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Pollutant Types

In the U.S., substances in the air that are harmful are classified in two ways, as criteria pollutants or as hazardous air pollutants (HAPs). Below is a discussion of each type. For more information, visit the Clean Air World site for general information on pollutants and links to numerous air pollutants.

Criteria Pollutants

U.S. EPA uses six "criteria pollutants" as indicators of air quality, and has established for each of them a maximum concentration above which adverse effects on human health may occur. These threshold concentrations are called National Ambient Air Quality Standards (NAAQS). The six criteria pollutants are:

  • Particulate matter
  • Ground-level ozone
  • Nitrogen dioxide
  • Carbon monoxide
  • Sulfur dioxide
  • Lead

When an area does not meet the air quality standard for one of the criteria pollutants, it may be subject to the formal rule-making process which designates it as non-attainment. The Clean Air Act further classifies ozone, carbon monoxide, and some particulate matter non-attainment areas based on the magnitude of an area's problem. Non-attainment classifications may be used to specify what air pollution reduction measures an area must adopt, and when the area must reach attainment. A major issue related to two of the criteria pollutants is acid rain because sulfur dioxide (SO2) and nitrogen oxides (NOx) are the primary causes of acid rain. See U.S. EPA's Acid Rain Program site for more information.

Hazardous Air Pollutants

Toxic air pollutants, also known as hazardous air pollutants (HAPs), are those pollutants that are known or suspected to cause cancer or other serious health effects, such as reproductive effects or birth defects, or adverse environmental effects. The U.S. EPA is working with state, local, and tribal governments to reduce air toxics releases of 188 pollutants to the environment. Examples of toxic air pollutants include benzene, which is found in gasoline; perchlorethlyene, which is emitted from some dry cleaning facilities; and methylene chloride, which is used as a solvent and paint stripper by a number of industries. Examples of other listed air toxics include dioxins, asbestos, toluene, and metals such as cadmium, mercury, chromium, and lead compounds.

Until the past few decades, air pollution was mostly considered on the local, urban level. It is now widely recognized that air pollution is not only a regional and national issue, but also international as air pollutants can travel great distances. The importance of the transboundary effects of air pollution has been explored by the United Nations in their Convention on Long-Range Transboundary Air Pollution and the Co-operative Program for Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe (EMEP). In addition, the U.S. has formed agreements with both Canada and Mexico related to transboundary pollution. Another major air quality pollution concern involves global warming or greenhouse gases.


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