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Taking Toxics Out of the Air

Part 1 - Main Body of Brochure

The technology- and performance-based standards issued by EPA over the past 10 years have proven extremely successful. Once fully implemented, these standards will cut emissions of toxic air pollutants by nearly 1.5 million tons per year-almost 15 times greater reductions than EPA was able to achieve in 20 years prior to 1990.


The air we breathe can be contaminated with pollutants from factories, vehicles, power plants, and many other sources. These pollutants have long been a major concern because of the harmful effects they sometimes have on people’s health and the environment. Their impact depends on many factors, including the quantity of air pollution to which people are exposed, the duration of the exposures, and the potency of the pollutants. The effects of air pollutants can be minor and reversible (such as eye irritation) or debilitating (such as aggravation of asthma) and even fatal (such as cancer).

Since 1970, the Clean Air Act has provided the primary framework for protecting people and the environment from the harmful effects of air pollution. A key component of the Clean Air Act is a requirement that the U.S. Environmental Protection Agency (EPA) significantly reduce daily, so-called "routine" emissions of the most potent air pollutants: those that are known or suspected to cause serious health problems such as cancer or birth defects. The Clean Air Act refers to these pollutants as "hazardous air pollutants," but they are also commonly known as toxic air pollutants or, simply, air toxics.

Prior to 1990, the Clean Air Act required EPA to set standards for each toxic air pollutant individually, based on its particular health risks. This approach proved difficult and minimally effective at reducing emissions. As a result, when amending the Clean Air Act in 1990, Congress directed EPA to use a "technology-based" and performance-based approach to significantly reduce emissions of air toxics from major sources of air pollution, followed by a risk-based approach to address any remaining, or residual, risks.

Under the "technology-based" approach, EPA develops standards for controlling the "routine" emissions of air toxics from each major type of facility within an industry group (or "source category"). These standards—known as "maximum achievable control technology (MACT) standards"—are based on emissions levels that are already being achieved by the better-controlled and lower-emitting sources in an industry. This approach assures citizens nationwide that each major source of toxic air pollution will be required to employ effective measures to limit its emissions. Also, this approach provides a level economic playing field by ensuring that facilities that employ cleaner processes and good emission controls are not disadvantaged relative to competitors with poorer controls.

In setting MACT standards, EPA does not generally prescribe a specific control technology. Instead, whenever feasible, the Agency sets a performance level based on technology or other practices already used by the industry. Facilities are free to achieve these performance levels in whatever way is most cost-effective for them. The MACT standards issued by EPA over the past 10 years have proven extremely successful. Once fully implemented, these standards will cut emissions of toxic air pollutants by nearly 1.5 million tons per year.

Eight years after each MACT standard is issued, EPA must assess the remaining health risks from source categories. If necessary, EPA may implement additional standards that address any significant remaining risk.

Sources of Air Toxics: Routine Emissions from Stationary Sources, Mobile Sources, Accidental Releases, and Forest Fires. Each year, millions of toxic polluntants are released into the air from both natural and man-made sources.

What Are Toxic Air Pollutants?

Toxic (also called hazardous) 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 effects. The degree to which a toxic air pollutant affects a person’s health depends on many factors, including the quantity of pollutant the person is exposed to, the duration and frequency of exposures, the toxicity of the chemical, and the person’s state of health and susceptibility.

The 1990 Clean Air Act Amendments list 188 toxic air pollutants that EPA is required to control. (The list originally included 189 chemicals. Based on new scientific information, EPA removed caprolactam from the list in 1996; thus, the current list includes 188 pollutants.) Examples of toxic air pollutants include benzene, which is found in gasoline; perchloroethylene, 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 dioxin, asbestos, toluene, and metals such as cadmium, mercury, chromium, and lead compounds.

Mobile Sources and Accidental Releases

While this document focuses on EPA's efforts to reduce routine emissions from stationary sources, EPA also is working to reduce toxic emissions from:

  • Mobile sources, such as cars and trucks. For example, EPA and state governments (e.g., California) have reduced emissions of benzene, toluene, and other toxic pollutants from mobile sources by requiring the use of reformulated gasoline and placing limits on tailpipe emissions. For more information, contact EPA's Office of Transportation and Air Quality at www.epa.gov/OMSWWW/toxics.htm or call (202) 564-1682.

