Hazardous Air Pollutants

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The U.S. Environmental Protection Agency (EPA) refers to air polluntants that can cause serious health and environmental hazards as "hazardous air pollutants" (HAPS) or "air toxics." The 1970 Clean Air Act authorized EPA to first list HAPs for regulation and then to regulate the chemicals. As of 1990, only seven chemicals (beryllium, arsenic, mercury, asbestos, benzene, vinyl chloride, and radionuclides) had been listed and regulated by the EPA. The 1990 Clean Air Act Amendment, however, included a list of 189 HAPs selected by Congress on the basis of potential health and environmental hazards. (The HAP list can be found on the EPA web site: www.epa.gov/ttn/uatw/188polls.html.) The 1990 amendments also allows EPA to add new chemicals to the HAP list as necessary. One of the major breakthroughs in the amendments is a permit program for large sources that release pollutants into the air. The permit program requires the application of maximum achievable control technology (MACT) to a nonutility source that emits over 10 tons per year of any one HAP, or 25 tons per year of any combination of HAPs.

The sources of HAPs include large stationary sources such as chemical factories and incinerators; small stationary sources such as dry cleaners and auto paint shops; and other sources such as tobacco smoking, fuel combustion, certain cooking practices, gasoline vaporization, and motor vehicle exhaust. Household products such as paints, carpets, and detergents can also release certain HAPs, and chemical reactions occurring in both outdoor and indoor air can generate HAPs.

The EPAs regulatory approach with HAPs, unlike the ambient-air-oriented approach with criteria air pollutants that are regulated through the National Ambient Air Quality Standards (NAAQS), is source oriented. The approach starts with identification of categories of sources that release the 189 HAPs listed under the 1990 amendments. Categories could be gasoline service stations, electrical repair shops, coal-burning power plants, oil refining plants, or chemical plants, and many others. The HAP producers are generally identified as "large" or "small" sources. The 1990 amendments deal with more strictly with large sources than small ones.

Mobile sources release large amount of HAPs such as benzene, formaldehyde, and toluene. Technical and regulatory efforts have been made to reduce HAPs from motor vehicles. These efforts include the use of cleaner fuels and improved engines, and the periodical examination of vehicle exhaust. The 1990 Clean Air Act amendments also requires that factories and other businesses develop plans to prevent accidental release of highly toxic chemicals. (This was largely inspired by the 1984 tragedy involving methyl isocyanate release, which killed about 4,000 people and injured more than 200,000 in Bhopal, India.) The amendments established the Chemical Safety Board to investigate and report on accidental releases of HAPs from industrial plants. However, small sources of HAPs will have to be directly or indirectly regulated as well in order to achieve effective reduction of exposure, because in many cases small sources contribute more to total human exposures than large sources. This is mainly due to the close proximity of people to small neighborhood polluters (e.g., auto shops and print shops) as well as household and personal sources (e.g., dry-cleaned clothes, deodorants).

Junfeng (Jim) Zhang

(see also: Airborne Particles; Ambient Air Quality [Air Pollution]; Automotive Emissions; Clean Air Act; Environmental Protection Agency; National Ambient Air Quality Standards; Pollution; Smog [Air Pollution]; Sulfur-Containing Air Pollutants [Particulates] )


Ott, W. R., and Roberts, J. H. (1998). "Everyday Exposure to Toxic Pollutants." Scientific American (February):8691.

Weschler, C. J.; Hodgson, A. T.; and Wooley, J. D. (1992). "Indoor Air Chemistry: Ozone, Volatile Organic Compounds, and Carpets." Environmental Science & Technology 23712377.

Zhang, J.; He, Q.; and Lioy, P. J. (1994). "Characteristics of Aldehydes: Concentrations, Sources, and Exposures for Indoor and Outdoor Residential Microenvironments." Environmental Science & Technology 28:146152.

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Hazardous Air Pollutants

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