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Frequently Asked Questions

General Background
  1. What is the national-scale assessment? What's its purpose?
  2. What are the steps in the national-scale assessment?
  3. How will EPA use the results of this national-scale assessment?
  4. Why are all the estimates from 1996 -- and not more recent?
  5. Which toxic air pollutants is EPA assessing?
  6. How did EPA decide which air toxics to focus on for this assessment?
  7. Are there any risks from air toxics that are not covered by the national-scale assessment?
  8. Who is responsible for controlling air toxics?
  9. What should I do if I am concerned about toxics in my area?
  10. What are some of the actions EPA has taken to control air toxics? Are you seeing results?
  11. Why is EPA using computer modeling techniques instead of actual measurements to estimate concentrations and exposure?
  12. Why doesn't the assessment include Alaska and Hawaii?
  13. Is this an update of the air toxics estimates EPA released in 1998 under the Cumulative Exposure Project (CEP)?
Results
  1. Based on the results of this assessment, does EPA think air toxics pose significant health risks?
  2. Based on this national-scale assessment, can EPA determine which areas and/or populations are at greatest risk from air toxics?
  3. What does EPA believe constitutes an acceptable level of risk?
  4. Why is there no cancer risk characterization for diesel particulate matter?
  5. What do these estimates mean to me?
  6. Will EPA use the results of the assessment to set new emissions limits?
  7. How accurate is the assessment?
  8. Why aren't results for dioxin included?
  9. What is the difference between the National Toxics Inventory (NTI) used in this assessment and the Toxics Release Inventory (TRI)?
  10. Has EPA compared the ambient concentrations estimates to actual monitored data?
Future Efforts/Schedule
  1. Does EPA plan to conduct the assessment for the other 154 toxic air pollutants included in the Clean Air Act list of air toxics?
  2. Are there emissions data more recent than 1996? When will it be analyzed?

General Background

1: What is the national-scale assessment? What's its purpose?

A: The national-scale assessment is designed to help EPA, state, local and tribal governments and the public better understand the air toxics problem in the U.S. The assessment includes four steps which focus on the year 1996:
  1. An inventory of air toxics emissions,
  2. Estimates of annual average outdoor air toxics concentrations,
  3. Estimates of exposure concentrations (what people are estimated to breathe),
  4. A characterization of potential public health risks.

This assessment looks at 32 of the 33 air toxics EPA identified in 1999 as potentially posing the greatest threat to public health in the largest number of urban areas. The assessment also includes diesel particulate matter, a surrogate measure of diesel exhaust. These pollutants were highlighted in EPA's Urban Strategy, which we released in July 1999.

2: What are the steps in the national-scale assessment?
A: The national-scale assessment includes the following four major steps for assessing air toxics across the contiguous United States (also Puerto Rico and the Virgin Islands) for the 32 air toxics plus diesel particulate matter.
  1. Compiling a 1996 national emissions inventory of air toxics emissions from outdoor sources. The types of emissions sources in the inventory include major stationary sources (e.g., large waste incinerators and factories), area and other sources (e.g., dry cleaners, small manufacturers, wildfires), and both onroad and nonroad mobile sources (e.g., cars, trucks, boats). EPA made some modifications to the 1996 National Toxics Inventory to prepare the emissions for computer modeling. See limitations in the emissions for more information about these modifications.

     

  2. Estimating 1996 ambient air concentrations based on an air dispersion model (the ASPEN model) and 1996 emissions as input to the model. As part of this modeling exercise, EPA compared estimated ambient concentrations to available ambient air toxics monitoring data to evaluate model performance. Learn more about this comparison.

     

  3. Estimating 1996 population exposures based on a screening-level inhalation exposure model (HAPEM4) and the estimated ambient concentrations (from the ASPEN model) as input to the exposure model. Estimating exposure is a key step in determining potential health risk. People move around from one location to another, outside to inside, etc., so exposure isn't the same as concentration at a static site. People also breathe at different rates depending on their activity levels, so the amounts of air they take in vary. For these reasons, the average concentration of a pollutant that people breathe (i.e., exposure concentration) may be significantly higher or lower than the concentration at a fixed location (i.e., ambient concentration).

