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Toxic Pollutants

Note: EPA no longer updates this information, but it may be useful as a reference or resource.

Please see www.epa.gov/airtrends for the latest information on Air Quality Trends.

Factory Smoke

Nature and Sources: Toxic air pollutants are those pollutants that are known or suspected to cause cancer or other serious health effects, such as reproductive effects or birth defects, or to cause adverse environmental and ecological effects. The EPA is required to address 188 toxic air pollutants. Examples of toxic air pollutants include benzene, found in gasoline; perchloroethlene, emitted from some dry cleaning facilities; and methylene chloride, used as a solvent and paint stripper by a number of industries. Some air toxics are released from natural sources such as volcanic eruptions and forest fires. Most, however, originate from human-made sources, including both mobile sources (ie., cars, trucks, buses) and stationary sources (ie., factories, refineries, power plants).

Control of toxic air pollutants differs from the control of the six widely monitored or so-called "criteria" pollutants for which EPA has established national air quality standards. For the six pollutants, the Clean Air Act requires states to develop plans to meet the air quality standards by specific deadlines. In contrast, for toxic air pollutants, the Act requires U.S. EPA to identify all sources that emit these pollutants and develop national standards to significantly reduce their emissions. The objective is to ensure that major sources of toxic air pollution are well controlled regardless of geographic location.

The toxic air pollutant program is especially important in reducing emissions at or near industrial locations and in controlling pollutants that are toxic even when emitted in small amounts. Companies handling toxic chemicals are required by EPA to develop plans to prevent accidental releases and to contain any releases in the event they should occur.

EPA's program to control toxics, ozone, and PM complement each other. Many toxic air pollutants are emitted in the form of particulates or as VOC. For example, EPA's final toxic air pollutant regulation for organic chemical manufacturing is not only expected to reduce benzene and other toxics, but also VOC emissions by an amount equivalent to removing millions of cars from the road.

Health and Environmental Effects: People exposed to toxic air pollutants at sufficient concentrations and for sufficient durations significantly increase their chances of getting cancer or experiencing other serious health effects. These health effects can include damage to the immune system, as well as neurological, reproductive (ie., reduced fertility), developmental, and respiratory problems. A growing body of evidence suggests that some air toxics disturb hormonal (or endocrine) systems, although it is not yet clear if this occurs at levels typically seen in the environment. Health effects associated with endocrine disruption include reduced fertility, birth defects, and breast cancer.

Toxic pollutants in the air or deposited on soils or surface waters can have a number of environmental impacts. Like humans, animals experience health problems if exposed to sufficient concentrations of air toxics over time. Numerous studies show that deposited air toxics contribute to birth defects, reproductive failure, and disease in animals. Persistent toxic air pollutants are of particular concern in aquatic ecosystems because the pollutants accumulate in sediments and can accumulate in tissues of animals at the top of the food chain. Toxic pollutants that disrupt the endocrine system also pose a potential threat. In some wildlife, for example, exposures to pollutants such as DDT, dioxins, and mercury have been associated with decreased fertility, decreased hatching success, damaged reproductive organs, and altered immune systems.

Trends in Toxic Air Pollutants: EPA is using the National Toxics Inventory (NTI) to track nationwide emissions trends for toxic air pollutants listed in the Clean Air Act. The NTI includes emissions information for 188 hazardous air pollutants from more than 900 stationary sources. There are approximately 3.7 million tons of air toxics released to the air each year according to NTI. As illustrated in the chart below, NTI includes emissions from large industrial or "point" sources, smaller stationary sources called "area" sources, and mobile sources. The NTI estimates of the area source and mobile source contributions to the national emissions of toxic air pollutants are approximately 35 and 41 percent, respectively.

Area, point, and mobile sources emit roughly equal proportions
to the total amount of toxic air pollutants
According to National Toxics Inventory data, area sources account for 34.6 percent of U.S. toxic emissions, mobile sources account for 41.5 percent, and point sources account for 23.9 percent.
Selected VOC Concentration Changes
From PAMS Sites (Summer 1994-1996)

These substances have high ozone forming potential and/or they are toxic
Pollutant Number of Sites Median % Change
Acetaldehyde 7 -35
Benzene 19 -41
Ethylene 19 -26
Formaldehyde 7 -26
M/P-Xylene 18 -18
O-xylene 19 -29
Toluene 19 -26

Data from the Toxic Release Inventory (TRI), a product of the Community Right-to-Know Act of 1986, were used as the foundation of NTI. The TRI data alone represent less than half of the total emissions from the point source category, with no data for mobile and area sources. Therefore, the NTI has incorporated other emissions data to create a more complete inventory.

As of January 1998, EPA has issued 23 air toxics standards under the Clean Air Act. These standards affect 48 categories of major industrial sources, such as chemical plants, oil refineries, aerospace manufacturers, and steel mills, as well as eight categories of smaller sources, such as dry cleaners, commercial sterilizers, secondary lead smelters, and chromium electroplating facilities. EPA has also issued two standards to control emissions from solid waste combustion facilities. Together these standards reduce emissions of over 100 different air toxics. When fully implemented, these standards will reduce air toxics emissions by about 1 million tons per year - almost ten times the reductions achieved prior to 1990. By the year 2005, EPA projects that the toxic air pollutant program will reduce toxic emissions by 75 percent. Because controls for toxic air pollutants also reduce VOC and PM emissions the program should realize reductions in VOC and PM emissions of more than 4 billion pounds per year, over the next ten years.

EPA collects data on concentrations of ozone and its precursors in 21 areas across the nation with the most significant ozone problems. The PAMS program requires routine measurement of ten toxic air pollutants: acetaldehyde, benzene, ethyl benzene, formaldehyde, hexane, styrene, toluene, m/p-xylene, o-xylene, and 2,2,4-trimethlypentane. Analysis of selected toxic air pollutants in PAMS areas indicate that concentrations of certain toxic VOCs appear to be declining. For example, as illustrated in the table, benzene levels showed a significant decline between 1994 and 1996 (approximately 41 percent), possibly as a result of the use of reformulated gasoline in those areas. It should be noted that PAMS measurements have only been taken for 4 years and that continued efforts in the PAMS program are expected to provide more confidence in evaluating the long-term trends of benzene and other toxic VOCs.


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