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Toxic Air Pollutants

Air pollution can be transported hundreds of miles downwind from its origin.

Since air pollutants do not recognize political boundaries, states and communities cannot independently solve all of their air pollution problems.

Each individual breathes nearly 13,000 liters (approximately 3,400 gallons) of air every day. Yet the air is being polluted by human activities like driving cars, burning fossil fuels, and manufacturing chemicals, and natural events such as forest fires. These add gases and particles to the air we breathe and, in high enough concentrations, can have harmful effects on people and the environment. Many air pollutants such as those that form urban smog, acid rain, and some toxic compounds remain in the environment for long periods of time and can be transported great distances from their origin.

The struggle for clean air is almost as old as industrialized society. In 1661, John Evelyn and John Graunt of England each published studies associating negative health effects with industrial air emissions. Both researchers described the transport of pollutants between England and France and suggested protecting human health by locating industrial facilities outside of towns and using taller smokestacks to spread "smoke" into "distant parts."

Research continues to show that air pollution can be carried hundreds of miles from its source and can cause health and environmental problems on a regional or even global scale. In people, air pollution can cause burning eyes, irritated throats, difficulty with breathing, long-term damage to the respiratory and reproductive systems, cancer, and, in extreme cases, death. Trees, lakes, crops, buildings, and statues can be damaged by air pollution. Air pollutants also cause haze, impairing visibility in cities, national parks, and other scenic areas.

Under the Clean Air Act, passed by Congress in 1970 and recently amended in 1990, the U.S. Environmental Protection Agency (EPA) sets and enforces air pollutant limits on sources such as power plants and industrial facilities to help protect against harmful health and environmental effects. Although the Clean Air Act is a Federal law, state and local agencies are responsible for implementing many of its requirements.

Specific air pollutants such as sulfur dioxide (SO2), particulate matter, ground-level ozone, and the emissions that form these pollutants can travel great distances from their sources. Since air pollutants do not recognize political boundaries, states and communities cannot independently solve all of their air pollution problems. Resolving air pollution control issues often requires state and local governments to work together to reduce air emissions. The Clean Air Act established groups such as the Ozone Transport Commission in the northeastern U.S. and the Grand Canyon Visibility Transport Commission in the western U.S. to develop regional strategies to address and control air pollution. Many other such groups have also been formed to address the regional transport of air pollutants.

Within the next 10 years, the national toxic air pollutant program is expected to lower emissions of toxic pollutants 75 percent and thus reduce adverse human health and ecosystem effects.

Toxic air pollutants are known to cause or are suspected of causing cancer, adverse reproductive, developmental, and central nervous system effects, and other serious health problems. The Clean Air Act lists 188 toxic air pollutants as hazardous. Examples of toxic air pollutants include heavy metals like mercury and lead, manmade chemicals like polychlorinated biphenyls (PCB), polycyclic organic matter (POM), dioxin and benzene, and pesticides like dichlorodiphenyl-trichloroethane (DDT). Some toxic air pollutants remain in the environment for only short periods of time. These pollutants, including compounds such as formaldehyde, toluene, and benzene, generally impact human health and the environment near emission sources. Other toxic air pollutants, such as lead, mercury, PCB, and DDT, break down slowly, if at all, in the environment and can be redeposited many times. Additionally, they build up in the body and concentrate as they rise through the food chain. Many of these "persistent" pollutants, emitted from various sources including motor vehicles and industrial facilities, are appearing in unexpected locations far away from their sources, including the Great Lakes, Lake Champlain, and the Chesapeake Bay.

Reducing Toxic Air Pollutants

EPA has identified 174 categories of sources that emit one or more of the 188 toxic air pollutants. These sources will be required to reduce emissions over the next 10 years. Since 1990, EPA's toxic air pollutant program has issued a number of rules to control toxic air releases from approximately 50 categories of sources. These include large industrial complexes such as chemical plants, oil refineries, and steel mills and smaller sources such as dry cleaners and commercial sterilizers. One of these rules applies to the organic chemical manufacturing industry, which produces chemicals used in many industrial processes. This rule alone will reduce emissions of toxic air pollutants by over half-a-million tons annually (a 90 percent reduction) and will lower smog-causing VOC by about 1 million tons annually (an 80 percent reduction). Within the next 10 years, EPA's national program is expected to lower emissions of toxic air pollutants 75 percent.


Metals and other toxic air pollutants that persist in the environment and are transported over broad regions come from a variety of sources. Mercury, for example, is a toxic metal that comes from both natural and manmade sources. Coal-fired power plants, municipal waste incinerators, medical waste incinerators, and cement kilns that burn hazardous waste or coal are among the major manmade sources of mercury. Natural sources of atmospheric mercury include gases released from the Earth's crust by geysers, volcanic eruptions, and forest fires. PCB are industrial chemicals used widely in the U.S. from 1929 until 1978 as coolants and lubricants and in electrical equipment. The manufacture of PCB in the U.S. stopped in 1977, and use was restricted in 1979. POM includes a number of cancer-causing products of incomplete combustion and can come from diesel engines and other motor vehicles, wood burning, and industrial burning of fossil fuels. DDT is an insecticide that was widely used in this country from 1946 until 1972. DDT is still used in other countries and, by special permit, in the U.S. Many VOC and fine particulates are also toxic air pollutants. Controlling air concentrations of ozone and particulate matter has the added benefit of reducing toxic air pollutants.

Health & Environmental Effects

At certain levels, toxic air pollutants can cause human health effects ranging from nausea and difficulty in breathing to cancer. Health effects can also include birth defects, serious developmental delays in children, and reduced immunity to disease in adults and children. Toxic air pollutants can also be deposited onto soil or into lakes and streams where they affect ecological systems and can eventually affect human health when consumed in contaminated food, particularly fish.

For example, people who regularly consume fish from the Great Lakes have been found to have higher concentrations of PCB, DDT, and other toxic chemicals in their bodies than people who do not. Fish-eating birds, mammals, and reptiles have experienced a variety of adverse effects associated with chemical pollution.

Long-Range Transport

Scientific studies conducted over the past 30 years consistently indicate that toxic air pollutants can be deposited at locations far from their sources. For example, a number of toxic air pollutants persist in the environment and concentrate through the food web, including toxaphene, a pesticide used primarily in the cotton belt, and have been found in fatty tissues of polar bears and other Arctic animals thousands of miles from any possible source. Lead and other trace metals have been measured in the air and rainfall at remote locations over the Atlantic and Pacific Oceans, great distances from likely sources. Core samples from peat bogs in the Great Lakes region show deposition of new releases of DDT. Since DDT is used only under special conditions in the U.S., this toxic compound may be originating from sources as far away as Mexico or Central America. Fortunately, Mexico has recently banned the use and production of DDT.

Toxic air pollutants can be deposited onto soil or into lakes and streams, where they affect ecological systems and can eventually affect human health when consumed in contaminated food, particularly fish.

U.S. Environmental Protection Agency
1200 Pennsylvania Avenue, NW
Washington, DC 20460