Criteria Pollutants
The six common air pollutants regulated by the EPA under the Clean Air Act: ozone, PM2.5, PM10, CO, NO2, and SO2.
Detailed Explanation
Criteria pollutants are the six common air pollutants for which the EPA has established National Ambient Air Quality Standards (NAAQS) under the Clean Air Act. The term "criteria" comes from the fact that the EPA sets these standards based on health-based criteria — scientific studies documenting the pollutants' effects on health and the environment. The six criteria pollutants are: ground-level ozone (O3), particulate matter (both PM2.5 and PM10), carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), and lead (Pb). Each has both a primary standard (protecting public health) and a secondary standard (protecting public welfare, including visibility and environmental damage). Of these six, ozone and PM2.5 are by far the most widespread concerns in the modern United States — they affect the most people and are the hardest to control. CO, SO2, and lead levels have declined dramatically since the 1970s due to catalytic converters, cleaner fuels, and the elimination of leaded gasoline. The AQI is calculated based on five of the six criteria pollutants (lead is excluded from AQI reporting). AirHistory tracks all pollutants reported in the EPA's annual AQI dataset, showing which pollutant is dominant in each city and how pollutant breakdowns change over time.
Related Terms
National Ambient Air Quality Standards (NAAQS)
EPA-established limits on concentrations of six criteria pollutants that must be met nationwide to protect public health and the environment.
Clean Air Act
The primary federal law governing air quality in the United States, first enacted in 1963 and significantly amended in 1970 and 1990.
PM2.5 (Fine Particulate Matter)
Airborne particles smaller than 2.5 micrometers in diameter — small enough to penetrate deep into the lungs and enter the bloodstream.
Ozone (O3)
A reactive gas formed at ground level when sunlight triggers chemical reactions between nitrogen oxides and volatile organic compounds.
Nitrogen Dioxide (NO2)
A reddish-brown gas produced primarily by vehicle engines and power plants that contributes to smog and respiratory problems.
Sulfur Dioxide (SO2)
A pungent gas released by burning sulfur-containing fossil fuels, primarily from power plants and industrial facilities.
Carbon Monoxide (CO)
A colorless, odorless gas produced by incomplete combustion of carbon-containing fuels, primarily from vehicles and industrial processes.
Frequently Asked Questions
The six common air pollutants regulated by the EPA under the Clean Air Act: ozone, PM2.5, PM10, CO, NO2, and SO2.
Criteria pollutants are the six common air pollutants for which the EPA has established National Ambient Air Quality Standards (NAAQS) under the Clean Air Act. The term "criteria" comes from the fact that the EPA sets these standards based on health-based criteria — scientific studies documenting the pollutants' effects on health and the environment. The six criteria pollutants are: ground-level ozone (O3), particulate matter (both PM2.5 and PM10), carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), and lead (Pb).