Criteria Pollutants

Criteria pollutants are pollutants for which National Ambient Air Quality Standards (NAAQS) have been established, under the federal Clean Air Act (CAA), in order to facilitate maintenance at low levels to protect public health. The criteria pollutants are: Carbon Monoxide, Lead, Nitrogen Dioxide, Ozone, Particulate Matter, and Sulfur Dioxide. The U.S. EPA can set both primary and secondary NAAQS for criteria pollutants.

Table of Contents

Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless gas that is regulated under the federal CAA as one of the six “criteria” pollutants. Carbon monoxide is primarily emitted from combustion processes and the majority of emissions come from mobile sources, especially in urban areas. It is considered very dangerous and harmful to human health and the environment because it can reduce the amount of oxygen delivered to the body’s organs, like the heart and the brain. At very high levels, it can lead to death. People with several types of heart disease may be more severely affected by short-term carbon monoxide exposure.


Lead (Pb) is a metal that is both naturally occurring and found in manufactured products, and is also one of the six “criteria” pollutants regulated under the federal CAA. Sources of lead include smelters, mining operations, waste incinerators, battery recycling, and the production of lead shot and fishing sinkers. Lead is also released by burning coal, oil, or solid waste. Lead-based paint may also be present in older homes. It is toxic to both humans and animals, with small children being at the highest risk for lead poisoning.

Nitrogen Dioxide

Nitrogen dioxide (NO2) is part of a group of highly reactive gases known as oxides of nitrogen (NOx) and is harmful to human health and the environment. Nitrogen dioxide is emitted from many sources when fuel is burned at high temperatures, including industrial, commercial and residential combustion, motor vehicles, and electric utilities. For this reason, individuals who spend time on or near major roadways are at higher risk for short-term exposure. Nitrogen dioxide is regulated as one of the six “criteria” pollutants under the federal CAA and is harmful to human health, particularly the respiratory system.


Ozone (O3), or “ground-level ozone”, is one of the six major pollutants identified by scientists as being harmful to both humans and the environment. Ground-level ozone, which is regulated by the federal CAA, is emitted by a chemical reaction between oxides of nitrogen (NOx) and volatile organic compounds (VOCs). Major contributors of NOx and VOCs that lead to the creation of ozone include motor vehicle exhaust from cars, trucks, off-road vehicles, agricultural and construction vehicles, industrial emissions, gasoline vapors, and chemical solvents. Ground-level ozone can inflame and damage lungs, increase the frequency of asthma attacks, and make the lungs more susceptible to infection.

Particulate Matter

Particulate matter (PM), also known as particle pollution, is a mixture of extremely small particles found in the air, including dust, dirt, soot, smoke, and liquid droplets. Two kinds of particulate matter are identified as being harmful to human health: fine particles (PM2.5), which are 2.5 micrometers in diameter or smaller and typically found in smoke or haze; or inhalable coarse particles (PM10), which are between 2.5 micrometers and 10 micrometers in diameter and can be found near roadways and dusty industries. Particulate matter is derived from many different sources, including residential combustion activities and vehicle exhaust. Particulate matter is regulated as one of the six “criteria” pollutants under the federal CAA. Exposure to particulate matter has been linked to a variety of health problems, particularly within the respiratory and cardiovascular systems.

Sulfur Dioxide

Sulfur dioxide (SO2) is part of a group of highly reactive gases known as oxides of sulfur (SOx) and is primarily derived from fossil fuel combustion at power plants and other industrial facilities. Other sources of SO2 include industrial processes like extracting metal from ore and the burning of high sulfur fuels. Sulfur dioxide is one of the six “criteria” air pollutants regulated under the federal CAA. Sulfur dioxide is considered to be harmful to human health and has been linked with many adverse health effects, particularly within the respiratory system.