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]
|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|
|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|
|24 hours||35 μg/m3||98th percentile, averaged over 3 years|
|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|
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.