The Clean Air Act, which was last amended in 1990, requires EPA to set National Ambient Air Quality Standards 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, parts per billion (ppb) by volume, milligrams per cubic meter of air (mg/m3), and micrograms per cubic meter of air (ug/m3).
| Pollutant | Standard Value | Standard Type |
|---|---|---|
| Carbon Monoxide | ||
| 8-Hour Average | 9 ppm | Primary |
| 1-Hour Average | 35 ppm | Primary |
| Nitrogen Dioxide (NO2) | ||
| 1-Hour Average | 100 ppb | Primary |
| 1-Year Average | 53 ppb | Primary & Secondary |
| Ozone (O3) | ||
| 8-Hour Average | 0.070 ppm | Primary & Secondary |
| Lead (Pb) | ||
| Rolling Three Month Average | 0.15 µg/m3 | Primary & Secondary |
| Fine Particulate (PM2.5) | ||
| 1-Year Average | 9 µg/m3 | Primary |
| 1-Year Average | 15 µg/m3 | Secondary |
| 24-Hour Average | 35 µg/m3 | Primary & Secondary |
| Particulate (PM10) | ||
| 24-Hour Average | 150 µg/m3 | Primary & Secondary |
| Sulfur Dioxide (SO2) | ||
| 1-Hour Average | 75 ppb | Primary |
| 3-Hour Average | 0.50 ppm | Secondary |