that will be published in
the near future, will facilitate appropriate and consistent implementation.
E. Averaging Time of Primary PM10-2.5 Standard
In the last review, EPA retained both 24-hour and annual
PM10 standards to provide protection against the known and
potential effects of short- and long-term exposures to thoracic coarse
particles (62 FR at 38,677-79). That decision was based in part on
qualitative considerations related to the expectation that deposition
of thoracic coarse particles in the respiratory system could aggravate
effects in individuals with asthma. In addition, quantitative support
for retaining a 24-hour standard came from limited epidemiologic
evidence suggesting that aggravation of asthma and respiratory
infection and symptoms may be associated with daily or episodic
increases in PM10, where dominated by thoracic coarse
particles including fugitive dust. The decision to retain an annual
standard as well was generally based on considerations of the
plausibility of the potential build-up of insoluble thoracic coarse
particles in the lung after long-term exposures to high levels of such
particles.
New information available in this review on thoracic coarse
particles, discussed above, includes several epidemiologic studies that
report statistically significant associations between short-term (24-
hour) exposure to PM10-2.5 and various morbidity effects and
mortality. With regard to long-term exposure studies, while one recent
study conducted in southern California reported a link between reduced
lung function growth and long-term exposure to PM10-2.5 and
PM2.5, other such studies reported no associations (EPA,
2005a, p. 3-19, 3-23-24). Thus, the Criteria Document concludes that
the available evidence does not suggest an association with long-term
exposure to PM10-2.5 (EPA, 2004, p. 9-79).
Based on these considerations, the Staff Paper concludes that the
newly available evidence continues to support a 24-hour averaging time
for a standard intended to control thoracic coarse particles, based
primarily on evidence suggestive of associations between short-term
(24-hour) exposure and morbidity effects and, to a lesser degree,
mortality. Noting the absence of evidence judged to be suggestive of an
association with long-term exposures, the Staff Paper concludes that
there is no quantitative evidence that directly supports an annual
standard, while recognizing that it could be appropriate to consider an
annual standard to provide a margin of safety against possible effects
related to long-term exposure to thoracic coarse particles that future
research may reveal. The Staff Paper observes, however, that a 24-hour
standard that would reduce 24-hour exposures would also likely reduce
long-term average exposures, thus providing some margin of safety
against the possibility of health effects associated with long-term
exposures (EPA, 2005a, p. 5-61).
Based on its review of the Staff Paper, CASAC recommends retention
of a 24-hour averaging time and agrees that an annual averaging time
for PM10-2.5 is not currently warranted (Henderson, 2005b).
Based on these considerations, the Administrator concurs with staff and
CASAC recommendations, and provisionally concludes that the newly
available evidence continues to support a 24-hour averaging time for a
PM10-2.5 standard, based primarily on evidence suggestive of
associations between
[[Page 2669]]
short-term (24-hour) exposure and morbidity effects and, to a lesser
degree, mortality. Further, the Administrator agrees that an annual
PM10-2.5 standard is not warranted at this time. Thus, the
Administrator proposes to revoke the annual PM10 standard
and is not proposing an annual PM10-2.5 standard.
F. Form of Primary PM10-2.5 Standard
For reasons similar to those discussed above in section II.F.2 on
the form of the 24-hour PM2.5 standard, the Staff Paper also
recommends consideration of either the 98th or 99th percentile form for
a 24-hour PM10-2.5 standard. The relative year-to-year
stability of the air quality statistic to be used as the basis for the
form of a PM10-2.5 standard is of particular importance for
a PM10-2.5 standard, since the nature and magnitude of the
uncertainties in the risk assessment conducted for thoracic coarse
particles weighed against considering risk estimates as a basis for
comparing alternative combinations of specific forms and levels of standards.
In considering the information provided in the Staff Paper, CASAC
strongly recommends use of the 98th percentile form because it is more
statistically robust than the 99th percentile form, together with the
use of a three-year average of this statistic (Henderson 2005b). In
making this recommendation, CASAC notes that the use of this statistic
will tend to minimize ``measurement error and spatial variability,
which are larger for coarse-mode particles than for fine PM'' as well
as ``the influence in arid areas of occasional but extreme excursion
contributions from rural, coarse-mode dust sources that are thought to
be inherently less toxic than coarse-mode particles heavily enriched
with urban source contaminants'' (Henderson, 2005b).
In considering the available information, the Administrator concurs
with the CASAC recommendation and proposes that the form of the 24-hour
PM10-2.5 standard be based on the annual 98th percentile
statistic, averaged over three years.
G. Level of Primary PM10-2.5 Standard
In considering the available evidence on associations between
short-term PM10-2.5 concentrations and morbidity and
mortality effects as a basis for setting a 24-hour standard for
thoracic coarse particles, the Staff Paper focuses on relevant U.S. and
Canadian epidemiologic studies, as discussed above in section II.A. As
an initial matter, the Staff Paper recognizes that these individual
short-term exposure studies provide no evidence of clear population
thresholds, or lowest-observed-effects levels, in terms of 24-hour
average concentrations. As a consequence, this body of evidence is
difficult to translate directly into a specific 24-hour standard that
would protect against the range of effects that have been associated
with short-term exposures.
In considering the evidence, the Staff Paper notes the significant
uncertainties and the limited nature of the available evidence. In
examining the available evidence to identify a basis for a range of
standard levels that would be appropriate for consideration, the Staff
Paper focuses on the upper end of the distributions of daily
PM10-2.5 concentrations in the relevant studies in terms of
the 98th and 99th percentile values.\68\
---------------------------------------------------------------------------
\68\ This examination of the evidence is based on air quality
information and analyses presented in two staff memos which were
part of the materials reviewed by CASAC (Ross and Langstaff, 2005;
Ross, 2005).
---------------------------------------------------------------------------
In looking first at the morbidity studies that report statistically
significant associations with respiratory- and cardiac-related hospital
admissions in Toronto (Burnett et al., 1997), Seattle (Sheppard et al.,
2003), and Detroit (Ito, 2003), the 98th percentile values reported in
these studies range from approximately 30 to 36 [mu]g/m\3\. To provide
some perspective on these PM10-2.5 levels, the Staff Paper
notes that the level of the 24-hour PM10 standard was
exceeded only on a few occasions during the time periods of the studies
in Detroit and Seattle.\69\ In looking also at the mortality studies
that report statistically significant and generally robust associations
with short-term exposures to PM10-2.5 in Phoenix (Mar et
al., 2003) and Coachella Valley, CA (Ostro et al., 2003), the reported
98th percentile values were approximately 70 and 107 [mu]g/m\3\,
respectively. These studies were conducted in areas with air quality
levels that did not meet the current PM10 standards. In
addition, a statistically significant association was reported between
PM10-2.5 and mortality in Steubenville as part of the
original Six Cities study (Schwartz et al., 1996), although in more
recent reanalyses, the association did not remain statistically
significant in most models (Schwartz, 2003a; Klemm and Mason, 2003)--
the PM10-2.5 concentrations in this eastern city were fairly
high, with a reported 98th percentile value of 53 [mu]g/m\3\. In
contrast to the statistically significant mortality associations with
PM10-2.5 reported in these studies, the Staff Paper notes
that no such associations were reported in a number of other studies,
including those in the five other cities that were part of the Six
Cities study (Boston, St. Louis, Knoxville, Topeka, and Portage), Santa
Clara County, CA, Detroit, Philadelphia, and Pittsburgh. With the
exception of Pittsburgh, these cities had much lower 98th percentile
PM10-2.5 values, ranging from 18 to 49 [mu]g/m\3\. Thus, in
mortality studies that reported statistically significant associations,
the reported 98th percentile PM10-2.5 values were all above
50 [mu]g/m\3\, whereas in the mortality studies that reported no
statistically significant associations, the reported 98th percentile
PM10-2.5 values were generally below 50 [mu]g/m\3\.
---------------------------------------------------------------------------
\69\ As shown in air quality data trends reports: for Seattle,
1997 Air Quality Annual Report for Washington State, p. 17, at
http://www.ecy.wa.gov/pubs/97208.pdf; for Detroit, Michigan's
2003 Annual Air Quality Report, p. 46, at http://www.deq.state.mi.us/
documents/deq-aqd-air-reports-03AQReport.pdf.
---------------------------------------------------------------------------
In looking more closely at air quality data used in the morbidity
and mortality studies discussed above, however, the Staff Paper
recognizes that the uncertainty related to exposure measurement error
associated with using ambient concentrations to represent area-wide
population exposure levels can be potentially quite large. For example,
in looking specifically at the Detroit study, the Staff Paper notes
that the PM10-2.5 air quality values were based on air
quality monitors located in Windsor, Canada. While the study authors
concluded that these monitors were appropriate for use in exploring the
association between air quality and hospital admissions in Detroit, a
close examination of air quality levels at Detroit and Windsor sites in
recent years led to the conclusion that the statistically significant,
generally robust association with hospital admissions in Detroit likely
reflects population exposures that may be appreciably higher in the
central city area, but not necessarily across the broader study area,
than would be estimated using data from the Windsor monitors (EPA,
2005a, p. 5-64).
The EPA staff also looked more specifically at the Coachella Valley
mortality study (Ostro et al., 2003), in which data were used from a
single monitoring site in one city, Indio, within the study area where
daily measurements were available. A close examination of air quality
levels across the Coachella Valley suggests that while the association
of mortality with PM10-2.5 measurements made at the Indio
site was statistically significant, a portion of the study population
would have been expected to experience appreciably lower ambient
exposure levels. In contrast to the Detroit study, air quality data
used in the mortality
[[Page 2670]]
study conducted in Coachella Valley appear to represent concentrations
on the high end of PM10-2.5 levels for Coachella Valley
communities. On the other hand, a close examination of the air quality
data used in the other studies discussed above generally shows less
disparity between air quality levels at the monitoring sites used in
the studies and the broader pattern of air quality levels across the
study areas than that described above in the Detroit and Coachella
Valley studies.
This close examination of air quality information generally
reinforces the view that exposure measurement error is potentially
quite large in these PM10-2.5 studies. As a consequence, the
air quality levels reported in these studies, as measured by ambient
concentrations at monitoring sites within the study areas, are not
necessarily good surrogates for population exposures that are likely
associated with the observed effects in the study areas or that would
likely be associated in other urban areas across the country. The
Detroit example suggests that population exposures were probably
appreciably underestimated in the Detroit morbidity study, such that
the observed effects are likely associated with higher
PM10-2.5 levels than reported. In contrast, the Coachella
Valley mortality study provides an example in which population levels
were probably appreciably overestimated, such that the observed effects
may well be associated with lower PM10-2.5 levels than
reported. At relatively low levels of air quality, population exposures
implied by these studies as being associated with the observed effects
likely become more uncertain, suggesting a high degree of caution in
interpreting the group of morbidity studies as a basis for identifying
a standard level that would protect against the observed effects.
Taking into account this close examination of the studies, the
Staff Paper concludes that this evidence suggests that EPA could
consider a standard for urban thoracic coarse particles at a
PM10-2.5 level at least down to 50 [mu]g/m\3\, in
conjunction with a 98th percentile form. This view takes into account
the conclusion that this evidence is particularly uncertain as to
population exposures, especially from the morbidity studies reporting
effects at relatively low concentrations, as well as the general lack
of evidence of associations from the group of mortality studies with
reported concentrations below these levels.