     

  4. Characterizing potential public health risks due to inhalation of air toxics. This includes both cancer and non-cancer health effects, using available information on air toxics health effects, current Agency risk assessment and risk characterization guidelines, and estimated population exposures. This characterization quantifies, as appropriate, potential cumulative risks to public health due to inhalation of air toxics from outdoor emission sources, discusses the uncertainties and limitations of the assessment, and identifies other potential risks to public health from air toxics that are beyond the scope of this quantitative assessment.

3: How will EPA use the results of this national-scale assessment?

A: The results of the national-scale assessment will provide important information to help EPA continue to develop and implement various aspects of the national air toxics program, but they will not be used by themselves to regulate sources of air toxics emissions. While regulatory priority setting will be informed by this and future national assessments, risk-based regulations will be based on more refined and source-specific data and assessments. More specifically, the national-scale assessment results will help to:

  • Identify air toxics of greatest potential concern.
  • Characterize the relative contributions to air toxics concentrations and population exposures of different types (e.g., major, mobile) of air toxics emissions sources.
  • Set priorities for the collection of additional air toxics data to improve estimates of air toxics concentrations and their potential public health impacts. Important additional data collection activities will include upgraded emission inventory information, ambient air toxics monitoring, and information on adverse effects to health and the environment.
  • Establish a baseline to track trends over time in modeled ambient concentrations of air toxics.
  • Establish a baseline for measuring progress toward meeting goals for inhalation risk reduction from ambient air toxics.

4: Why are all the estimates from 1996 -- and not more recent?

A: We used 1996 data because emissions inventories from that year are the most complete and up-to-date available. Working with the States, we update our air toxics emissions inventories every 3 years and are now gathering and compiling 1999 data.

Although actual current concentrations and risks may be higher or lower than our estimates (depending on the quality of emission inventories, on the effects of Federal, State and local regulations since 1996, etc.), these are the most comprehensive estimates we have developed to date.

5: Which toxic air pollutants is EPA assessing?
A: The National-Scale Air Toxics Assessment will help to characterize the potential health risks associated with inhalation exposures to 33 air pollutants identified in EPA's Integrated Urban Air Toxics Strategy. This includes 32 air toxics that are a subset of EPA's list of 188 toxic air pollutants plus diesel particulate matter, which is used as a surrogate measure of diesel exhaust. Diesel exhaust is addressed in several regulatory actions and EPA listed diesel particulate matter plus diesel exhaust organic gases as a mobile source air toxic.

6: How did EPA decide which air toxics to focus on for this assessment?

A: We are focusing on the 32 air toxics because they were highlighted as priority pollutants in EPA's Urban Strategy. These 32 toxic air pollutants are a subset of the 188 toxic air pollutants for which EPA must develop emissions standards. The 32 were selected based on a number of factors, including toxicity-weighted emissions, monitoring data, past air quality modeling analysis, and a review of existing risk assessment literature.

The national-scale assessment also includes diesel particulate matter, which is used as a surrogate measure of diesel exhaust. EPA recently concluded that diesel exhaust is likely to be carcinogenic to humans by inhalation at environmental levels of exposure. Diesel exhaust is addressed in several regulatory actions and diesel particulate matter plus diesel organic gases is listed by EPA as a mobile source air toxic. The concentration estimates EPA is modeling in this assessment will provide information for our continuing health assessment analysis.

7: Are there any risks from air toxics that are not covered by the national-scale assessment?

A: As mentioned above, the assessment only addresses the risks from 32 air toxics and diesel particulate matter. While these pollutants are expected to dominate the inhalation risks from outdoor sources of air toxics, the remaining 156 toxic air pollutants can also contribute to risks, especially in localized areas around individual sources.

Non-inhalation exposures, such as ingestion and dermal exposures, are not included. Evaluation of these additional pathways for exposure is especially important for pollutants that persist in the environment and bioaccumulate, such as mercury and PCBs.

The highest localized inhalation exposures and risks are not captured by the national-scale approach. It is important to use local-scale assessments to characterize exposures and risks very close to specific sources.

Predicted inhalation exposures associated with indoor sources of air toxics are not included in the current assessment due to lack of emissions data. For certain air toxics, total long-term human exposures can be significantly influenced and sometimes dominated by exposures from indoor sources.

