Criteria Pollutants
The six common air pollutants regulated under NAAQS: ground-level ozone, particulate matter, carbon monoxide, lead, sulfur dioxide, and nitrogen dioxide.
Related terms
National Ambient Air Quality Standards Federal standards set by the EPA that establish the maximum allowable levels of six common air pollutants (the criteria pollutants) considered harmful to public health and the environment. Particulate Matter Microscopic solid particles or liquid droplets suspended in air, including dust, soot, and smoke. PM2.5 (fine particles) is small enough to enter the bloodstream through the lungs. Ground-Level Ozone A harmful air pollutant formed when emissions from cars, factories, and chemicals react in sunlight. Different from the protective ozone layer high in the atmosphere—at ground level it triggers asthma and lung damage. Sulfur Dioxide A toxic gas produced mainly by burning fossil fuels at power plants and industrial facilities. Contributes to acid rain and aggravates respiratory illness. Nitrogen Dioxide A reddish-brown toxic gas, primarily a byproduct of vehicle exhaust and power plants. Inflames airways and helps form smog and acid rain. Hazardous Air Pollutants Also called air toxics. A group of 187+ chemicals known or suspected to cause cancer, birth defects, or other serious health problems—regulated separately from criteria pollutants.