Another view that reflects a more cautious or restrained approach
to interpreting the limited body of PM10-2.5 epidemiologic
evidence would be to judge that the uncertainties in this whole group
of studies as to population exposures that are associated with the
observed effects are too large to use the reported air quality levels
directly as a basis for setting a specific standard level. Such a
judgment would be consistent with concluding that these studies,
together with other dosimetric and toxicologic evidence, provide
support for retaining standards for thoracic coarse particles at some
level to protect against the morbidity and mortality effects observed
in the studies, regardless of whether an associated population exposure
level can be clearly discerned from the studies.
Based on this more cautious approach, the Staff Paper concludes
that it would be reasonable to interpret the available epidemiologic
evidence more qualitatively. Considering the available evidence in this
way leads to the following observations:
(1) The statistically significant mortality associations with
short-term exposure to PM10-2.5 reported in the Phoenix and
Coachella Valley studies were observed in areas that did not meet the
current PM10 standards.
(2) The statistically significant morbidity associations with
short-term exposure to PM10-2.5 reported in the Detroit and
Seattle studies were observed in areas that exceeded the level of the
current 24-hour PM10 standard on just a few occasions during
the time periods of the studies.
(3) All but one of the statistically significant morbidity and
mortality associations with short-term exposure to PM10
reported in areas in which the thoracic coarse particle fraction of
PM10 was much greater than was the fine fraction (including
Reno/Sparks, NV, Tucson, AZ, Anchorage, AK, and the Utah Valley area)
were observed in areas that did not meet the current PM10
standards.
Based on these considerations, the Staff Paper finds little basis
for concluding that the degree of protection afforded by the current
PM10 standards in urban areas is greater than warranted,
since potential mortality effects have been associated with air quality
levels not allowed by the current standards, but have not been
associated with air quality levels that would generally meet the
current standards, and morbidity effects have been associated with air
quality levels that exceeded the current standards only a few times.
Further, the Staff Paper finds little basis for concluding that a
greater degree of protection is warranted in light of the very high
degree of uncertainty in the relevant population exposures implied by
the morbidity studies. The Staff Paper concludes, therefore, that it is
reasonable to interpret the available evidence as supporting
consideration of a short-term standard for thoracic coarse particles,
so as to provide generally ``equivalent'' protection to that afforded
by the current PM10 standards, recognizing that no one
PM10-2.5 level will be strictly equivalent to a specific
PM10 level in all areas (EPA, 2005a, p. 5-67). Such a
standard would likely provide protection against morbidity effects
especially in urban areas where, unlike the study areas,
PM10 is generally dominated by coarse-fraction rather than
fine-fraction particles. Such a standard would also likely provide
protection against the more serious, but more uncertain,
PM10-2.5-related mortality effects generally observed at
somewhat higher air quality levels.
To identify a range of levels for consideration for a 24-hour
PM10-2.5 standard, based on the indicator proposed above and
set so as to afford generally ``equivalent'' protection as the current
PM10 standards, the Staff Paper presents the results of
analyses of relevant data on PM10-2.5 and PM10
24-hour average concentrations.\70\ In one such analysis of 205
monitoring sites (Schmidt et al., 2005),\71\ a PM10-2.5
level of approximately 60 [mu]g/m\3\, in terms of a 98th percentile
form, would be roughly equivalent on average across the U.S. to the
current PM10 standard level of 150 [mu]g/m\3\, in terms of
the current one-expected-exceedance form.\72\ While noting appreciable
variability in the estimated point of equivalence across individual
sites, these levels of approximate average equivalence are quite
consistent across each of the five regions in which all of the areas
that do not meet the current PM10 standards are located
(including the southern California, southwest, northwest, upper mid-
west, and southeast regions). Notably different average equivalence
[[Page 2671]]
levels were observed in the other two regions, i.e., approximately 40
[mu]g/m\3\ in the northeast and over 70 [mu]g/m\3\ in the industrial
mid-west.
---------------------------------------------------------------------------
\70\ Consistent with PM10-2.5 monitoring network
design criteria discussed in section 5.4.2.2 of the Staff Paper,
monitors included in this analysis are those in CBSAs with at least
100,000 population and in census block groups with a population
density of at least 500, and that also had 3 years of complete data
in each quarter for both PM10 and PM10-2.5
(EPA, 2005a, p. 5-67).
\71\ These analyses were based on collocated PM10 and
PM10-2.5 data, and used linear regression methods to
predict PM10-2.5 concentrations (98th percentile form)
equivalent to the 24-hour PM10 standard level of 150
[mu]g/m\3\ (one expected exceedence form) at a national and at
regional levels.
\72\ Across the U.S., the 95 percent confidence intervals around
these point estimates are approximately ± 3 [mu]g/m\3\,
while region-specific intervals are approximately ± 10
[mu]g/m\3\ in the five regions in which all of the areas that do not
meet the current PM10 standards are located (EPA, 2005a, p. 5-68).
---------------------------------------------------------------------------
Another such analysis was based on comparing the number of areas,
and the population in those areas, that would likely not meet a
specific PM10-2.5 standard, set at a given level and form,
with the same measures in areas that do not meet the current
PM10 standards. This analysis, based on 2001 to 2003 data,
provides some rough indication of the breadth of protection potentially
afforded by alternative standards. The results of this analysis
indicate that a PM10-2.5 standard of about 70 or 65 [mu]g/
m\3\, 98th percentile form, would impact approximately the same number
of counties or number of people, respectively, as would the current
PM10 standards.\73\
---------------------------------------------------------------------------
\73\ As shown in Tables 5B-2(a) and (b) of the Staff Paper,
there are 585 counties with PM10 monitoring sites used in
determining compliance with the PM10 standards, whereas
only 309 of those counties have monitor sites that would be included
in the monitoring network design criteria discussed in section
5.4.2.2 of the Staff Paper. Of these 309 counties, 259 have
PM10 and PM10-2.5 air quality data that meet
the data completeness criteria defined for this analysis, which are
somewhat less restrictive than the criteria that were applied in the
regression analysis described above.
---------------------------------------------------------------------------
In considering the relevant dosimetric, toxicologic, and
epidemiologic evidence, related limitations and uncertainties, and
analyses of relevant air quality information, the Staff Paper concludes
that it is appropriate to consider a 24-hour PM10-2.5
standard in the range of 50 to 70 [mu]g/m\3\, with a 98th percentile
form.\74\ The lower end of this range is based on a close examination
of the air quality patterns related to the limited number of relevant
epidemiologic studies. The upper part of this range is based on a more
cautious approach to interpreting the available information and
reflects a generally ``equivalent'' degree of protection to that
afforded by the current PM10 standards. The upper end of
this range is also below the 98th percentile PM10-2.5
concentrations in the two mortality studies that reported statistically
significant associations. Consideration of a generally ``equivalent''
PM10-2.5 standard would reflect a judgment that while the
epidemiologic evidence supports establishing a short-term standard for
urban thoracic coarse particles at such a generally ``equivalent''
level, the evidence concerning air quality levels of thoracic coarse
particles in the studies is not strong enough to provide a basis for
changing the level of protection generally afforded by the current
PM10 standards.
---------------------------------------------------------------------------
\74\ Beyond looking directly at the relevant epidemiologic
evidence and related air quality information, the Staff Paper also
considers the extent to which the PM10-2.5 risk
assessment, discussed above in section III.B, can help inform
consideration of alternative 24-hour PM10-2.5 standards.
The Staff Paper concludes that the nature and magnitude of the
uncertainties and concerns associated with this portion of the risk
assessment weigh against use of these risk estimates as a basis for
recommending specific standard levels (EPA, 2005a, p. 5-69).
---------------------------------------------------------------------------
Based on its review of the Staff Paper, there was general agreement
among the CASAC Panel members that the Staff Paper-recommended range of
50 to 70 [mu]g/m\3\, with a 98th percentile form, for a 24-hour
PM10-2.5 standard was reasonably justified. Most CASAC Panel
members favored levels at the upper end of that range, while several
members supported the lower end of the range (Henderson, 2005b).
Because of the significant uncertainties resulting from the limited
number of studies to date in which PM10-2.5 has been
measured and the potentially large exposure measurement errors in such
studies, the CASAC Panel did not generally support a level below the
Staff Paper-recommended range.
In considering an appropriate level for a 24-hour
PM10-2.5 standard intended to afford requisite protection of
public health from health effects associated with exposure to thoracic
coarse particles of concern, the Administrator has carefully considered
the rationale and recommendations contained in the Staff Paper, the
advice and recommendations of CASAC, and public comments to date on
this issue. Taking these considerations into account, the Administrator
proposes to set the level of the primary 24-hour PM10-2.5
standard at 70 [mu]g/m\3\. In the Administrator's provisional judgment,
based on the currently available evidence, a standard set at this level
would be requisite to protect public health with an adequate margin of
safety from the morbidity and possibly mortality effects that have been
associated with short-term exposures to thoracic coarse particles of
concern. This proposed standard is expected to have the most impact in
areas that do not meet the current 24-hour PM10 standard.
In reaching this judgment, the Administrator recognizes that the
epidemiologic evidence on morbidity and possible mortality effects
related to PM10-2.5 exposure is very limited at this time,
and that there are potentially quite large uncertainties inherent in
interpreting the available evidence for PM10-2.5 as compared
with the evidence related to fine particles. For example,
PM10-2.5 concentrations can vary substantially across a
metropolitan area and thoracic coarse particles are less able to
penetrate into buildings than fine particles; thus, the ambient
concentrations reported in epidemiologic studies may not well represent
area-wide population exposure levels. It may also be difficult to
disentangle effects associated with PM10-2.5 and
PM2.5 in epidemiologic studies. Further, the Administrator
is mindful that considering what standard is requisite to protect
public health with an adequate margin of safety requires judgments that
neither overstate nor understate the strength and limitations of the
evidence or the appropriate inferences to be drawn from the evidence.
Thus, the Administrator provisionally concludes that the selection of a
level that provides generally equivalent protection to that provided by
the current PM10 standards is an appropriate policy response
to the very limited body of evidence that is available at this time.
The EPA intends to address the considerable uncertainties in the
currently available information on thoracic coarse particles as part of
the Agency's ongoing PM research program.
The Administrator also recognizes that there is no one level for a
PM10-2.5 standard that would be equivalent to the current
PM10 standards in every area across the country, and that
there are likely additional approaches to identifying a generally
equivalent standard level beyond those approaches considered in the
Staff Paper upon which the proposed level is based. Thus, the
Administrator also solicits comment on alternative approaches to
identifying a generally ``equivalent'' standard level. While proposing
to set the PM10-2.5 standard at a level that is generally
equivalent to the 1987 PM10 standard, the Administrator
solicits comment on whether it would be more appropriate to set the
PM10-2.5 standard at a level that is generally equivalent to
the PM10 standard set in 1997.