See "Limitations, Variability and Uncertainty" for a more detailed discussion.

8: Who is responsible for controlling air toxics?

A: The responsibility is shared between EPA and State, local and Tribal air programs. EPA sets standards for air toxics emissions, which state and local programs are responsible for carrying out. In addition, some state and local programs have their own air toxics rules.

9: What should I do if I am concerned about toxics in my area?

A: Contact your State, local or Tribal air program. A list of State and local programs is available at: www.4cleanair.org

Information on the tribal programs and EPA's Regional Tribal Program coordinators can be found at : http://www.epa.gov/oar/tribal

10: What are some of the actions EPA has taken to control air toxics? Are you seeing results?

A: Since 1990, EPA has taken many actions to significantly reduce air toxics across the country. See Air Toxics Reductions to learn more.

11: Why is EPA using computer modeling techniques instead of actual measurements to estimate concentrations and exposure?

A: The assessment approach is fundamentally based on using computer models to estimate ambient air toxics concentrations and population exposures nationwide.

While such computer models necessarily require simplifying assumptions and introduce significant uncertainties, they are needed to conduct such a large scale assessment since direct measurements of ambient air toxics concentrations are limited, and direct personal exposure measurements are even more limited. Such measurements are available for only a subset of air toxics in relatively few locations and for small study populations.

Although EPA is working to expand the number and locations of ambient air toxics monitors and the study of personal exposures, direct measurement of air toxics concentrations is not practical for all air toxics of interest across all areas of the country. Over time, such measurement data can and will be used, however, to evaluate the models so as to better understand some of the uncertainties in such assessments and to improve modeling tools.

12. Why doesn't the assessment include Alaska and Hawaii?

A: This assessment builds off of a previous EPA effort, called the Cumulative Exposure Project (CEP), that looked at 1990 emissions and concentrations of air toxics. Because Alaska and Hawaii were not included in the CEP, the meteorological data from the numerous National Weather Service stations in Alaska and Hawaii would need to be processed to be able to enter the emissions from these states into the model. In subsequent assessments, EPA may be able to develop the databases needed to add Alaska and Hawaii.

13. Is this an update of the air toxics estimates EPA released in 1998 under the Cumulative Exposure Project (CEP)?

A: The Cumulative Exposure Project estimated ambient concentrations of 148 air toxics across the contiguous United States for the year 1990. This 1996 national-scale assessment uses the same computer dispersion model developed for the CEP to estimate ambient concentrations of 32 air toxics and diesel particulate matter across the contiguous U.S., plus Puerto Rico and the Virgin Islands, for the year 1996. The national-scale assessment includes an additional step that the CEP did not have -- use of another computer model to factor in the extent to which people are exposed to these pollutants.

Although the CEP and the current national-scale assessment relied on the same computer model, the two used different emissions inventory inputs. The emissions inventory used to estimate concentrations for 1996 is much improved over the 1990 data used in the CEP. It is based on extensive state and local input and includes specific information (exact locations and emission characteristics) about many more sources than did the inventory used in the CEP. For this reason, EPA does not recommend comparing the two assessments since any changes in estimated toxic air pollutant concentrations could as likely be a result of the improved estimates of emissions as it could be due to a change in pollutant concentrations.

Results

1. Based on the results of this assessment, does EPA think air toxics pose significant health risks?

A: Based on the results of this national-scale assessment and other studies, millions of people live in areas where air toxics may pose potentially significant health concerns.

Although air quality continues to improve, we feel that more needs to be done to reduce the potential for harm from exposures to these chemicals.

For this reason, EPA will continue to use and improve air toxics information and tools (i.e., monitoring networks, reporting requirements, inventories, and both national- and more local-scale assessment approaches) to support the evaluation, characterization, and tracking of risk.

EPA will also continue to develop air toxics regulations as well as cost-effective pollution prevention and other control options to address indoor and urban pollutant sources that significantly contribute to risk.

2. Based on this national-scale assessment, can EPA determine which areas and/or populations are at greatest risk from air toxics?

A: This assessment has not focused on the identification of geographic areas or populations that have significantly higher risks than others. Rather, it has focused on characterizing ranges of risks across the country independent of their location.

