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Technology Transfer Network - Air Toxics Web Site

Pollutants and Sources

The Pollutants

Hazardous air pollutants, also known as toxic air pollutants or air toxics, are those pollutants that cause or may cause cancer or other serious health effects, such as reproductive effects or birth defects, or adverse environmental and ecological effects. EPA is required to control 187 hazardous air pollutants. 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. Through appropriate rulemaking, the Clean Air Act list can be modified. A current list of modifications is available. Some clarification on certain pollutant aggregation (PDF) (8pp, 33k) is also available.

The Sources

Most air toxics originate from human-made sources, including mobile sources (e.g., cars, trucks, buses) and stationary sources (e.g., factories, refineries, power plants), as well as indoor sources (e.g., building materials and activities such as cleaning). There are two types of stationary sources that generate routine emissions of air toxics:

  • "Major" sources are defined as sources that emit 10 tons per year of any of the listed toxic air pollutants, or 25 tons per year of a mixture of air toxics. These sources may release air toxics from equipment leaks, when materials are transferred from one location to another, or during discharge through emission stacks or vents
  • "Area" sources consist of smaller-size facilities that release lesser quantities of toxic pollutants into the air. Area sources are defined as sources that emit less than 10 tons per year of a single air toxic, or less than 25 tons per year of a combination of air toxics. Though emissions from individual area sources are often relatively small, collectively their emissions can be of concern - particularly where large numbers of sources are located in heavily populated areas.

EPA published the initial list of "source categories" in 1992 (57FR31576 , July 16, 1992) and since that time has issued several revisions and updates to the list and promulgation schedule. For each listed source category, EPA indicates whether the sources are considered to be "major" sources or "area" sources. The 1990 Clean Air Act Amendments direct EPA to set standards for all major sources of air toxics (and some area sources that are of particular concern).


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