Having decided to propose the 24-hour PM10-2.5 standard
described above, the Administrator recognizes that there are important
views on the information relating to the effects of coarse fraction PM
that warrant consideration. For example, an alternative interpretation
of the available health evidence presented in the Criteria Document and
the Staff Paper questions the conclusions about PM10-2.5
associations drawn from one-pollutant models. This interpretation of
the available epidemiological evidence suggests that the results from
one-pollutant PM10-2.5 models are confounded by fine
particles and gaseous co-pollutants.
The key PM10-2.5 epidemiologic results discussed in the
Criteria Document and
[[Page 2672]]
Staff Paper are drawn from one-pollutant models; i.e.,
PM10-2.5 is the only variable used in the statistical model
reflecting exposure to air pollution. There are four studies cited in
these documents as being suggestive of a statistically significant role
for PM10-2.5 in the reported associations: Ito (2003),
Burnett et al. (1997), Mar et al. (2003), and Ostro et al. (2003).
However, there is strong evidence that adverse health effects similar
to those observed in these studies, including both cardiovascular and/
or respiratory health effects are associated with exposure to
PM2.5. The authors of several of these studies focus on fine
particles (and in some cases one or more of the gaseous pollutants) as
playing an important role in ``explaining'' the association between PM
and various health endpoints. For example, in these key epidemiologic
studies, the correlation coefficients between PM2.5 and
PM10-2.5 concentrations range from moderate to high (i.e.,
0.4 to 0.7), which increases the likelihood that associations between
health effects and PM10-2.5 identified in one-pollutant
models may instead simply reflect the effects of exposure to
PM2.5 rather than independent health effects. With the
positive correlations between pollutants and similar health effects, it
generally would be appropriate for any assessment of the effect of
exposure to PM10-2.5 to control for exposure to the PM2.5.
In this light, it is important to review how the authors of the
four key PM10-2.5 epidemiology studies have accounted for
co-pollutants in their analysis. Ito (2003) noted significant estimates
of the health effects of associations in one-pollutant models, but in a
two-pollutant model with PM2.5 the PM10-2.5
associations lost statistical significance. Burnett et al. (1997)
concluded that the effect of PM10-2.5 in a one-pollutant
model could be explained by gaseous co-pollutants. Mar et al. (2003)
found PM10-2.5 to be positively associated with adverse
health effects in a one-pollutant model, but also found similar
associations with a range of other air pollutants. In addition, Mar et
al. (2003) noted that even though all PM mass metrics included in the
study were associated with an excess risk of cardiovascular death, the
strongest associations were with PM2.5, followed by
PM10 and PM10-2.5. Ostro et al. (2003) used a
one-pollutant model to estimate the association between
PM10-2.5 on mortality using an effectively linear construct
of PM10 (as observed in Indio, CA) to represent
PM10-2.5 for the entire study area. By using such a
construct of PM10, the estimated associations simply reflect
a PM10 association (i.e., the construct does not provide
additional information on the effect of PM10-2.5). Moreover,
roughly 75 percent of the cardiovascular mortality in this study
occurred in or near Palm Springs, CA and PM characteristics differ
significantly between Palm Springs and Indio (e.g., average
PM10 concentrations are roughly 30 percent lower in Palm
Springs and PM2.5 represents a higher fraction of
PM10, with a correlation coefficient between
PM2.5 and PM10-2.5 of 0.46 in Palm Springs).
Thus, the Ostro et al. (2003) study suggests a positive association
between PM10 monitored in Indio and mortality in Palm
Springs, but some view this study as offering little basis for
attributing significant mortality association to PM10-2.5 as
observed in either city.
The Criteria Document and Staff Paper also present and discuss
other epidemiology studies in support of the proposal for both the
PM2.5 and PM10-2.5 standards (as shown in Figure
2 and discussed in Section III.A above): Burnett (1997), Fairley
(2003), Ito (2003), Lipfert et al (2000), Mar et al (2003), Moolgavkar
(2000), Sheppard et al (2003), Thurston et al (1994), Burnett (2000,
2003), Klemm and Mason (2003), and Schwartz and Neas (2000). However,
these studies report positive, statistically significant associations
with PM2.5 that are more consistent and robust than the
associations thus far identified for PM10-2.5. Indeed,
several of these and other studies that specifically considered
PM10-2.5, but did not find statistically significant
associations, including Schwartz et al (1996), Thurston et al. (1994),
Sheppard et al. (2003), Fairley (2003), Schwartz et al (1996) and
Lipfert et al. (2000). With respect to mortality effects in the Six-
City study, Schwartz et al. (1996) concluded that the PM associations
(in the six metropolitan areas--including Steubenville) were
specifically associated with PM2.5, with little additional
contribution from the PM10-2.5. Sheppard et al. (2003) noted
that bias in model selection and reporting can result in inflated
excess risk estimates for PM. Fairley (1999) noted that
PM10-2.5 effects become negative and insignificant when
modeled jointly with PM2.5. Lipfert et al. (2000) showed
insignificant effects for PM10-2.5 in one- and two-pollutant
models with O3. The authors also caution against drawing
causal interpretations from results when comparing health effects from
one region in a metropolitan area to air quality observations in
another region. In addition, several of these studies also report
positive, statistically significant associations with one or more of
the gaseous pollutants. Both Thurston et al. (1994) and Burnett et al.
(1997) reported substantial confounding with gaseous co-pollutants in
Toronto, and Thurston et al. (1994, p. 282) reported that ``it seems
clear that these apparent associations were merely a statistical by-
product of interpollutant confounding resulting from the shared day-to-
day variations in dispersion conditions.'' In addition, Burnett et al.
(2000) concluded that gaseous pollutants played an important role in
explaining the effect of urban air pollution on health. Similarly,
Moolgavkar (2000) concludes that gases were more strongly associated
with respiratory effects than PM in Los Angeles.
Taken as a whole, evidence from PM10-2.5 epidemiologic
studies could be interpreted to suggest that one-pollutant
PM10-2.5 models suffer from bias due to omitting co-
pollutants in the statistical model, especially given the much stronger
evidence (discussed above) that these effects are associated with
exposure to PM2.5. As noted by many of the aforementioned
authors, while significant health associations may be noted for coarse
fraction PM in one-pollutant models, the actual association may be
insignificant from zero due to confounding co-pollutants. Of course,
the Administrator must conclude in the final rule that the evidence
about the health effects of PM10-2.5 is sufficiently robust
to finalize a standard for PM10-2.5.
The Administrator, recognizing notably large uncertainties in the
underlying evidence and information that formed the basis for this
proposal as well as the challenges associated with moving toward a new
PM10-2.5 indicator and a related new monitoring network,
solicits comment on this and other alternative interpretations of the
available health evidence and alternative policy responses. Several
such alternative interpretations and policy responses are discussed below.
(1) In light of the large uncertainties in the evidence and the
challenges of moving to a new indicator, and provisionally recognizing
the need for a standard to provide a requisite level of protection from
the risks associated with thoracic coarse particles, the Administrator
also believes it appropriate to consider a policy option that would
retain the current 24-hour PM10 standard (with a one-
expected-exceedance form), while addressing issues such as the
appropriateness of the indicator and the level of the standard.
As discussed in section I.D, in response to a challenge to the 1997
standards for thoracic coarse PM, the
[[Page 2673]]
U.S. Court of Appeals for the District of Columbia vacated the Agency's
1997 PM10 standards. In its decision the Court noted that
use of PM10 as an indicator to protect against the public
health risks associated with thoracic coarse particles resulted in
double regulation of PM2.5, since this size fraction is both
a component of PM10 and the subject of its own standard. The
Court further reasoned that, since PM2.5 concentrations vary
from area to area, use of PM10 as a thoracic coarse particle
indicator results in an arbitrary level of protection in public health
from the risks associated with thoracic coarse particles on a national
basis, as the level of protection would vary based on the concentration
of PM2.5 in an area. See American Trucking Associations v.
EPA, 175 F.3d at 1054-55.
Under this option to retain the 24-hour PM10 standard,
EPA would modify the standard to exclude the double-counted
PM2.5 contribution in circumstances where this could present
a concern. First, there will be some areas that may be in nonattainment
with the PM10 standard because, and only because, they are
in nonattainment with the PM2.5 standard. To remedy the
double counting in this situation, EPA is requesting comment on
subtracting from a daily measured PM10 concentration the
value by which the concentration of PM2.5 measured at a
collocated monitor is in excess of 35 [mu]g/m3 (i.e., the
proposed level for the 24-hour PM2.5 standard). This
adjustment would need to be made only on days when a 24-hour average
PM10 concentration is measured in excess of 150 [mu]g/
m3. In such a case, the amount by which the PM2.5
concentration exceeds 35 [mu]g/m3 would be subtracted from
the measured PM10 concentration. The EPA would then use this
adjusted value in any comparison to the PM10 standard.
The second situation where the overlap between the PM2.5
and PM10 standards may cause some concern is in areas where
a daily PM2.5 level is below 35 [mu]g/m3. In
those areas, the level of the PM10 standard would allow a
higher concentration of thoracic coarse particles before triggering an
exceedance than it would in other areas. The EPA is requesting comment
on not requiring any adjustment to the daily measured PM10
concentration in this situation, on the basis that any additional risk
to public health that may be associated with this higher allowable
concentration of thoracic coarse particles would reasonably be expected
to present less concern from a public health perspective than would the
otherwise allowable equivalent increase in the concentration of PM2.5.
The EPA also believes that it would be appropriate in this option
to focus the PM10 standard in a manner similar to that
proposed above for the PM10-2.5 standard. While the
indicator would remain specified as PM10, the focus would be
on including only the mix of ambient thoracic coarse particles that are
of concern to public health (and to exclude the mix for which
information is not sufficient to infer a public health concern) and
would be achieved in practice through the data handling requirements
associated with the standard, which are linked to the proposed
monitoring network design criteria (in the part 58 rule proposed
elsewhere in today's Federal Register).
The EPA invites comment on whether this option would provide the
requisite level of public health protection from risks associated with
thoracic coarse particles. Given the difference in form between the 24-
hour PM10 standard (one-expected-exceedance form) and the
proposed PM10-2.5 standard (98th percentile form), and the
adjustments noted above, in practice there may not be an appreciable
difference in the degree of public health protection afforded by this
option relative to that afforded by the proposed PM10-2.5
standard. The EPA invites comment on whether this approach addresses
one of the concerns about use of a PM10 indicator for
thoracic coarse particles noted by the Court in its ATA decision,
namely that the level of public health protection from thoracic coarse
particles in an area would vary depending on the relative proportions
of fine and thoracic coarse particles, by recognizing that the
PM10 indicator and standard would cover both fine and
thoracic coarse particles.
With respect to revocation of the 1987 24-hour PM10
standard, under this option EPA would apply the same approach to
revocation as that proposed below in section III.H. in conjunction with
the proposed PM10-2.5 standard. Since the 24-hour
PM10 standard would be focused in basically the same manner
as the proposed PM10-2.5 standard, it would be appropriate
to follow the same approach to revocation of the current 24-hour
PM10 standard under this option as well.