  • Accidental releases, including leaks and spills. For example, EPA has established regulations under the Clean Air Act requiring certain facilities to implement risk management programs that will help prevent accidental releases of toxic chemicals. For more information, contact EPA's Office of Chemical Emergency Preparedness and Prevention at www.epa.gov/swercepp or call (800) 424-9346.

Where Do Air Toxics Come From?

Scientists estimate that millions of tons of toxic pollutants are released into the air each year. Most air toxics originate from manmade sources, including both mobile sources (e.g., cars, buses, trucks) and stationary sources (e.g., factories, refineries, power plants). However, some are released in major amounts from natural sources such as forest fires. This document focuses on EPA's efforts, as of August 2000, to reduce routine (as opposed to accidental) emissions of toxic air pollutants from stationary sources. Routine emissions from stationary sources constitute almost one-half of all manmade air toxics emissions.

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. Examples include chemical plants, steel mills, oil refineries, and hazardous waste incinerators. These sources may release air toxics from equipment leaks, when materials are transferred from one location to another, or during discharge through emissions stacks or vents. One key public health concern regarding major sources is the health effects on populations located downwind from them.

  • "Area" sources consist of smaller sources, each releasing smaller amounts 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 mixture of air toxics. Examples include neighborhood dry cleaners and gas stations. 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’s published list of "source categories" now contains 175 categories of industrial and commercial sources that emit one or more toxic air pollutants. For each of these source categories, EPA indicated whether the sources are considered to be "major" sources or "area" sources. The 1990 Clean Air Act Amendments direct EPA to set standards requiring all major sources of air toxics (and some area sources that are of particular concern) to significantly reduce their air toxics emissions.

Pie Chart, Mobile Source 50%, Major Source, 25%, Area Source, 25%, Based on 1996 National Toxics Inventory data, major sources account for about 25 percent of air toxics emissions, area sources for 25 percent, and mobile sources for 50 per-cent.  Accidental releases and natural sources, which also contribute air toxics to the atmosphere, are not included these estimates.
Based on 1996 National Toxics Inventory data, major sources account for about 26 percent of air toxics emissions, smaller area sources and other sources (such as forest fires) for 24 percent, and mobile sources for 50 percent. Accidental releases, which also contribute air toxics to the atmosphere, are not included in these estimates.

Where Do Air Toxics Go?

Once released, toxic pollutants can be carried by the wind, away from their sources, to other locations. Factors such as weather, the terrain (i.e., mountains, plains, valleys), and the chemical and physical properties of a pollutant determine how far it is transported, its concentration at various distances from the source, what kind of physical and chemical changes it undergoes, and whether it will degrade, remain airborne, or deposit to land or water.

Some pollutants remain airborne and contribute to air pollution problems far from the pollution source. Other pollutants released into the air can be deposited to land and water bodies through precipitation, or by settling directly out of the air onto land or water. Eventually, a large portion of those pollutants deposited near water bodies or small tributaries will reach the water bodies via stormwater runoff or inflow from the tributary streams.

Some toxic air pollutants are of particular concern because they degrade very slowly or not at all, as in the case of metals such as mercury or lead. These persistent air toxics (as they are called) can remain in the environment for a long time (or forever, in the case of metals) and can be transported great distances.

Toxic air pollutants can be deposited to land and water bodies through precipitation (wet deposition) or by settling directly out of the air (dry deposition). Repeated cycles of transport, deposition, and evaporation can move toxic air pollutants very long distances.
Toxic air pollutants can be deposited to land and water bodies through precipitation (wet deposition) or by settling directly out of the air (dry deposition). Repeated cycles of transport, deposition, and evaporation can move toxic air pollutants very long distances.

Often, persistent air toxics reach the ground, evaporate back into the atmosphere, and are then transported further until they are deposited on the ground again. Repeated cycles of transport, deposition, and evaporation can move toxic air pollutants very long distances. For example, toxic pollutants such as toxaphene, a pesticide used primarily in the cotton belt, have been found in the Antarctic, thousands of miles from their likely emissions sources.

How Are People Exposed to Air Toxics?

People are exposed to toxic air pollutants in many ways that can pose health risks, such as by:

  • Breathing contaminated air.

