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The Clean Air Act, which was last amended in 1990, requires EPA to set National Ambient Air Quality Standards (40 CFR part 50) for six principal pollutants ("criteria" air pollutants) which can be harmful to public health and the environment. The Clean Air Act identifies two types of national ambient air quality standards. Primary standards provide public health protection, including protecting the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards provide public welfare protection, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings.

Periodically, the standards are reviewed and sometimes may be revised, establishing new standards. The most recently established standards are listed below. In some areas of the U.S., certain regulatory requirements may also remain for implementation of previously established standards.

Units of measure for the standards are parts per million (ppm) by volume, parts per billion (ppb) by volume, and micrograms per cubic meter of air (µg/m3). 

[links to historical tables of NAAQS reviews]
Averaging Time Level Form
Carbon Monoxide (CO) primary 8 hours 9 ppm Not to be exceeded more than once per year
1 hour 35 ppm
Lead (Pb) primary and
Rolling 3 month average 0.15 μg/m3 (1) Not to be exceeded
Nitrogen Dioxide (NO2) primary 1 hour 100 ppb 98th percentile of 1-hour daily maximum concentrations, averaged over 3 years
primary and
1 year 53 ppb (2) Annual Mean
Ozone (O3) primary and
8 hours 0.070 ppm (3) Annual fourth-highest daily maximum 8-hour concentration, averaged over 3 years
Particle Pollution (PM) PM2.5 primary 1 year 12.0 μg/m3 annual mean, averaged over 3 years
secondary 1 year 15.0 μg/m3 annual mean, averaged over 3 years
primary and
24 hours 35 μg/m3 98th percentile, averaged over 3 years
PM10 primary and
24 hours 150 μg/m3 Not to be exceeded more than once per year on average over 3 years
Sulfur Dioxide (SO2) primary 1 hour 75 ppb (4) 99th percentile of 1-hour daily maximum concentrations, averaged over 3 years
secondary 3 hours 0.5 ppm Not to be exceeded more than once per year

(1) In areas designated nonattainment for the Pb standards prior to the promulgation of the current (2008) standards, and for which implementation plans to attain or maintain the current (2008) standards have not been submitted and approved, the previous standards (1.5 µg/m3 as a calendar quarter average) also remain in effect.

(2) The level of the annual NO2 standard is 0.053 ppm. It is shown here in terms of ppb for the purposes of clearer comparison to the 1-hour standard level.

(3) Final rule signed October 1, 2015, and effective December 28, 2015. The previous (2008) O3 standards are not revoked and remain in effect for designated areas. Additionally, some areas may have certain continuing implementation obligations under the prior revoked 1-hour (1979) and 8-hour (1997) O3 standards.

(4) The previous SO2 standards (0.14 ppm 24-hour and 0.03 ppm annual) will additionally remain in effect in certain areas: (1) any area for which it is not yet 1 year since the effective date of designation under the current (2010) standards, and (2)any area for which an implementation plan providing for attainment of the current (2010) standard has not been submitted and approved and which is designated nonattainment under the previous SO2 standards or is not meeting the requirements of a SIP call under the previous SO2 standards (40 CFR 50.4(3)).  A SIP call is an EPA action requiring a state to resubmit all or part of its State Implementation Plan to demonstrate attainment of the required NAAQS.

The Menu of Control Measures (MCM) provides state, local and tribal air agencies with the existing emission reduction measures as well as relevant information concerning the efficiency and cost effectiveness of the measures. State, local and tribal agencies will be able to use this information in developing emission reduction strategies, plans and programs to assure they attain and maintain the National Ambient Air Quality Standards (NAAQS). The MCM is a living document that can be updated with newly available or more current data as it becomes available.