However, in general, we see that larger urban areas tend to carry larger risk burdens than smaller urban and rural areas because the emissions of air toxics tend to be higher in areas with more people. This trend is not universal, however, and can vary from pollutant to pollutant, according to its sources.

In addition, there are two reasons why EPA would not choose to rank areas of the country based on this assessment:

  • First, some areas have done better jobs of reporting/monitoring than others. This thoroughness could give the incorrect impression of higher air toxics concentrations. Wisconsin and Minnesota, for example, have worked hard to ensure that their inventories are complete. As a result, these two states could appear to have relatively higher air toxics levels (e.g., for PCBs) when, in fact, their inventories are just better (i.e., more complete). As another example, Florida has developed more complete estimates for forest acreage burned within the State. As a result, Florida may appear to have relatively higher air toxics levels for those air toxics associated with wildfires and prescribed burning (e.g., Acrolein, PAHs).

  • Second, these numbers are estimates of the concentrations in 1996, they do not reflect potentially significant air toxics reductions since that time.

3. What does EPA believe constitutes an acceptable level of risk?

A: Unlike other pollutants which EPA regulates, for air toxics there are no risk levels that represent acceptable or unacceptable regulatory thresholds.

However, EPA has made case-specific determinations such as the 1989 Benzene National Emission Standard for Hazardous Air Pollutants (NESHAP) that set up a two part risk-based decision framework. First, it set an upper limit of acceptability of 1 in 10,000 lifetime cancer risk for highly exposed individuals. Second, it set a target of protecting the greatest number of persons possible to an individual lifetime risk level no higher than approximately 1 in 1,000,000. In addition, these determinations called for considering other health and risk factors, including the uncertainty in the risk assessment, in making an overall judgment on acceptability.

This assessment, however, is not designed to be a definitive tool for determining risk since it has many limitations in data and methods. In addition, this assessment estimates average population exposures rather than the exposures experienced by the most exposed individuals. Thus, it contains significant uncertainties, e.g., emissions levels, exposure concentrations, toxicity, and lacks the level of refinement which would enable us to adequately assess the highest exposures found in localized "hot spots."

Consequently, the results should not be used as absolute measures to determine whether risks are acceptable. Rather, they should be used to focus or target further measurement and assessment activities.

4. Why is there no cancer risk characterization for diesel particulate matter?

A: While available evidence supports EPA's conclusion that diesel exhaust is likely to be a human carcinogen, EPA has concluded that the available data are not sufficient to develop a confident estimate of cancer unit risk (i.e., unit risk estimate or URE). (The cancer unit risk is a health assessment value that can be matched with environmental exposure data to estimate environmental risk.) Therefore, EPA cannot provide a quantitative estimate of potential cancer risk associated with environmental exposures to diesel particulate matter as it has for the other 29 carcinogens in the 1996 national-scale assessment.

However, the current diesel exhaust hazard information is sufficient to indicate that this pollutant presents a potentially significant public health concern. First, the Agency has concluded that it is a likely human carcinogen. In addition, the general population is exposed to levels close to or overlapping with apparent levels that result in increased cancer risk in epidemiological studies. Furthermore, the Agency has concluded that national average lifetime cancer risks from exposure to diesel exhaust may exceed one in one hundred thousand (1/100,000) and could be as high as one in one thousand (1/1,000), although the lower end of the risk range includes zero.

5: What do these estimates mean to me?

A: Since EPA does not have an extensive network to monitor levels of toxic air pollutants across the country, the results of the assessment provide estimates that will give you an idea of the total amount of air toxics in your area for 1996 as well as a general estimate of the potential risk in your state and county in 1996. However, it does not provide risk measures for any particular neighborhood. (Refer to question 7 for more information about the accuracy of the results.)

6: Will EPA use the results of the assessment to set new emissions limits?

A: Not directly. But the results will provide important information to help EPA continue to develop and implement various aspects of our national program. Specifically, we will use the results of this national-scale assessments to:
  • Identify air toxics of greatest potential concern;
  • Set priorities for collection of additional air toxics data at the EPA, state, local, and tribal level;
  • Roughly characterize the relative contributions to air toxics concentrations and population exposures of different types of air toxics emissions sources (e.g., mobile, large industrial, smaller industrial);
  • Establish a baseline for tracking trends over time in modeled ambient concentrations of air toxics; and
  • Establish a baseline to measure progress toward meeting goals for reductions of inhalation risk from ambient air toxics.