The EPA solicits comment on all aspects of this approach, including
views on whether a 24-hour PM10 standard revised as noted
above would be requisite to protect public health from the risks
associated with thoracic coarse particles, with an adequate margin of
safety, as well as views on any legal, scientific, or policy issues
associated with this alternative, and including comments on the
consistency of this option with CASAC's recommendations. The EPA also
solicits comment on whether a 98th percentile form should be considered
for a 24-hour PM10 standard and on the appropriate level of
such a standard.
(2) The Administrator recognizes that some commenters hold the view
that the uncertainties that exist at the present time are so great that
no standards for thoracic coarse particles are warranted. Some such
commenters point to conclusions reached in the Staff Paper in part as a
basis for their view, including, for example, the conclusion that the
``substantial uncertainties associated with this limited body of
epidemiological evidence on health effects related to
PM10-2.5 * * * suggests a high degree of caution in
interpreting this evidence * * *.'' (EPA 2005, pp. 5-50). This view
generally places significant weight on the issue of confounding between
PM2.5 and PM10-2.5 (discussed above in section
III.A), with some commenters stating that the correlation coefficients
between fine and thoracic coarse particle levels are modest to high for
all studies for which such data are available, increasing the
possibility that the positive association identified in the
PM10-2.5 one-pollutant models may instead reflect the
effects of fine particles. Noting that the Staff Paper puts little
weight on the health risk assessment because of the significant
uncertainties in the underlying health studies, some commenters suggest
that the risk assessment therefore does not provide a basis for
determining whether the health effects possibly associated with
PM10-2.5 constitute a meaningful public health risk. Some
commenters take the view that, based either on the studies or the risk
assessment, the magnitude of the health effects possibly associated
with PM10-2.5 do not constitute a meaningful risk to public
health. These commenters also maintain that significant uncertainty
remains as to an appropriate level of a standard, even assuming that a
meaningful public health risk exists. In consideration of these views,
the Administrator also solicits comment on revoking the current 24-hour
PM10 standard at this time (as well as the current annual
PM10 standard, as proposed above), not adopting a thoracic
coarse particle standard at this time, and taking into account any new
relevant research that becomes available as a basis for considering a
more targeted standard for thoracic coarse particles in the next
periodic review of the PM NAAQS.
[[Page 2674]]
(3) In sharp contrast to the views noted above, another view that
the Administrator takes note of would place greater weight on the
available epidemiologic evidence as a basis for selecting a level down
to 50 [mu]g/m3 or below and/or for selecting an unqualified
PM10-2.5 indicator. While recognizing that important
uncertainties are present in the available evidence, this view would
support incorporating a larger margin of safety consistent with a more
highly precautionary policy response. In soliciting comments on a wide
array of views, the Administrator solicits comment on this view and on
standard levels that are consistent with this view.
H. Proposed Decisions on Primary PM10-2.5 Standard
For the reasons discussed above, and taking into account the
information and assessments presented in the Criteria Document and
Staff Paper, the advice and recommendations of CASAC, and public
comments to date, the Administrator proposes to revise the current
primary PM10 standards. In particular, to provide more
targeted protection from thoracic coarse particles that are of concern
to public health, the Administrator proposes to establish a new
indicator for thoracic coarse particles in terms of
PM10-2.5, the definition of which includes qualifications
that identify both the mix of such particles that are of concern to
public health, and are thus included in the indicator, and those for
which currently available information is not sufficient to infer a
public health concern, and are thus excluded. More specifically, the
proposed PM10-2.5 indicator is qualified so as to include
any ambient mix of PM10-2.5 that is dominated by particles
generated by high-density traffic on paved roads, industrial sources,
and construction sources, and to exclude any ambient mix of particles
dominated by rural windblown dust and soils and agricultural and mining
sources. The Administrator proposes to replace the current primary 24-
hour PM10 standard with a 24-hour standard defined in terms
of this new PM10-2.5 indicator and set at a level of 70
[mu]g/m3, which would generally maintain the degree of
public health protection afforded by the current PM10
standards from short-term exposure to thoracic coarse particles of
concern. The proposed new standard would be met at an ambient air
quality monitoring site \75\ when the 3-year average of the annual 98th
percentile 24-hour average PM10-2.5 concentration is less
than or equal to 70 [mu]g/m3.\76\ The Administrator also
proposes to revoke and not replace the annual PM10 standard.
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\75\ Monitoring sites that are appropriate for determining
compliance with this standard are those that are consistent with the
proposed indicator. Guidance on this can be found in the proposed
monitoring network design criteria published elsewhere in today's
Federal Register.
\76\ Data handling conventions are specified in a new proposed
Appendix P, as discussed in Section V below, and the reference
method for monitoring PM as PM10-2.5 is specified in a
new proposed Appendix L, as discussed in Section VI below.
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In recognition of alternative views of the currently available
scientific information and the appropriate policy response to this
information, the Administrator also solicits comments on (1)
alternative approaches to selecting the level of a 24-hour
PM10-2.5 standard or to selecting an unqualified
PM10-2.5 indicator, and (2) alternative approaches to
providing continued protection from thoracic coarse particles based on
retaining the current 24-hour PM10 standard. Alternatively,
the Administrator also solicits comment on revoking and not replacing
the 24-hour PM10 standard. Based on the comments received
and the accompanying rationale, the Administrator may adopt other
standards within the range of the alternatives identified above in lieu
of the standard he is proposing today.
The Administrator is also proposing to revoke the current annual
PM10 standard upon promulgation of this rule. Further, if
EPA finalizes a 24-hour primary PM10-2.5 standard, the
Administrator is proposing to revoke the current 24-hour
PM10 standard everywhere except in areas where there is at
least one monitor that is located in an urbanized area \77\ with a
minimum population of 100,000 people and that violates the 24-hour
PM10 standard based on the most recent three years of data.
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\77\ As defined by the U.S. Bureau of the Census, an urbanized
area has ``a minimum residential population of at least 50,000
people'' and generally includes ``core census block groups or blocks
that have a population density of at least 1,000 people per square
mile and surrounding census blocks that have an overall density of
at least 500 people per square mile.'' The Census Bureau notes that
``under certain conditions, less densely settled territory may be
part of each UA.'' See http://www.census.gov/geo/www/ua/ua_2k.html.
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EPA specifically proposes that the 24-hour PM10 standard
would be revoked in this rulemaking in all areas except the following:
1. Birmingham urban area (Jefferson County, AL)
2. Maricopa and Pinal Counties; Phoenix planning area (AZ)
3. Riverside, Los Angeles, Orange and San Bernardino Counties; South
Coast Air Basin (CA)
4. Fresno, Kern, Kings, Tulare, San Joaquin, Stanislaus, Maderia
Counties; San Joaquin Valley planning area (CA)
5. San Bernardino County (part); excluding Searles Valley Planning Area
and South Coast Air Basin (CA)
6. Riverside County; Coachella Valley Planning Area (CA)
7. Simi Valley urban area (CA)
8. Lake County; Cities of East Chicago, Hammond, Whiting, and Gary (IN)
9. Wayne County (part) (MI)
10. St. Louis urban area (MO)
11. Albuquerque urban area (NM)
12. Clark County; Las Vegas planning area (NV)
13. Columbia urban area (SC)
14. El Paso urban area (including those portions in TX and those
portions in NM)
15. Salt Lake County (UT)
A separate memorandum explaining the factual basis for our proposed
determinations regarding each PM10 area where we are
proposing to retain the current 24-hour standard is part of the
administrative record for this proposed rule (Rosendahl, 2005).
In essence, we are proposing to retain the current 24-hour
PM10 standard only in areas which could be in violation of
the proposed PM10-2.5 standard. While it is possible that
some existing PM10 monitors may not be sited in accordance
with all of the criteria for PM10-2.5 monitor siting
proposed elsewhere in today's Federal Register (see section IV.E.2.b.ii
of the preamble to the proposed changes to Part 53/58), it is not
possible for EPA to make a case-by-case assessment of monitor placement
within each area at this time. Therefore, EPA believes that all areas
with violating PM10 monitors located in urbanized areas with
a minimum population of 100,000 people should be considered areas that
may violate the PM10-2.5 standard.
For those areas where we propose to retain the 24-hour
PM10 standard which were previously designated nonattainment
for PM10 or which are currently designated nonattainment for
PM10, EPA proposes, in the alternative, either that the
standard would continue to apply in the entire attainment/nonattainment
area, or that the area to which the standard would continue to apply
should be limited to the urbanized area containing the violating
monitor(s). For areas with violating monitor(s) which were never
designated nonattainment, EPA proposes that the boundaries of the area
to which the standard would continue to apply should be limited to the
urbanized area containing the violating monitor(s). For
[[Page 2675]]
all areas in which the 24-hour PM10 standard would be
retained, EPA invites comments on the appropriate boundaries within
which the standard should continue to apply.
Consistent with our request for comment in the Part 53/58 proposal,
section IV.E.2.b.ii, on whether we should establish criteria for
locating discretionary monitors appropriate for comparison with the
proposed 24-hour PM10-2.5 standard in locations other than
urbanized areas with population of at least 100,000 people, we also
request comment on whether the 24-hour PM10 standard should
be retained in areas that are either urbanized areas with a population
less than 100,000 people or non-urbanized areas (i.e. population less
than 50,000) but where the majority of the ambient mix of
PM10-2.5 is generated by high density traffic on paved
roads, industrial sources, and construction activities, and which have
at least one monitor that violates the 24-hour PM10
standard. The EPA requests comment on the criteria that should be used
to determine whether such an area with a violating monitor must retain
the 24-hour PM10 standard. Such criteria could include
whether the area has one (or more) industrial source(s) listed in
either the National Emissions Inventory or the Toxics Release Inventory
located within a certain radius of the violating monitor, and whether
these sources are in industrial categories that do not include
agricultural or mining sources. One approach to defining such
categories would be to utilize the U.S. Census Bureau's North American
Industry Classification System,\78\ which defines separate
classifications for agricultural and mining activities such as Crop
Production (111), Animal Production (112), and Mining (112). The EPA
requests comments on how this or another classification system,
combined with information on the location of sources relative to the
violating PM10 monitor, could be used to identify additional
areas to which the 24-hour PM10 standard should continue to
apply due to the presence of industrial sources. The EPA also requests
comments on which areas would meet these criteria or other criteria
that may be appropriate to determine in which, if any, areas the 24-
hour PM10 standard should be retained, and the appropriate
boundaries within which the standard should continue to apply for these
areas. A more detailed example of criteria that could be used to
identify areas to which the standard should continue to apply, along
with a list of all areas with violating PM10 monitors that
meet these criteria, are part of the administrative record for this
proposed rule (Rosendahl, 2005). For all areas where the 24-hour
PM10 standard would be retained under this proposal, we
contemplate that the 24-hour PM10 standard would be revoked
after designations are completed under a final 24-hour PM10-2.5 standard.
---------------------------------------------------------------------------
\78\ http://www.census.gov/epcd/naics02/naicod02.htm#N21.
---------------------------------------------------------------------------
The EPA also recognizes that it is possible that some areas for
which we are proposing to retain the PM10 daily standard
would, upon a case-specific investigation (see section IV.E.2.c of the
Part 53/58 preamble), warrant revocation as not being an area where the
ambient coarse PM mix is dominated by the type of coarse PM described
by the proposed indicator. The EPA is not in a position to conduct such
case-by-case evaluation for this proposal, but could address revocation
in such situations in a future rulemaking. The EPA invites comment on
this issue.