  • Eating contaminated food products, such as fish from contaminated waters; meat, milk, or eggs from animals that fed on contaminated plants; and fruits and vegetables grown in contaminated soil on which air toxics have been deposited.

  • Drinking water contaminated by toxic air pollutants.

  • Eating contaminated soil. Young children are especially vulnerable because they may ingest contaminated soil from their hands or from objects they place in their mouths.

  • Touching (skin contact) contaminated soil, dust, or water (for example, during recreational use of contaminated water bodies).

Once toxic air pollutants enter the body, some persistent toxic air pollutants accumulate in body tissues. Also, through a phenomenon called biomagnification, predators typically accumulate even greater pollutant concentrations than their contaminated prey. As a result, people and other animals at the "top" of the food chain who eat contaminated fish or meat are exposed to concentrations that are much higher than the concentrations in the water, air, or soil.

Fish consumption advisories have been issued for thousands of water bodies nationwide, including over 52,000 lakes and over 238,000 miles of rivers. As of December 1999, 40 states have consumption advisories about mercury-contaminated fish for specific water bodies. Eleven of those states have issued state-wide advisories for freshwater lakes and rivers. Many of these water bodies were once thought to be relatively pristine. However, EPA is now finding that deposition from the air may be a major source of the pollution in these water bodies.

Health Effects

People who are exposed to toxic air pollutants at sufficient concentrations and for sufficient durations may increase their chances of getting cancer or experiencing other serious health effects. Depending on which air toxics an individual is exposed to, these health effects can include damage to the immune system, as well as neurological, reproductive (e.g., reduced fertility), developmental, and respiratory problems. A growing body of evidence indicates that some air toxics (e.g., DDT, dioxins, and mercury) may disturb hormonal (or endocrine) systems. In some cases this happens by pollutants either mimicking or blocking the action of natural hormones. Health effects associated with endocrine disruption include reduced male fertility, birth defects, and breast cancer.

How Do Air Toxics Affect the Environment?

Toxic pollutants in the air, or deposited on soils or surface waters, can have a number of environmental impacts. Like humans, animals can experience health problems if they are exposed to sufficient concentrations of air toxics over time. Numerous studies conclude that deposited air toxics are contributing 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 may biomagnify in tissues of animals at the top of the food chain to concentrations many times higher than in the water or air.

Toxic pollutants that mimic hormones also pose a threat to the environment. In some wildlife (e.g., birds, shellfish, fish, and mammals), 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.

What Has EPA Done to Reduce Air Toxics?

The Pre-1990 "Risk-Only" Approach

Prior to 1990, the Clean Air Act directed EPA to regulate toxic air pollutants based on the risks each pollutant posed to human health. Specifically, the Act directed EPA to:

  • Identify all pollutants that caused "serious and irreversible illness or death."

  • Develop standards to reduce emissions of these pollutants to levels that provided an "ample margin of safety" for the public.

While attempting to control air toxics during the 1970s and 1980s, EPA became involved in many legal, scientific, and policy debates over which pollutants to regulate and how stringently to regulate them. Debates focused on risk assessment methods and assumptions, the amount of health risk data needed to justify regulation, analyses of the costs to industry and benefits to human health and the environment, and decisions about "how safe is safe."

During this time, EPA was still developing methods to assess risk. These methods were essential tools that would be needed to establish the scientific basis for making risk-based decisions about air toxics. While EPA and the scientific community gained valuable knowledge about risk assessment methods through this work, the chemical-by-chemical regulatory approach—an approach based solely on risk—proved difficult. In fact, in 20 years, EPA regulated only seven pollutants (asbestos, benzene, beryllium, inorganic arsenic, mercury, radionuclides, and vinyl chloride). Collectively, these standards cut annual air toxics emissions by an estimated 125,000 tons.

The 1990 Clean Air Act Amendments:
A "Technology First, Then Risk" Approach

Realizing the limitations of a chemical-by-chemical decision framework based solely on risk, and acknowledging the gaps in scientific and analytical information, Congress adopted a new strategy in 1990, when the Clean Air Act was amended. Specifically, Congress revised Section 112 of the Clean Air Act to mandate a more practical approach to reducing emissions of toxic air pollutants.

