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National Ambient Air Quality Standards (NAAQS)
The Clean Air Act, which was last amended in 1990, requires EPA to set National Ambient Air Quality Standards (40 CFR part 50) for pollutants considered harmful to public health and the environment. The Clean Air Act established two types of national air quality standards. Primary standards set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection against decreased visibility, damage to animals, crops, vegetation, and buildings.
The EPA Office of Air Quality Planning and Standards (OAQPS) has set National Ambient Air Quality Standards for six principal pollutants, which are called "criteria" pollutants. They are listed below. Units of measure for the standards are parts per million (ppm) by volume, milligrams per cubic meter of air (mg/m3), and micrograms per cubic meter of air (µg/m3).
National Ambient Air Quality Standards
| Primary Standards | Secondary Standards | |||
|---|---|---|---|---|
| Pollutant | Level | Averaging Time | Level | Averaging Time |
| Carbon Monoxide |
9 ppm (10 mg/m3) |
8-hour (1) | None |
|
| 35 ppm (40 mg/m3) |
1-hour (1) | |||
| Lead | 0.15 µg/m3 (2) | Rolling 3-Month Average | Same as Primary |
|
| 1.5 µg/m3 | Quarterly Average | Same as Primary |
||
| Nitrogen Dioxide |
0.053 ppm (100 µg/m3) |
Annual (Arithmetic Mean) |
Same as Primary |
|
| Particulate Matter (PM10) |
150 µg/m3 | 24-hour (3) | Same as Primary |
|
| Particulate Matter (PM2.5) |
15.0 µg/m3 |
Annual (4) (Arithmetic Mean) |
Same as Primary |
|
| 35 µg/m3 | 24-hour (5) | Same as Primary |
||
| Ozone | 0.075 ppm (2008 std) | 8-hour (6) | Same as Primary |
|
| 0.08 ppm (1997 std) | 8-hour (7) | Same as Primary |
||
| 0.12 ppm | 1-hour (8)
(Applies only in limited areas) |
Same as Primary |
||
| Sulfur Dioxide |
0.03 ppm | Annual (Arithmetic Mean) |
0.5 ppm (1300 µg/m3) |
|
| 0.14 ppm | 24-hour (1) | |||
(1) Not to be exceeded more than once per year.
(2) Final rule signed October 15, 2008.
(3) Not to be exceeded more than once per year on average over 3 years.
(4) To attain this standard, the 3-year average of the weighted annual mean PM2.5 concentrations from single or multiple community-oriented monitors must not exceed 15.0 µg/m3.
(5) To attain this standard, the 3-year average of the 98th percentile of 24-hour concentrations at each population-oriented monitor within an area must not exceed 35 µg/m3 (effective December 17, 2006).
(6) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.075 ppm. (effective May 27, 2008)
(7) (a) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area over each year must not exceed 0.08 ppm.
(b) The 1997 standard—and the implementation rules for that standard—will remain in place for implementation purposes as EPA undertakes rulemaking to address the transition from the 1997 ozone standard to the 2008 ozone standard.
(8) (a) The standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is < 1.
(b) As of June 15, 2005 EPA revoked the 1-hour ozone standard in all areas except the 8-hour ozone nonattainment Early Action Compact (EAC) Areas.
