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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 Standard (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 (PM_2.5).

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

National AmbientAir Quality Standards

	Primary Standard		Secondary Standard
Criteria Pollutant	Concentration Limit	Averaging Time	Concentration Limit	Averaging Time
Carbon monoxide	9 ppmv ( 10 mg/m3 )	8-hour (1)	None
	35 ppmv ( 40 mg/m3 )	1-hour (1)
Sulfur dioxide	0.03 ppmv ( 80 μg/m3 )	Annual (arithmetic mean)	0.5 ppmv ( 1300 μg/m3 )	3-hour (1)
	0.14 ppmv ( 365 μg/m3	24-hour (1)
Nitrogen dioxide	0.053 ppmv ( 100 μg/m3 )	Annual (arithmetic mean)	Same as primary
Ozone	0.075 ppmv ( 150 μg/m3 )	8-hour (2)	Same as primary
	0.12 ppmv ( 235 μg/m3 )	1-hour (3)	Same as primary
Lead	0.15 μg/m3	Rolling 3-month average	Same as primary
	1.5 μg/m3	Quarterly average	Same as primary
Particulate Matter (PM10)	150 μg/m3	24-hour (4)	Same as primary
Particulate Matter (PM2.5)	15.0 μg/m3	Annual (5) (arithmetic mean)	Same as primary
	35 μg/m3	24-hour (6)	Same as primary
(1) Not to be exceeded more than once per year. (2) The 3-year average of the fourth-highest daily maximum 8-hour average at each monitor within the area over each year must not exceed 0.075 ppmv. (3a) The expected number of days per calendar year with maximum hourly averages above 0.12 ppm must be equal to or less than 1. (3b) As of June 15, 2007 , the U.S. EPA revoked the 1-hour ozone standard in all areas except for certain parts of 10 states. (4) Not to be exceeded more than once per year on average over 3 years. (5) The 3-year average of the weighted annual mean PM2.5 concentrations from single or multiple community-oriented monitors must not exceed 15 μg/m3. (6) The 3-year average of the 98th percentile of 24-hour concentrations at each population-oriented monitor within the area must not exceed 35.5 μg/m3.

The units of measure used for the concentrations in the above table are parts per million by volume (ppmv), milligrams per cubic meter of air (mg/m³), and micrograms per cubic meter of air (µg/m³). For gases, the conversions between ppmv and mg/m³ are available at Air pollutant concentrations.