This approach has two components. In the first phase, EPA develops regulations—MACT standards—requiring sources to meet specific emissions limits that are based on emissions levels already being achieved by many similar sources in the country. Even in its earliest stages, this new "technology-based" approach clearly produced real, measurable reductions. In the second phase, EPA applies a risk-based approach to assess how these technology-based emissions limits are reducing health and environmental risks. Based on this assessment, EPA may implement additional standards to address any significant remaining, or residual, health or environmental risks. EPA completed development of its strategy for addressing residual risks from air toxics in March of 1999.

Maximum Achievable Control Technology

EPA's MACT standards are based on the emissions levels already achieved by the best-performing similar facilities. This straightforward, performance-based approach yields standards that are both reasonable and effective in reducing toxic emissions. This approach also provides a level economic playing field by ensuring that facilities with good controls are not disadvantaged relative to competitors with poorer controls.

When developing a MACT standard for a particular source category, EPA looks at the level of emissions currently being achieved by the best-performing similar sources through clean processes, control devices, work practices, or other methods. These emissions levels set a baseline (often referred to as the "MACT floor" ) for the new standard. At a minimum, a MACT standard must achieve, throughout the industry, a level of emissions control that is at least equivalent to the MACT floor. EPA can establish a more stringent standard when this makes economic, environmental, and public health sense.

The MACT floor is established differently for existing sources and new sources:

  • For existing sources, the MACT floor must equal the average emissions limitations currently achieved by the best-performing 12 percent of sources in that source category, if there are 30 or more existing sources. If there are fewer than 30 existing sources, then the MACT floor must equal the average emissions limitation achieved by the best-performing five sources in the category.

  • For new sources, the MACT floor must equal the level of emissions control currently achieved by the best-controlled similar source.

Wherever feasible, EPA writes the final MACT standard as an emissions limit (i.e., as a percent reduction in emissions or a concentration limit that regulated sources must achieve). Emissions limits provide flexibility for industry to determine the most effective way to comply with the standard.

What Progress Has Been Made in Reducing Toxic Air Pollution?

As of August 2000, EPA has issued 45 air toxics MACT standards under Section 112 of the Clean Air Act Amendments. These standards affect 82 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 under Section 129 of the Clean Air Act to control emissions, including certain toxic pollutants, from solid waste combustion facilities (one standard for municipal waste combustors and the other for medical waste incinerators). Together, these standards reduce emissions of over 100 different air toxics. When fully implemented, all of these standards will reduce air toxics emissions by about 1.5 million tons per year—almost 15 times the reductions achieved prior to 1990. Each of the final rules developed since 1990 is summarized in an appendix to this document. These summaries describe the sources for which final rules have been issued as of August 2000, the types of pollutants the sources emit, and how EPA’s rules are reducing their emissions.

Some of these air toxics rules have the added benefit of reducing ground-level ozone (urban smog) and particulate matter. This occurs because some air toxics are also smog-causing volatile organic compounds (VOCs) (e.g., toluene) or particulate matter (e.g., chromium). In addition, some of the technologies and practices designed to control air toxics also reduce VOCs or types of particulate matter that are not currently among the 188 listed air toxics. Reductions of smog-causing pollutants and particulate matter are important because of the health and environmental problems they can cause. Most notably, urban smog can cause respiratory problems and can damage vegetation, and particulate matter can cause many detrimental impacts on human health, such as bronchitis, lung damage, increased infection, aggravation of asthma, and premature death. In addition many of these pollutants can contribute significantly to impaired visibility in places, such as national parks, that are valued for their scenic views and recreational opportunities.

EPA has consistently worked to develop air toxics standards that achieve the required reductions in air pollution while providing regulated communities with as much flexibility as possible in deciding how to comply with the standards. For example, under a flexible regulation, industries may reduce their emissions by redesigning their processes, capturing and recycling emissions, changing work practices, or installing any of a variety of control technologies. Flexibility helps industries minimize the cost of compliance and encourages pollution prevention. To provide flexibility, EPA makes every effort to develop standards that are based on performance measures rather than specific control devices, and that allow for equivalent alternative control measures.

