This database reports air quality data for U.S. cities based on the EPA's monitoring and enforcement of the Clean Air Act. The EPA reports air quality data for six common air pollutants, also called criteria pollutants. These include carbon monoxide (CO), ozone (O3), lead (Pb), nitrogen dioxide (NO2), particulate matter (PM), and sulfur dioxide (SO2).

Some pollutants, notably CO, SO2, and lead, are emitted directly from a variety of sources, including industry. In contrast, ground-level ozone (O3) forms in the air from chemical reactions involving nitrogen oxides (NOx), volatile organic compounds (VOCs), and sunlight. NO2 is formed in the air through the oxidation of nitric oxide (NO). PM, also known as particle pollution, can be directly emitted, or it can be formed when emissions of NOx, sulfur oxides (SOx), ammonia, organic compounds, and other gases react in the atmosphere. Particle pollution is regulated as PM2.5, or "fine particles" with diameters less than or equal to 2.5 micrometers, and PM10, which includes all particles with diameters less than or equal to 10 micrometers. Measurements below are provided in parts per million (ppm), parts per billion (ppb), or in micrograms per cubic meter (µg/m3) (1,000 ppb = 1 ppm). Primary standards are to protect health, and secondary standards are to to protect the public welfare. Standards are subject to change, but in fact rarely do. Standards in mid-2011 include:

• CO2: 8-hour primary standard at 9 parts per million (ppm) and a 1-hour primary standard at 35 ppm
• O3: 8-hour primary and secondary standard at 0.08 ppm (1997 standard), lowered to 0.075 (2008 standard); 1-hour primary and secondary standard of 0.12 ppm
• Pb: rolling three-month average primary and secondary standards of 0.15 µg/m3
• NO2: annual average of 0.053 ppm
• PM2.5: annual primary and secondary standards of 15.0 µg/m3; 24-hour primary and secondary standards of 35.0 µg/m3

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• PM 10: 24-hour primary and secondary standards of 150 µg/m3
• SO2: 24-hour primary standard at 0.140 ppm and a primary annual average standard at 0.030 ppm.

See category definitions and the EPA's air trends for additional information. 

Note that data from exceptional events are not included and that data represent the quality of air in the vicinity of the monitoring site and, for some pollutants, may not necessarily represent urban-wide air quality. Data may not be reported in all years.