To address issues related to the transition from the current
PM10 standards to a new PM10-2.5 standard, the
Administrator intends to seek public comment on EPA's plans for
assuring an effective transition as part of an ANPR that EPA intends to
issue by the end of January 2006. In the forthcoming ANPR dealing with
transition issues, EPA intends to address, among other things, the
timing for revocation of the PM10 standard in areas in which
we are proposing to retain that standard, and the consequences of
revoking the PM10 standards on the PM10 PSD
program (including PM10 increments), on the PM10
nonattainment New Source Review (NSR) program, and on our existing
policy of using PM10 as a surrogate for the PM2.5
NSR program.
IV. Rationale for Proposed Decisions on Secondary PM Standards
The Criteria Document and Staff Paper examined the effects of PM on
such aspects of public welfare as visibility, vegetation and
ecosystems, materials damage and soiling, and climate change. The
existing suite of secondary PM standards, which is identical to the
suite of primary PM standards, includes annual and 24-hour
PM2.5 standards and annual and 24-hour PM10
standards. This existing suite of secondary standards is intended to
address visibility impairment associated with fine particles and
materials damage and soiling related to both fine and coarse particles.
The following discussion of the rationale for the proposed decisions on
secondary PM standards focuses on those considerations most influential
in the Administrator's proposed decisions, first addressing visibility
impairment followed by the other welfare effects considered in this
review.\79\
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\79\ As noted in section I.A above, in establishing secondary
standards that are requisite to protect the public welfare from any
known or anticipated adverse effects, EPA may not consider the costs
of implementing the standards.
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A. Visibility Impairment
This section presents the rationale for the Administrator's
proposed revision of the current secondary PM2.5 standard to
address PM-related visibility impairment. As discussed below, the
rationale includes consideration of: (1) The latest scientific
information on visibility effects associated with PM; (2) insights
gained from assessments of correlations between ambient
PM2.5 and visibility impairment prepared by EPA staff; and
(3) specific conclusions regarding the need for revisions to the
current standards (i.e., indicator, averaging time, form, and level)
that, taken together, would be requisite to protect the public welfare
from adverse effects on visual air quality.
1. Visibility Impairment Related to Ambient PM
This section outlines key information contained in the Criteria
Document and Staff Paper on: (1) The nature of visibility impairment,
including trends in visual air quality and the characterization of
current visibility conditions; (2) quantitative relationships between
ambient PM and visibility; (3) the impacts of visibility impairment on
public welfare; and (4) approaches to evaluating public perceptions and
attitudes about visibility impairment.
a. Nature of Visibility Impairment
Visibility can be defined as the degree to which the atmosphere is
transparent to visible light. Visibility conditions are determined by
the scattering and absorption of light by particles and gases, from
both natural and anthropogenic sources. Visibility is often described
in terms of visual range, light extinction, or deciviews.\80\ The
classes of fine particles principally responsible for visibility
impairment are sulfates, nitrates, organic matter, elemental carbon,
and soil dust. Fine
[[Page 2676]]
particles are more efficient per unit mass at scattering light than
coarse particles. The scattering efficiency of certain classes of fine
particles, such as sulfates, nitrates, and some organics, increases as
relative humidity rises because these particles can absorb water and
grow to sizes comparable to the wavelength of visible light. In
addition to limiting the distance that one can see, the scattering and
absorption of light caused by air pollution can also degrade the color,
clarity, and contrast of scenes.
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\80\ Visual range can be defined as the maximum distance at
which one can identify a black object against the horizon sky. It is
typically described in kilometers or miles. Light extinction is the
sum of light scattering and absorption by particles and gases in the
atmosphere. It is typically expressed in terms of inverse megameters
(Mm-1), with larger values representing poorer
visibility. The deciview metric describes perceived visual changes
in a linear fashion over its entire range, analogous to the decibel
scale for sound.
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Visibility impairment is manifested in two principal ways: As local
visibility impairment and as regional haze. Local visibility impairment
may take the form of a localized plume, a band or layer of
discoloration appearing well above the terrain that results from
complex local meteorological conditions. Alternatively, local
visibility impairment may manifest as an urban haze, sometimes referred
to as a ``brown cloud.'' A ``brown cloud'' is predominantly caused by
emissions from multiple sources in the urban area and is not typically
attributable to a single nearby source or to long-range transport from
more distant sources. The second type of visibility impairment,
regional haze, generally results from pollutant emissions from a
multitude of sources located across a broad geographic region. Regional
haze impairs visibility in every direction over a large area, in some
cases over multi-state regions. It is regional haze that is principally
responsible for impairment in national parks and wilderness areas
across the country (NRC, 1993).
While visibility impairment in urban areas at times may be
dominated by local sources, it often may be significantly affected by
long-range transport of haze due to the multi-day residence times of
fine particles in the atmosphere. Fine particles transported from urban
and industrialized areas, in turn, may, in some cases, be significant
contributors to regional-scale impairment in Class I areas \81\ and
other rural areas.
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\81\ There are 156 mandatory Class I Federal areas protected by
the visibility provisions in sections 169A and 169B of the Act.
These areas are defined in section 163 of the Act as those national
parks exceeding 6000 acres, wilderness areas and memorial parks
exceeding 5000 acres, and all international parks which were in
existence on August 7, 1977.
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As discussed in the Staff Paper (EPA, 2004, section 6.2), in Class
I areas, visibility levels on the 20 percent haziest days in the West
are about equal to levels on the 20 percent best days in the East.
Despite improvement through the 1990's, visibility in the rural East
remains significantly impaired, with an average visual range of
approximately 20 km on the 20 percent haziest days (compared to the
naturally occurring visual range in the eastern U.S. of about 150
± 45 km). In the rural West, the average visual range showed
little change over this period, with an average visual range of
approximately 100 km on the 20 percent haziest days (compared to the
naturally occurring visual range in the western U.S. of about 230
± 40 km).
In urban areas, visibility levels show far less difference between
eastern and western regions. For example, the average visual ranges on
the 20 percent haziest days in eastern and western urban areas are
approximately 20 km and 27 km, respectively (Schmidt et al., 2005).
Even more similarity is seen in considering 4-hour (12 to 4 p.m.)
average PM2.5 concentrations, for which the average visual
ranges on the 20 percent haziest days in eastern and western urban
areas are approximately 26 km and 31 km, respectively (Schmidt et al.,
2005).
Data on visibility conditions indicate that urban areas generally
have higher loadings of PM2.5 and, thus, higher visibility
impairment than monitored Class I areas. Since efforts are now underway
to address all human-caused visibility in Class I areas through the
regional haze program (EPA, 1999; 65 FR 35713), implemented under
sections 169A and 169B of the CAA, and since the Clean Air Interstate
Rule (CAIR) (70 FR 25162) is expected to result in improvements to
visual air quality, particularly in eastern Class I and non-urban
areas, new assessments included in the Staff Paper were primarily
focused on visibility impairment in urban areas.
b. Correlations Between Urban Visibility and PM2.5 Mass
Direct relationships exist between measured ambient pollutant
concentrations and their contributions to light extinction and thus to
visibility impairment. The contribution of each PM constituent to total
light extinction is derived by multiplying the constituent
concentration by its extinction efficiency to calculate a
``reconstructed'' light extinction.\82\ For certain fine particle
constituents, extinction efficiencies increase significantly with
increases in relative humidity. As a consequence, while higher
PM2.5 mass concentrations generally indicate higher levels
of visibility impairment, it is not as precise a metric as the light
extinction coefficient. Nonetheless, by using historic averages,
regional estimates, or actual day-specific measurements of the
component-specific percentage of total mass, one can develop reasonable
estimates of light extinction from PM mass concentrations. As discussed
below, the Staff Paper concludes that fine particle mass concentrations
can be used as a general surrogate for visibility impairment (EPA,
2005a, p. 2-74).
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\82\ Extinction efficiencies vary by type of constituent and
have been obtained for typical atmospheric aerosols by a combination
of empirical approaches and theoretical calculations. As discussed
in the Staff Paper, EPA's guidance for tracking progress under the
regional haze program specifies an algorithm for calculating total
light extinction as a function of the major fine particle components
(EPA, 2005a, section 2.8.1). ``Reconstructed'' light extinction
simply refers to the calculation of PM-related light extinction by
the use of that formula.
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In an effort to better characterize urban visibility, the Staff
Paper presents results of analyses of the extensive new data now
available on PM2.5 primarily in urban areas. This rapidly
expanding national database includes federal reference method (FRM)
\83\ measurements of PM2.5 mass, continuous measurements of
hourly PM2.5 mass, and PM2.5 chemical speciation
measurements. These data allowed for analyses that explored factors
that have historically complicated efforts to address visibility
impairment nationally, including regional differences related to levels
of primarily fine particles and to relative humidity. These analyses
show a consistently high correlation between visibility, in terms of
reconstructed light extinction, and hourly PM2.5
concentrations for urban areas in a number of regions across the U.S.
and, more generally, in the eastern and western U.S. These correlations
in urban areas are generally similar in the East and West, in sharp
contrast to the East/West differences observed in rural areas.
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\83\ The PM2.5 Federal Reference Method (FRM)
monitoring network provides 24-hour average PM2.5
concentrations.
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While the average daily relative humidity levels are generally
higher in the East than in the West, in both regions relative humidity
levels are appreciably lower during daylight as compared to night time
hours. The reconstructed light extinction coefficient, for a given mass
and concentration, increases sharply as relative humidity rises. Thus,
with lower relative humidity levels, visibility impacts related to
East/West differences in average relative humidity are minimized during
daylight hours, when relative humidity is generally lower.
Both 24-hour and shorter-term daylight hour averaging periods were
[[Page 2677]]
considered in evaluations of correlations between PM2.5
concentrations in urban areas and visibility in eastern and western
areas, as well as nationwide. Clear and similarly strong correlations
are found between visibility and 24-hour average PM2.5 in
eastern, western, and all urban areas (EPA, 2005a, Figure 6-3).
Somewhat stronger correlations are observed between visibility and
PM2.5 concentrations averaged over a 4-hour time period
(EPA, 2005a, Figure 6-5). The correlations between visibility and
PM2.5 concentrations during daylight hours in urban areas
are relatively more reflective of PM2.5 mass rather than
relative humidity effects, in comparison to correlations based on a 24-
hour averaging time.
c. Impacts of Urban Visibility Impairment on Public Welfare
EPA has long recognized that impairment of visibility is an
important effect of PM on public welfare, and that it is experienced
throughout the U.S. in urban areas as well as in remote Class I areas
(62 FR 38680). Visibility is an important welfare effect because it has
direct significance to people's enjoyment of daily activities in all
parts of the country. Individuals value good visibility for the sense
of well-being it provides them directly, both in places where they live
and work, and in places where they enjoy recreational opportunities.
Survey research on public awareness of visual air quality using
direct questioning typically reveals that 80 percent or more of the
respondents are aware of poor visual air quality (Cohen et al., 1986).