An important component of our future activities will be to repeat this type of national assessment every 3 years, with the next such assessment focusing on 1999 air toxics data.

7. How accurate is the assessment?

A: Large uncertainties, e.g., emissions levels, exposure, toxicity, are inherent in this analysis; therefore, we suggest interpreting the results in a relative rather than absolute manner.

Thus, the results are appropriate to answer questions such as which pollutants or source sectors may be associated with higher risks than others (e.g., priority setting for data collection), but not for determining exactly how many people are exposed to certain levels of absolute risk (e.g., to determine what's safe and what's not).

8. Why aren't results for dioxin included?

A: EPA has identified further effort that is needed to assure the quality of the emissions data. EPA is working to include dioxin in the next national-scale assessment which will focus on 1999 emissions.

9. What is the difference between the National Toxics Inventory (NTI) used in this assessment and the Toxics Release Inventory (TRI)?

A: The Toxics Release Inventory (TRI) is a Community Right-to-Know Inventory. Facilities that manufacture or process 25,000 pounds or more of listed substances or use 10,000 pounds or more of listed substances, that are in the manufacturing sector (Standard Industrial Classification (SIC) codes 20-39), electric utility sector, mining sector, solvent recovery sector, petroleum bulk storage sector, chemical wholesale sector and the treatment, storage and disposal sector, and that have 10 or more full-time employees are required to report annually. (Beginning in reporting year 2000, certain persistent, bioaccumulative toxic chemicals are subject to lower reporting thresholds).

Under TRI, for each chemical that meets the threshold, a facility reports the quantity that is released to the air, water, land, or managed as waste on-site or off-site (disposal, treatment, energy recovery, recycling). This provides a comprehensive picture of how a facility manages its toxic chemicals in waste.

There are over 640 chemicals and chemical categories on the TRI list. The TRI facilities use the best available information to make their estimates, including monitoring data, emission factors, mass balance and engineering calculations.

While TRI provides information on point sources, it does not address many other important sources of toxics emissions, including mobile sources, combustion sources such as certain incinerators, and agricultural sources.

By contrast, this national-scale assessment uses the National Toxics Inventory (NTI). The NTI is a complete national inventory of stationary and mobile sources that emit any of the air toxics listed in The Clean Air Act.

EPA works with states, local governments, and tribes to update the NTI every three years (1993, 1996, 1999, etc.). It includes facility data with emissions reported for individual stacks, area source data, and mobile source data.

While TRI is based on emissions data only, NTI contains stack parameters, control device information, and location data for individual stacks, which are the parameters needed for modeling.

Furthermore, the NTI includes more facilities than the TRI because of TRI reporting thresholds and the limitations in the types of facilities required to report to TRI. The NTI is compiled primarily using state and local agency and tribal toxic air pollutant emission inventories.

EPA supplements state and local agency and tribal data with inventory data collected during regulatory development, TRI data, mobile source data generated by EPA, and area source data.

10: Has EPA compared the ambient concentrations estimates to actual monitored data?

A: In order to understand the overall performance and limitations of the modeled ambient concentration estimates, EPA compared these estimates to available monitoring data from 1996 as a "reality check." EPA has done this for seven pollutants (benzene, perchloroethylene, formaldehyde, acetaldehyde, cadmium, chromium and lead). The comparisons generally show that the model estimates are lower than the monitored concentrations for these pollutants. Learn more about these comparisons.

Future Efforts/Schedule

1: Does EPA plan to conduct the assessment for the other 154 toxic air pollutants included in the Clean Air Act list of air toxics?

A: EPA does not have all of the scientific information necessary to perform the full exposure assessment for the remaining 154 air toxics. As this information is developed, EPA will determine when the assessment can be completed for the remaining pollutants (1999 national-scale assessment or later).

2: Are there emissions data more recent than 1996? When will it be analyzed?

A: EPA is currently gathering 1999 air toxics emission estimates as part of the National Emission Toxics Inventory effort. We expect to complete this inventory and begin a 1999 national-scale assessment in late 2002. We expect to publicly release the 1999 national-scale assessment by the summer of 2004.

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