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The U.S. Clean Air Act, enacted by the U.S. Congress and last amended in 1990, required the U.S. Environmental Protection Agency (U.S. EPA) to set National Ambient Air Quality Standards (NAAQS) for outdoor ambient air pollutants considered harmful to public health and the environment.

The Clean Air Act established two types of national air quality standards. Primary standards that define air pollutant concentration limits intended to protect the public health, including the health of sensitive populations such as asthmatics, children and the elderly. Secondary standards that define limits intended to protect the public welfare and environment, including damage to animals, crops, vegetation and buildings as well as providing protection against decreased visibility.

Summary of the NAAQS
The NAAQS set air pollutant concentration limits for six pollutants, called the criteria air pollutants, namely carbon monoxide (CO2), sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3), lead (Pb), Particulate Matter (PM10) and Particulate Matter (PM2.5).

The NAAQS for the six criteria pollutants are listed in the table below:

The units of measure used for concentrations in the above table are parts per million by volume (ppmv), milligrams per cubic meter of air at 101.325 kPa and 25 °C (mg/m3), and micrograms per cubic meter of air at 101.325 kPa and 25 °C (µg/m3). See Air pollutant concentrations for the conversions between gas concentrations ppmv and mg/m3.