The importance of visual air quality to public welfare across the
country has been demonstrated by a number of studies designed to
quantify the benefits (or willingness to pay) associated with potential
improvements in visibility (Chestnut and Dennis, 1997; Chestnut and
Rowe, 1991). Economists have performed many studies in an attempt to
quantify the economic benefits associated with improvements in current
visibility conditions both in national parks and in urban areas
(Chestnut and Dennis, 1997). These economic benefits may include the
value of improved aesthetics during daily activities (e.g., driving or
walking, daily recreations), for special activities (e.g., visiting
parks and scenic vistas, hiking, hunting), and for viewing scenic
photography. They may also include the value of improved road and air
safety, and/or preservation of the resource for its own sake. As
discussed in the Staff Paper and below, the value placed on protecting
visual air quality is further demonstrated by the existence of a number
of programs, goals, standards, and planning efforts that have been
established in the U.S. and abroad to address visibility concerns in
urban and non-urban areas.
Protection against visibility impairment in special areas is
provided for in sections 169A, 169B, and 165 of the CAA, in addition to
that provided by the secondary NAAQS. Section 169A, added by the 1977
CAA Amendments, established a national visibility goal to ``remedy
existing impairment and prevent future impairment'' in 156 national
parks and wilderness areas (Class I areas). The Amendments also called
for EPA to issue regulations requiring States to develop long-term
strategies to make ``reasonable progress'' toward the national goal.
EPA issued initial regulations in 1980 focusing on visibility problems
that could be linked to a single source or small group of sources. The
1990 CAA Amendments placed additional emphasis on regional haze issues
through the addition of section 169B. In accordance with this section,
EPA established the Grand Canyon Visibility Transport Commission
(GCVTC) in 1991 to address adverse visibility impacts on 16 Class I
national parks and wilderness areas on the Colorado Plateau. The GCVTC
issued its recommendations to EPA in 1996, triggering a requirement in
section 169B for EPA issuance of regional haze regulations.
EPA accordingly promulgated a final regional haze rule in 1999
(U.S. EPA, 1999; 65 FR 35713). Under the regional haze program, States
are required to establish goals for improving visibility on the 20
percent most impaired days in each Class I area, and for allowing no
degradation on the 20 percent least impaired days. Each state must also
adopt emission reduction strategies which, in combination with the
strategies of contributing States, assure that Class I area visibility
improvement goals are met. The first State implementation plans are to
be adopted in the 2003-2008 time period, with the first implementation
period extending until 2018. Five multi-state planning organizations
are evaluating the sources of PM2.5 contributing to Class I
area visibility impairment to lay the technical foundation for
developing strategies, coordinated among many States, in order to make
reasonable progress in Class I areas across the country.
A number of other programs, goals, standards, and planning efforts
have also been established in the U.S. and abroad to address visibility
concerns in urban and non-urban areas. These regulatory and planning
activities are of interest because they are illustrative of the
significant value that the public places on improving visibility, and
because they have developed and applied methods for evaluating public
perceptions and judgments about the acceptability of varying degrees of
visibility impairment, as discussed below in the next section.
Several state and local governments have developed programs to
improve visual air quality in specific urban areas, including Denver,
CO; Phoenix, AZ; and, Lake Tahoe, CA. At least two States have
established statewide standards to protect visibility. In addition,
interest in visibility protection in other countries, including Canada,
Australia, and New Zealand has resulted in various studies, surveys,
and programs. Examples of these efforts are highlighted below.
In 1990, the State of Colorado adopted a visibility standard for
the city of Denver. The Denver standard is a short-term standard that
establishes a limit of a four-hour average light extinction level of 76
Mm-1 (equivalent to a visual range of approximately 50 km)
during the hours between 8 a.m. and 4 p.m. (Ely et al., 1991). In 2003,
the Arizona Department of Environmental Quality created the Phoenix
Region Visibility Index, which focuses on an averaging time of 4 hours
during actual daylight hours. This visibility index establishes visual
air quality categories (i.e., excellent to very poor) and establishes
the goals of moving days in the poor/very poor categories up to the
fair category, and moving days in the fair category up to the good/
excellent categories (Arizona Department of Environmental Quality,
2003). This approach results in a focus on improving visibility to a
visual range of approximately 48-36 km. In 1989, the state of
California revised the visibility standard for the Lake Tahoe Air Basin
and established an 8-hour visibility standard equal to a visual range
of 30 miles (approximately 48 km) (California Code of Regulations).
California and Vermont each have standards to protect visibility,
though they are based on different measures. Since 1959, the state of
California has had an air quality standard for particle pollution where
the ``adverse'' level was defined as the ``level at which there will be
* * * reduction in visibility or similar effects.'' California's
general statewide visibility standard is a visual range of 10 miles
(approximately 16 km) (California Code of Regulations). In 1985,
Vermont established a state visibility standard that is expressed as a
summer seasonal sulfate concentration of 2 [mu]g/m\3\, that equates to
a visual range
[[Page 2678]]
of approximately 50 km. This standard was established to represent
``reasonable progress'' toward attaining the congressional visibility
goal for the Class 1 Lye Brook National Wilderness Area, and applies to
this Class 1 area and to all other areas of the state with elevations
greater than 2500 ft.
Outside of the U.S., efforts have also been made to protect
visibility. The Australian state of Victoria has established a
visibility objective (State Government of Victoria, 1999 and 2000), and
a visibility guideline is under consideration in New Zealand (New
Zealand National Institute of Water & Atmospheric Research, 2000a and
2000b; New Zealand Ministry of Environment, 2000). A survey was
undertaken for the Lower Fraser Valley in British Columbia, with
responses from this pilot study being supportive of a standard in terms
of a visual range of approximately 40 km for the suburban township of
Chilliwack and 60 km for the suburban township of Abbotsford, although
no visibility standard has been adopted for the Lower Fraser Valley at
this time.
d. Approaches to Evaluating Public Perceptions and Attitudes
New methods and tools have been developed to communicate and
evaluate public perceptions of varying visual effects associated with
alternative levels of visibility impairment relative to varying
pollution levels and environmental conditions. New survey methods have
been applied and evaluated in various studies, such as those done in
Denver, Phoenix, and the Lower Fraser Valley in British Columbia. These
methods are intended to assess public perceptions as to the
acceptability of varying levels of visual air quality, considered in
these studies to be an appropriate basis for developing goals and
standards for visibility protection. A pilot study was also conducted
in Washington, DC by EPA staff.\84\ Even with variations in each
study's approaches, the public perception survey methods used for the
Denver, Phoenix, and British Columbia studies produced reasonably
consistent results from location to location, with each study
indicating that a majority of participants find visual ranges within
about 40 to 60 km to be acceptable.
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\84\ This small pilot study was briefly discussed in the
preliminary draft staff paper (Abt Associates, 2001).
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These public perception studies use images of urban and distant
scenic views under different visibility conditions together with survey
techniques designed to elicit judgments from members of the public
about the acceptability of differing levels of visual air quality.
Images used are either photographs or computer simulations using the
WinHaze program.\85\ Examples of images that illustrate visual air
quality in Denver, Phoenix, Washington, DC, and Chicago under a range
of visibility conditions associated with a range of PM2.5
concentrations are available at http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_cr_sp.html
(labeled as Appendix 6A: Images of Visual Air
Quality in Selected Urban Areas in the U.S.). These examples include
simulated images for Denver, Phoenix, and Washington, DC, and
photographs of Chicago.
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\85\ The Criteria Document discusses methods available to
represent different levels of visual air quality (EPA, 2004, p. 4-
174). In particular, Molenar et al. (1994) describe a sophisticated
visual air quality simulation technique, incorporated into the
WinHaze program developed by Air Resources Specialists, Inc., which
combined various modeling systems under development for the past 20
years to produce images that standardize non-pollution related
effects on visibility so that perceptions of these images are not
biased due to these other factors.
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Survey techniques were developed in conjunction with the Denver
study and relied on citizen judgments of acceptable and unacceptable
levels of visual air quality (Ely et al., 1991; EPA, 2005a, section
6.2.6.2). The studies in Phoenix and British Columbia, and the pilot
study in Washington, DC used survey approaches based on that used in
Denver. This approach involves conducting a series of meetings with
civic and community groups to elicit individual ratings of a number of
images of well-known local vistas having varying levels of visual air
quality. Participants are told that the results are intended to provide
input on setting a visibility standard, and they are asked to base
their judgments on three factors: (1) The standard is for an urban
area, not a pristine national park area where the standards might be
more strict; (2) standard violations should be at visual air quality
levels considered to be unreasonable, objectionable, and unacceptable
visually; and (3) judgments of standard violations should be based on
visual air quality only, not on any health effects that some may
perceive as being linked with poor visual air quality. The Denver
visibility survey process produced the following findings: (1)
Individuals' judgments of an images's visual air quality and whether
the image should be considered to violate a visibility standard are
highly correlated with the group average; (2) when participants judged
duplicate slides, group averages of the first and second ratings were
highly correlated; and (3) group averages of visual air quality ratings
and ``standard violations'' were highly correlated. The strong
relationship of standard violation judgments with the visual air
quality ratings is cited as the best evidence available from this study
for the validity of this approach as input to a standard setting
process (Ely et al., 1991).
The Denver visibility standard was established based on a 50
percent acceptability criterion. That is, under this approach, the
standard was identified as the light extinction level that divides the
images into two groups: those found to be acceptable and those found to
be unacceptable by a majority of study participants. In fact, when
researchers evaluated all citizen judgments made on all the
photographic images at this level and above as a single group, more
than 85 percent of the participants found visibility impairment at and
above the level of the selected standard to be unacceptable.
Generally consistent results were found in the Phoenix study, which
used simulated images from the WinHaze program. The study carefully
selected participants to be demographically representative of the
Phoenix population. The Phoenix survey demonstrates that the rating
methodology developed for gathering citizen input for establishing the
Denver visibility standard can be reliably transferred to another city
while relying on updated imaging technology to simulate a range of
visibility impairment levels. Similarly, the British Columbia study
reinforces the conclusion that the methodology originally developed for
the Denver standard setting process is a sound and effective one for
obtaining public participation in a standard setting process (EPA,
2005a, p. 6-22).
2. Need for Revision of the Current Secondary PM Standards for
Visibility Protection
The initial issue to be addressed in the current review of the
secondary PM standards is whether, in view of the information now
available, the existing secondary standards should be revised to
provide requisite protection from PM-related adverse effects on visual
air quality. As discussed in the Criteria Document and Staff Paper,
while new research has led to improved understanding of the optical
properties of particles and the effects of relative humidity on those
properties, it has not changed the fundamental characterization of the
role of PM, especially fine particles, in visibility impairment from
the last review. However, extensive new information
[[Page 2679]]
now available from visibility and fine particle monitoring networks has
allowed for updated characterizations of visibility trends and current
levels in urban areas, as well as Class I areas. As discussed above,
these new data are a critical component of analyses that better
characterize visibility impairment in urban areas and the relationships
between visibility and PM2.5 concentrations, finding that
PM2.5 concentrations can be used as a general surrogate for
visibility impairment in urban areas.
