In the United States, the Clean Air Act sets national ambient air quality standards for six common air pollutants:

In the United States, the Clean Air Act sets national ambient air quality standards for six common air pollutants: particle pollution (also known as particulate matter), ground-level ozone (O3), carbon monoxide (CO), sulfur oxides (SOx), nitrogen oxides (NOx), and lead (Pb). Read about each of these, their common sources, and the Air Quality Index (AQI) at this Environmental Protection Agency (EPA) site: