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Criteria Pollutants

The Clean Air Act requires United States Environmental Protection Agency (U.S. EPA) to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants known as “criteria pollutants”. These pollutants are carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), particulate matter (PM), and sulfur dioxide (SO2). The NAAQS define allowable concentrations of these substances in ambient (outdoor) air. U.S. EPA sets primary standards for human health (including protection for sensitive populations such as individuals who suffer from asthma, young children, and the elderly) and secondary standards for public welfare (including protection against decreased visibility and damage to animals, crops, vegetation, and buildings). There is a NAAQS for each pollutant.

All states must have a State Implementation Plan (SIP), which is generally a collection of documents describing the state’s measures for complying with the NAAQS. States must demonstrate that their air quality meets the NAAQS by monitoring air quality and reporting this data to U.S. EPA.

U.S. EPA periodically reviews the NAAQS and may revise them according to the latest information concerning the pollutant’s potential health and environmental effects. When a NAAQS is revised, U.S. EPA must issue nonattainment designations for areas that fail to comply with the new standard and/or are determined to contribute significantly to nearby violations. States amend their SIPs appropriately and submit documents to U.S. EPA such as statewide Infrastructure SIPs describing the state’s legal authority and resources for NAAQS implementation; attainment demonstrations for areas that must improve air quality; and redesignations and maintenance plans for nonattainment areas that achieve the NAAQS.

Indiana’s Map of Current Nonattainment Areas [PDF] shows areas currently designated by U.S. EPA as nonattainment under the NAAQS.

Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless gas that is primarily emitted from combustion processes. The majority of CO emissions come from mobile sources (motor vehicles), especially in urban areas, and machinery that burns fossil fuel. CO is very dangerous and harmful to human health and the environment because it can reduce the amount of oxygen delivered to the body’s organs such as the heart and the brain. At very high levels, which are possible indoors and in other enclosed spaces, CO can lead to dizziness, confusion, unconsciousness and death. People with certain types of heart disease may be more severely affected by short-term exposure to elevated CO outdoors.


Lead (Pb) is a metal that is both naturally occurring and found in manufactured products. It is toxic to both humans and animals, with small children being at the highest risk for lead poisoning. Sources of lead emissions in the atmosphere 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. In older homes and buildings, lead-based paint may be present and, if not maintained or properly removed, can produce lead dust. The plumbing in some older buildings may contain lead pipes, or lead solder may have been used to fuse pipe joints.

Nitrogen Dioxide

Nitrogen dioxide (NO2) is part of a group of highly reactive gases known as oxides of nitrogen (NOx). NO2 is harmful to human health, particularly the respiratory system and contributes to the formation of other air pollutants including PM, ozone, acid rain and regional haze, which blocks natural sunlight and reduces visibility. NO2 is emitted from many sources when fuel is burned at high temperatures, including industrial, commercial and residential combustion, motor vehicles, and electric utilities. Individuals who spend time on or near major roadways are at higher risk for short-term exposure.

While all of the NOx gases are harmful to human health and the environment, U.S. EPA has determined NO2 to be of greater concern. U.S. EPA has determined that NO2 is an appropriate indicator for NOx and that measures for reducing exposure to NO2 will also reduce exposures to NOx.


Ozone (O3), or “ground-level ozone”, is identified by scientists as being harmful to both humans and the environment. Where it naturally occurs high in the atmosphere, ozone protects the earth from the sun’s ultraviolet rays. However, at ground level, ozone can inflame and damage lungs, increase the frequency of asthma attacks, and make the lungs more susceptible to infection. Ground-level ozone is not emitted directly by any one source but instead, is formed in the atmosphere by a chemical reaction between other directly emitted pollutants including 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.

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. (For comparison, the width of a human hair is approximately 70 micrometers.) Particulate matter is derived from many different sources, including residential combustion activities and vehicle exhaust. 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 gaseous pollutants known as oxides of sulfur (SOx) and is emitted primarily by industrial furnaces or power plants burning coal or oil-containing sulfur. Other sources of SO2 include industrial processes such as extracting metal from ore and the burning of high sulfur fuels. SO2 is considered harmful to human health and has been linked with many adverse health effects, particularly within the respiratory system. In the atmosphere, SO2 pollution damages vegetation and contributes to harmful PM, regional haze, and acid rain. While all of the SOx gases are harmful to human health and the environment, U.S. EPA has determined SO2 to be of greater concern. U.S. EPA has determined that SO2 is an appropriate indicator for SOx and that measures for reducing exposure to SO2 will also reduce exposures to SOx.

More Information

Additional information about criteria air pollutants is available on U.S. EPA’s Criteria Air Pollutants page.