Taking into account the most recent monitoring information and
analyses, and recognizing that efforts are now underway to address all
human-caused visibility impairment in Class I areas through the
regional haze program implemented under sections 169A and 169B of the
CAA, as discussed above, this review focuses on visibility impairment
primarily in urban areas. In so doing, consideration is first given to
the question of whether visibility impairment in urban areas allowed by
the current 24-hour secondary PM2.5 standard can be
considered adverse to public welfare.
As discussed above, studies in the U.S. and abroad have provided
the basis for the establishment of standards and programs to address
specific visibility concerns in a number of local areas. These studies
(e.g., in Denver, Phoenix, British Columbia) have produced reasonably
consistent results in terms of the visual ranges found to be generally
acceptable by the participants in the various studies, which ranged
from approximately 40 to 60 km in visual range. Standards targeting
protection within this range have also been set by the State of Vermont
and by California for the Lake Tahoe area, in contrast to the statewide
California standard that targets a visual range of approximately 16 km.
In addition to the information available from such programs,
photographic representations (simulated images and actual photographs)
of visibility impairment are available, as discussed above, to help
inform judgments about the acceptability of varying levels of visual
air quality in urban areas across the U.S. In considering these images
for Phoenix, Washington, DC, and Chicago (for which PM2.5
concentrations are reported), the Staff Paper observes that:
(1) At concentrations at or near the level of the current 24-hour
PM2.5 standard (65 [mu]g/m3), which equates to
visual ranges roughly around 10 km, scenic views (e.g., mountains,
historic monuments), as depicted in these images around and within the
urban areas, are significantly obscured from view.
(2) Appreciable improvement in the visual clarity of the scenic
views depicted in these images occurs at PM2.5
concentrations below 35 to 40 [mu]g/m3, which equate to
visual ranges generally above 20 km for the urban areas considered
(EPA, 2005a, p. 7-6).
(3) Visual air quality appears to be good in these images at
PM2.5 concentrations generally below 20 [mu]g/m3,
corresponding to visual ranges of approximately 25 to 35 km (EPA,
2005a, p. 7-8).
While being mindful of the limitations in using visual
representations from a small number of areas as a basis for considering
national visibility-based secondary standards, the Staff Paper
nonetheless concludes that these observations, together with
information from the analyses and other programs discussed above,
support revising the current secondary PM2.5 standards to
improve visual air quality, particularly in urban areas. As discussed
in the following sections, the Staff Paper recommends the establishment
of a new short-term secondary PM2.5 standard to provide
increased and more targeted protection primarily in urban areas from
visibility impairment related to fine particles (EPA, 2005a, p. 7-12).
Based on its review of the Staff Paper, the CASAC advised the
Administrator that most CASAC PM Panel members strongly supported the
Staff Paper recommendation to establish a new, secondary
PM2.5 standard to protect urban visibility (Henderson,
2005a).\86\ Most Panel members considered such a standard to be a
reasonable complement to the Regional Haze Rules that protect Class I areas.
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\86\ A dissenting view was expressed in one Panel member's
invididual review comments to the effect that any urban visibility
standard should be voluntary and locally adopted (Henderson, 2005a).
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In considering whether the secondary PM standards should be revised
to target PM-related visibility impairment primarily in urban areas,
the Administrator has carefully considered the rationale and
recommendation in the Staff Paper, the advice and recommendations from
CASAC, and public comments to date on this issue. In so doing, the
Administrator first recognizes that PM-related visibility impairment is
principally related to fine particle levels, such that it is
appropriate to focus in this review on the current secondary
PM2.5 standards to provide such targeted protection. The
Administrator also recognizes that visibility is most directly related
to instantaneous levels of visual air quality, such that it is
appropriate to focus on a standard with a short-term averaging time
(e.g., 24-hours or less). Thus, the Administrator has considered
whether the current 24-hour secondary PM2.5 standard should
be revised to provide a requisite level of protection from visibility
impairment, principally in urban areas, in conjunction with the
regional haze program for protection of visual air quality in Class I
areas. The Administrator observes that at concentrations at or near the
level of the current 24-hour PM2.5 standard (65 [mu]g/
m3), corresponding to visual ranges of about 10 km, images
of scenic views (e.g., mountains, historic monuments, urban skylines)
around and within a number of urban areas are significantly obscured
from view. Further, the Administrator notes the various State and local
standards and programs that have been established protect visual air
quality beyond the degree of protection that would be afforded by the
current 24-hour secondary PM2.5 standard. Based on all of
the above considerations, the Administrator provisionally concludes
that it is appropriate to revise the current 24-hour secondary
PM2.5 standard to provide requisite protection from
visibility impairment principally in urban areas.
3. Indicator of PM for Secondary Standard To Address Visibility Impairment
As discussed in the Staff Paper, fine particles contribute to
visibility impairment directly in proportion to their concentration in
the ambient air. Hygroscopic components of fine particles, in
particular sulfates and nitrates, contribute disproportionately to
visibility impairment under high humidity conditions. Particles in the
coarse mode generally contribute only marginally to visibility
impairment in urban areas. In analyzing how well PM2.5
concentrations correlate with visibility in urban locations across the
U.S. (see EPA, 2005a, section 6.2.3), the Staff Paper concludes that
the observed correlations are strong enough to support the use of
PM2.5 as the indicator for such standards. More
specifically, clear correlations exist between 24-hour average
PM2.5 concentrations and reconstructed light extinction,
which is directly related to visual range. These correlations are
similar in the eastern and western regions of the U.S.. Further, these
correlations are less influenced by relative humidity and more
consistent across regions when PM2.5 concentrations are
averaged over shorter, daylight time periods (e.g., 4 to
[[Page 2680]]
8 hours). Thus, the Staff Paper concludes that it is appropriate to use
PM2.5 as an indicator for standards to address visibility
impairment in urban areas, especially when the indicator is defined for
a relatively short period of daylight hours. Based on its review of the
Staff Paper, most CASAC PM Panel members endorsed a PM2.5
indicator for a secondary standard to address visibility impairment.
The Administrator concurs with the EPA staff and CASAC
recommendations, and concludes that PM2.5 should be retained
as the indicator for fine particles as part of a secondary standard to
address visibility protection. In the Administrator's view,
PM2.5 is the appropriate indicator for any such standard,
whether averaged over 24-hours or over a shorter, sub-daily time period.
4. Averaging Time of a Secondary PM2.5 Standard for
Visibility Protection
As discussed in the Staff Paper, averaging times from 24 to 4 hours
have been considered for a standard to address visibility impairment.
Within this range, as noted above, clear and similarly strong
correlations are found between visibility and 24-hour average
PM2.5 concentrations in eastern and western areas, while
somewhat stronger correlations are found with PM2.5
concentrations averaged over a 4-hour time period. In general,
correlations between PM2.5 concentrations and light
extinction are generally less influenced by relative humidity and more
consistent across regions as shorter, sub-daily averaging times, within
daylight hours from approximately 10 a.m. to 6 p.m., are considered.
The Staff Paper concludes that an averaging time from 4 to 8 hours,
generally within this daylight time period, should be considered for a
standard to address visibility impairment.
In reaching this conclusion, the Staff Paper recognizes that the
PM2.5 Federal Reference Method (FRM) monitoring network
provides 24-hour average concentrations, and, in some cases, on a
third- or sixth-day sample schedule, such that implementing a standard
with a less-than-24-hour averaging time would necessitate the use of
continuous monitors that can provide hourly time resolution. Given that
the data used in the analysis discussed above are from commercially
available PM2.5 continuous monitors, such monitors clearly
could provide the hourly data that would be needed for comparison with
a potential visibility standard with a less-than-24-hour averaging
time.\87\
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\87\ Decisions as to which PM2.5 continuous monitors
are providing data of sufficient quality to be used in a sub-daily
visibility standard would follow protocols for approval of Federal
equivalent methods (FEMs) that can provide data in at least hourly
intervals, as proposed in the revisions to Part 53, published
elsewhere in today's Federal Register.
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Most CASAC PM Panel members supported the Staff Paper
recommendation of a sub-daily (4 to 8 daylight hours) averaging time,
finding it to be an innovative approach that strengthens the quality of
the PM2.5 indicator by targeting the driest part of the day
(Henderson, 2005a). In its advice to the Administrator, CASAC noted an
indirect but important benefit to advancing EPA's monitoring program
goals that would come from the direct use of hourly data from a network
of continuous PM2.5 mass monitors.
In considering the Staff Paper recommendation and CASAC's advice,
the Administrator provisionally concludes that averaging times from 24
hours to 4 daylight hours would represent a reasonable range of choices
for a standard to address urban visibility impairment. A 24-hour
averaging time could be selected and applied based on the extensive
data base currently available from the existing PM2.5 FRM
monitoring network, whereas a sub-daily averaging time would
necessarily depend upon an expanded network of continuous
PM2.5 mass monitors. While the Administrator agrees that
broader deployment of continuous PM2.5 mass monitors is a
desirable goal, working toward that goal does not depend upon nor
provide a basis for setting a sub-daily standard. The Administrator
believes that it is appropriate to evaluate averaging time in
conjunction with reaching decisions on the form and level of a
standard, as discussed below.
5. Elements of a Secondary PM2.5 Standard for Visibility
Protection
In considering PM2.5 standards that would provide
requisite protection against PM-related impairment of visibility
primarily in urban areas, the Administrator has taken into account the
results of public perception and attitude surveys in the U.S. and
Canada, State and local visibility standards within the U.S., and
visual inspection of photographic representations of several urban
areas across the U.S. In the Administrator's judgment, these sources
provide useful but still quite limited information on the range of
levels appropriate for consideration in setting a national visibility
standard primarily for urban areas, given the generally subjective
nature of the public welfare effect involved. In considering
alternative forms for such standards, the Administrator has also taken
into account the same general factors that were considered in selecting
an appropriate form for the 24-hour primary PM2.5 standard,
as well as additional information on the percent of areas not likely to
meet various alternative PM2.5 standards, consistent with
CASAC advice to consider such information (Henderson, 2005a).
In considering elements of a secondary PM2.5 standard,
the Administrator has looked to the rationale presented in the Staff
Paper and to CASAC's advice and recommendations for such a standard.
Based on photographic representations of varying levels of visual air
quality, public perception studies, and local and State visibility
standards, as discussed above, the Staff Paper concludes that 30 to 20
[mu]g/m3 PM2.5 represents a reasonable range for
a national visibility standard primarily for urban areas, based on a
sub-daily averaging time. The upper end of this range is below the
levels at which the illustrative scenic views are significantly
obscured, and the lower end is around the level at which visual air
quality generally appears to be good based on observation of the
illustrative views. Analyses of 4-hour average PM2.5
concentrations indicate that this concentration range can be expected
generally to correspond to median visual ranges in urban areas within
regions across the U.S. of approximately 25 to 35 km (see EPA, 2005a,
Figure 7-1).\88\ This range of visual range values is bounded above by
the visual range targets selected in specific areas where State or
local agencies placed particular emphasis on protecting visual air quality.
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\88\ The Staff Paper notes that a standard set at any specific
PM2.5 concentration will necessarily result in visual
ranges that vary somewhat in urban areas across the country,
reflecting the variability in the correlations between
PM2.5 concentrations and light extinction (EPA, 2005a, p. 7-8).
