The U.S. National Ambient Air Quality Standards (NAAQS, pronounced /ˈnæks/ naks) are limits on atmospheric concentration of six pollutants that cause smog, acid rain, and other health hazards.  Established by the United States Environmental Protection Agency (EPA) under authority of the Clean Air Act (42 U.S.C. 7401 et seq.), NAAQS is applied for outdoor air throughout the country. 
The six criteria air pollutants (CAP), or criteria pollutants, for which limits are set in the NAAQS are ozone (O3), atmospheric particulate matter, lead, carbon monoxide (CO), sulfur oxides (SOx), and nitrogen oxides (NOx).  These are typically emitted from many sources in industry, mining, transportation, electricity generation and agriculture. In many cases they are the products of the combustion of fossil fuels or industrial processes. 
The National Emissions Standards for Hazardous Air Pollutants cover many other chemicals, and require the maximum achievable reduction that the EPA determines is feasible.
The six criteria air pollutants were the first set of pollutants recognized by the United States Environmental Protection Agency as needing standards on a national level.  The Clean Air Act requires the EPA to set US National Ambient Air Quality Standards (NAAQS) for the six CAPs.  The NAAQS are health based and the EPA sets two types of standards: primary and secondary. The primary standards are designed to protect the health of 'sensitive' populations such as asthmatics, children, and the elderly. The secondary standards are concerned with protecting the environment. They are designed to address visibility, damage to crops, vegetation, buildings, and animals. 
The EPA established the NAAQS according to Sections 108 and 109 of the U.S. Clean Air Act, which was last amended in 1990.  These sections require the EPA "(1) to list widespread air pollutants that reasonably may be expected to endanger public health or welfare; (2) to issue air quality criteria for them that assess the latest available scientific information on nature and effects of ambient exposure to them; (3) to set primary NAAQS to protect human health with adequate margin of safety and to set secondary NAAQS to protect against welfare effects (e.g., effects on vegetation, ecosystems, visibility, climate, manmade materials, etc); and (5) to periodically review and revise, as appropriate, the criteria and NAAQS for a given listed pollutant or class of pollutants." 
The standards are listed in 40 CFR 50 . Primary standards are designed to protect human health,  with an adequate margin of safety, including sensitive populations such as children, the elderly, and individuals suffering from respiratory diseases. Secondary standards are designed to protect public welfare, damage to property, transportation hazards, economic values, and personal comfort and well-being from any known or anticipated adverse effects of a pollutant. A district meeting a given standard is known as an "attainment area" for that standard, and otherwise a "non-attainment area". 
Standards are required to "accurately reflect the latest scientific knowledge," and are reviewed every five years by a Clean Air Scientific Advisory Committee (CASAC), consisting of "seven members appointed by the EPA administrator." 
EPA has set NAAQS for six major pollutants listed as below. These six are also the criteria air pollutants. 
|Pollutant||Type||Standard||Averaging Time||Form a||Regulatory Citation|
|Sulfur dioxide (SO2)||Primary||75 ppb||1-hour||99th Percentile of 1-hour daily maximum concentrations, averaged over 3 years||40 CFR 50.17|
|Secondary||0.5 ppm (1,300 μg/m3)||3-hour||Not to be exceeded more than once per year||40 CFR 50.5|
|Particulate matter (PM10)||Primary and Secondary||150 μg/m3||24-hour||Not to be exceeded more than once per year on average over 3 years||40 CFR 50.6|
|Fine particulate matter (PM2.5)||Primary||12 μg/m3||annual||Annual mean, averaged over 3 years||40 CFR 50.18|
|Secondary||15 μg/m3||annual||Annual mean, averaged over 3 years||40 CFR 50.7|
|Primary and Secondary||35 μg/m3||24-hour||98th percentile, averaged over 3 years||40 CFR 50.18|
|Carbon monoxide (CO)||Primary||35 ppm (40 mg/m3)||1-hour||Not to be exceeded more than once per year||40 CFR 50.8|
|Primary||9 ppm (10 mg/m3)||8-hour||Not to be exceeded more than once per year||40 CFR 50.8|
|Ozone (O3)||Primary and Secondary||0.12 ppm (235 μg/m3)||1-hour b||expected number of days per calendar year, with maximum hourly average concentration greater than 0.12 ppm, is equal to or less than 1||40 CFR 50.9|
|Primary and Secondary||0.070 ppm (140 μg/m3)||8-hour||Annual fourth-highest daily maximum 8-hour concentration, averaged over 3 years||40 CFR 50.19|
|Nitrogen dioxide (NO2)||Primary and Secondary||0.053 ppm (100 μg/m3)||annual||Annual mean||40 CFR 50.11|
|Primary||0.100 ppm (188 μg/m3)||1-hour||98th percentile of 1-hour daily maximum, averaged over 3 years||40 CFR 50.11|
|Lead (Pb)||Primary and Secondary||0.15 μg/m3||Rolling 3 months||Not to be exceeded||40 CFR 50.12|
The EPA National Exposure Research Laboratory can designate a measurement device using an established technological basis as a Federal Reference Method (FRM) to certify that the device has undergone a testing and analysis protocol, and can be used to monitor NAAQS compliance. Devices based on new technologies can be designated as a Federal Equivalent Method (FEM).[ citation needed ] FEMs are based on different sampling and/or analyzing technologies than FRMs, but are required to provide the same decision making quality when making NAAQS attainment determinations. Approved new methods are formally announced through publication in the Federal Register.  A complete list of FRMs and FEMs is available. 
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