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 air pollutant concentration limits as 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.

The Clean Air Act requires periodic review of the science upon which the standards are based and the standards themselves.

Worldwide, other air quality standards have been adopted, such as the European Union's Air Quality Directive, the United Kingdom's Air Quality Strategy, and the Air Quality Guidelines of the World Health Organization (WHO).

Summary of the NAAQS

The NAAQS set air pollutant concentration limits for six pollutants, called the criteria air pollutants, namely carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone 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 ). For the conversions between gas concentrations in ppmv and mg/m3, see Air pollutant concentrations.

Air quality areas not in compliance with the NAAQS as of early 2011

A network of about 4,000 State and Local Air Monitoring Stations (SLAMS) is used to determine if geographic areas are meeting or exceeding the NAAQS. Attainment areas are those geographic areas that stay at or below the NAAQS. Nonattainment areas are areas in which the NAAQS for a criteria pollutant are being exceeded.

The map below depicts the counties in the United States that have not attained compliance with the NAAQS standards for one or more of the criteria pollutants as of the end 2010:

References

• Clean Air Act, The entire text as of February 24, 2004).
• Clean Air Act, From the U.S. EPA website: "Legislation passed since 1990 has made several minor changes".
• National Air Quality Standards, As summarized on the website of the U.S. EPA.
• New Air Quality Directive, From the European Union's website.
• Air Quality Strategy, From the website of the UK's Department for Environment, Food and Rural Affairs.
• Air Quality Guidelines, From the website of the World Health Organization.
• Counties Designated As Nonattainment Areas, From the EPA's website.
• Note: PM10 is particulate matter having an aerodynamic diameter of 10 micrometer (μm). PM2.5 is particulate matter having an aerodynamic diameter of 2.5 μm or less.