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In considering a reasonable range of forms for a PM2.5
standard within this range of levels, the Staff Paper concludes that a
concentration-based percentile form is appropriate for the same reasons
as discussed above in section II.F.1 (on the form of the 24-hour
primary PM2.5 standard). The Staff Paper also concludes that
the upper end of the range of concentration percentiles should be
consistent with the percentile used for the primary standard, which is
proposed to be the 98th percentile, and that the lower end of the range
should be the 92nd percentile, which represents the mean of the distribution
[[Page 2681]]
of the 20 percent worst day, as targeted in the regional haze program
(EPA, 2005a, p. 7-11 to 12).
In its letter to the Administrator (Henderson, 2005a), the CASAC PM
Panel recognizes that it is difficult to select any specific level and
form based on currently available information. Some Panel members felt
that the range of levels recommended in the Staff Paper was on the high
side, but recognized that developing a more specific (and more
protective) level in future reviews would require updated and refined
public visibility valuation studies, which CASAC strongly encouraged
the Agency to support prior to the next review. With regard to the form
of the standard, the recommendations in the final Staff Paper reflected
CASAC's advice to consider percentiles in the range of the 92nd to the
98th percentile. Some Panel members recommend considering a percentile
within this range in conjunction with a level toward the upper end of
the range recommended in the Staff Paper.\89\
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\89\ Some CASAC Panel members also recommend that such a
standard be implemented in conjunction an ``exceptional events''
policy so as to avoid having non-compliance with the standard be
driven by natural source influences such as dust storms and wild
fires (Henderson, 2005a).
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Based on the above considerations, the Administrator believes that
it is appropriate to first consider the level of protection that would
be afforded by the suite of primary PM2.5 standards proposed
today. The limited and uncertain evidence currently available for use
in evaluating the appropriate level of protection suggests that a
cautious approach is warranted in establishing a secondary standard.
While significantly more information is available since the last review
concerning the relationship between fine PM levels and visibility
across the country, there is still little available information for use
in making the relatively subjective value judgment needed in setting
the secondary standard. Given this, it is appropriate to first evaluate
the level of protection that the proposed primary standards would
likely provide, and then determine whether the available evidence
warrants adopting a standard with a different level, form, or averaging
time. In comparing the extent to which the proposed suite of primary
standards would require areas across the country to improve visual air
quality with the extent of increased protection likely to be afforded
by a standard based on a sub-daily averaging time, the Administrator
has looked to information on the predicted percent of areas not likely
to meet various alternative secondary and primary PM2.5
standards (EPA, 2005a, Tables 7A-1 and 5B-1(a) \90\). In so doing, the
Administrator observes that the predicted percent of counties with
monitors not likely to meet the proposed suite of primary
PM2.5 standards (i.e., a 24-hour standard set at 35 [mu]g/
m3, with a 98th percentile form, and an annual standard of
15 [mu]g/m3) is somewhat higher (27 percent) than the
predicted percent of counties with monitors not likely to meet a sub-
daily secondary standard with an averaging time of 4 to 8 daylight
hours, a level toward the upper end of the range recommended in the
Staff Paper (e.g., up to 30 [mu]g/m3), and a form within the
recommended range (e.g., around the 95th percentile) (24 percent). A
similar comparison is seen in considering the predicted percentages of
the population living in such areas.
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\90\ The information in these Tables is based on analysis of
2001-2003 air quality data, including 562 counties with FRM monitors
that met specific data completeness criteria for developing
predicted percentages of counties not likely to meet the suite of
primary PM2.5 standards and 168 counties with continuous
PM2.5 monitors that met less restrictive data
completeness criteria for developing predicted percentages for a 4-
hour secondary PM2.5 standard.
---------------------------------------------------------------------------
The Administrator provisionally concludes that revising the current
secondary PM2.5 to be identical to the proposed suite of
primary PM2.5 standards is a reasonable policy approach to
addressing visibility protection primarily in urban areas. Such an
approach would result in improvements in visual air quality in as many
or more urban areas across the country as would the alternative
approach of setting a sub-daily standard consistent with that generally
recommended by CASAC. Such an approach also takes into account the
substantial limitations in the available hourly air quality data and in
available studies of public perception and attitudes with regard to the
acceptability of various degrees of visibility impairment in urban
areas across the country. Given these limitations, the Administrator
concludes, subject to consideration of public comment, that a secondary
standard with a different averaging time, level, or form is not
warranted, because the available evidence does not support a decision
to achieve a level of protection different from that provided by the
current primary standards, and because no change in averaging time,
level, or form appears needed to achieve a comparable level of protection.
The Administrator believes that a secondary NAAQS should be
considered in conjunction with the regional haze program as a means of
achieving appropriate levels of protection against PM-related
visibility impairment in urban, non-urban, and Class I areas across the
country. Programs implemented to meet a national standard focused
primarily on urban areas can be expected to improve visual air quality
in surrounding non-urban areas as well, as would programs now being
developed to address the requirements of the regional haze rule
established for protection of visual air quality in Class I areas. The
Administrator further believes that the development of local programs
continues to be an effective and appropriate approach to provide
additional protection for unique scenic resources in and around certain
urban areas that are particularly highly valued by people living in
those areas. Based on these considerations, and taking into account the
observations, analyses, and recommendations discussed above, the
Administrator proposes to revise the current secondary PM2.5
standards by making them identical in all respects to the proposed
suite of primary PM2.5 standards.
As discussed above, most CASAC PM Panel members strongly supported
a sub-daily (4- to 8-hour averaging time) PM2.5 standard.
The Administrator places great importance on the advice of CASAC, and
therefore solicits public comment on such a standard.
B. Other PM-Related Welfare Effects
This section presents the rationale for the Administrator's
proposed revision of the current secondary PM standards to address PM-
related effects other than visibility impairment, including vegetation
and ecosystems, materials damage and soiling, and climate change. In
considering the currently available evidence on each of these types of
PM-related welfare effects, the Staff Paper notes that there is much
information linking ambient PM to potentially adverse effects on
materials and ecosystems and vegetation, and on characterizing the role
of atmospheric particles in climatic and radiative processes. However,
given the evaluation of this information in the Criteria Document and
Staff Paper which highlighted the substantial limitations in the
evidence, especially the lack of evidence linking various effects to
specific levels of ambient PM, the Administrator provisionally
concludes that the available evidence does not provide a sufficient
basis for establishing distinct secondary standards for PM based on any
of these effects alone.
[[Page 2682]]
The Administrator has also addressed the question of whether
reductions in PM likely to result from the current secondary PM
standards, or from the range of proposed revisions to the primary PM
standards, would provide requisite protection against any of these PM-
related welfare effects. As discussed below, these considerations
include the latest scientific information characterizing the nature of
these PM-related effects and judgments as to whether revision of the
current secondary standards are appropriate based on that information.
1. Nature of Effects
Particulate matter contributes to adverse effects on a number of
welfare effects categories other than visibility impairment, including
vegetation and ecosystems, soiling and materials damage and climate.
These welfare effects result predominantly from exposure to excess
amounts of specific chemical species, regardless of their source or
predominant form (particle, gas or liquid). Reflecting this fact, the
Criteria Document concludes that regardless of size fraction, particles
containing nitrates and sulfates have the greatest potential for
widespread environmental significance, while effects are also related
to other chemical constituents found in ambient PM, such as trace
metals and organics.\91\ The following characterizations of the nature
of these welfare effects are based on the information contained in the
Criteria Document and Staff Paper.
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\91\ The Staff Paper notes that some of these other components
are regulated under separate statutory authorities, e.g., section
112 of the CAA.
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a. Effects on Vegetation and Ecosystems
Potentially adverse PM-related effects on vegetation and ecosystems
are principally associated with particulate nitrate and sulfate
deposition. In characterizing such effects, it is important to
recognize that nitrogen and sulfur are necessary and beneficial
nutrients for most organisms that make up ecosystems, with optimal
amounts of these nutrients varying across organisms, populations,
communities, ecosystems and time scales. Therefore, it is impossible to
generalize to all species in all circumstances as to the amount at
which inputs of these nutrients or acidifying compounds become
stressors. The Staff Paper recognizes that the public welfare benefits
from the use of nitrogen (N) and sulfur (S) nutrients in fertilizers in
managed agricultural and commercial forest settings. The focus of this
review, therefore, is on identifying risks to sensitive species and
ecosystems where unintentional additions of these atmospherically
derived nutrient and acidifying compounds may be contributing to
undesired change in the nation's ecosystems and resulting in adverse
impacts on essential ecological attributes such as species shifts, loss
of species richness and diversity, impacts on threatened and endangered
species, and alteration of native fire cycles. In these cases,
deposited particulate nitrate and sulfate are appropriately termed
ecosystem ``stressors.''
i. Vegetation Effects
At current ambient levels, risks to vegetation from short-term
exposures to dry deposited particulate nitrate or sulfate are low.
However, when found in acid or acidifying deposition, such particles do
have the potential to cause direct foliar injury. Specifically, the
responses of forest trees to acid precipitation (rain, snow) include
accelerated weathering of leaf cuticular surfaces, increased
permeability of leaf surfaces to toxic materials, water, and disease
agents; increased leaching of nutrients from foliage; and altered
reproductive processes--all which serve to weaken trees so that they
are more susceptible to other stresses (e.g., extreme weather, pests,
pathogens). Acid deposition with levels of acidity associated with the
foliar effects described above are currently found in some locations in
the eastern U.S. (EPA, 2003). Even higher concentrations of acidity can
be present in occult deposition (e.g. fog, mist or clouds) which more
frequently impacts higher elevations. Thus, the risks of foliar injury
occurring from acid deposition in some areas of the eastern U.S. is
high. However, based on currently available information, the
contribution of particulate sulfates and nitrates to the total acidity
found at these locations is not clear.
ii. Ecosystem Effects
The N- and S-containing components of PM have been associated with
a broad spectrum of terrestrial and aquatic ecosystem impacts that
result from either the nutrient or acidifying characteristics of the
deposited compounds.
Reactive nitrogen (Nr) is the form of N that is available to
support the growth of plants and microorganisms. Since the mid-1960's,
Nr creation through natural terrestrial processes has been overtaken by
Nr creation as a result of human processes, and is now accumulating in
the environment on all spatial scales--local, regional and global. Some
Nr emissions are transformed into ambient PM and deposited onto
sensitive ecosystems. Some of the most significant detrimental effects
associated with excess Nr deposition are those associated with a
syndrome known as ``nitrogen saturation.'' These effects include: (1)
Decreased productivity, increased mortality, and/or shifts in
terrestrial plant community composition, often leading to decreased
biodiversity in many natural habitats wherever atmospheric Nr
deposition increases significantly and critical thresholds are
exceeded; (2) leaching of excess nitrate and associated base cations
from terrestrial soils into streams, lakes and rivers and mobilization
of soil aluminum; and (3) alteration of ecosystem processes such as
nutrient and energy cycles through changes in the functioning and
species composition of beneficial soil organisms (Galloway and Cowling
2002). Thus, through its effects on habitat suitability, genetic
diversity, community dynamics and composition, nutrient status, energy
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