Looking Ahead

To date, EPA has primarily focused efforts to reduce emissions of toxic air pollutants on technology-based or MACT emission standards. Over the next few years, EPA will continue to work with industry; environmental groups; state, local, and tribal agencies; and other interested groups to develop standards for the remaining source categories that will reduce air toxics emissions even further. By 2002, EPA is scheduled to issue 62 technology-based standards covering 96 remaining source categories.

EPA anticipates that its technology-based approach will continue to prove successful at reducing air toxics. Additional air toxics reductions are also expected to indirectly reduce toxics. To identify additional measures beyond the technology standards that may be needed to protect the public health and the environment from toxic air pollutants, EPA will use a more risk-based focus. EPA’s efforts underway include national air toxics assessment activities, residual risk standards, evaluation of the impacts of air toxics deposition, data-gathering on mercury emissions from coal-fired electric utilities, and implementation of an urban air toxics strategy. These efforts are explained below.

National Air Toxics Assessment

The National Air Toxics Assessment is an ongoing comprehensive evaluation by EPA of air toxics in the United States. EPA, states, and others are working to improve the national toxics inventory of emissions and to expand air toxics monitoring networks to obtain more air toxics data. This work is expected to help focus future efforts to reduce air toxics and resultant health effects.

Residual Risk Standards

After EPA develops technology-based standards, the 1990 Clean Air Act Amendments require EPA to assess their effectiveness at reducing the health and environmental risks posed by air toxics. Based on this assessment, the Agency may implement additional standards that address any significant remaining, or "residual," risk. After setting a MACT standard, EPA has 8 years (9 years for the earliest standards) to examine the risk posed by continued emissions from regulated facilities and issue requirements for additional controls if necessary to reduce unacceptable residual risk. EPA has begun to assess residual risk for several source categories, including coke ovens, dry cleaning, gasoline distribution, commercial ethylene oxide sterilizers, halogenated solvent cleaning, industrial cooling towers, and magnetic tape manufacturing. The first residual risk evaluation is scheduled to be completed by EPA in 2001 for the coke oven industry.

Air Toxics Deposition to the Great Waters

Since 1990, EPA has issued three reports to Congress on the deposition of air toxics and their detrimental effects on the Great Waters (i.e., the Great Lakes, Chesapeake Bay, Lake Champlain, and coastal waters). In these reports, EPA lists 15 pollutants of greatest concern, most of which have a tendency to persist in the environment and accumulate in organisms such as fish. The pollutants of concern are: metals (mercury, cadmium, lead), dioxins, furans, polycyclic organic matter, polychlorinated biphenyls (PCBs), pesticides (such as chlordane and DDT/DDE), and nitrogen compounds. Nitrogen compounds from the deposition of air toxics can intensify nutrient enrichment (or eutrophication) of coastal waterbodies. EPA's most recent report, issued in 2000, provides an update on atmospheric deposition of pollutants to the Great Waters and identifies activities that will reduce these pollutants. Several of the MACT standards described in the appendix to this document are expected to substantially cut emissions of mercury, dioxins, and other pollutants of concern to the Great Waters from sources such as municipal waste combustors and medical waste incinerators, which alone account for almost 30 percent of the mercury emissions and over 70 percent of the dioxin emissions nationwide (1990 baselines).

Mercury Emissions from Coal-Fired Electric Utilities

Mercury is one of the 188 listed toxic air pollutants. It is of concern because it does not degrade but persists in the environment. The largest emitter of mercury is electric utility plants (primarily coal-fired plants), which are estimated to emit approximately one-third of all manmade mercury in the United States. EPA is currently gathering monitoring data on mercury emissions from coal-fired electric utility plants and expects to complete its evaluation no later than December 2000 on the need to reduce mercury from electric utilities.

Integrated Urban Air Toxics Strategy

A key component of future efforts to reduce air toxics is the Integrated Urban Air Toxics Strategy released by EPA in July 1999. The Strategy presents a framework to address air toxics in urban areas and builds on the substantial emission reductions already achieved from cars, trucks, fuels, and industries such as chemical plants and oil refineries. The Strategy outlines actions to further reduce emissions of air toxics and to improve EPA's understanding of the health risks posed by air toxics in urban areas. The goals of the Strategy are to reduce the risk of cancer by 75 percent and to substantially reduce non-cancer risks associated with air toxics from commercial and industrial sources. The Strategy also reflects the need to address any disproportionate impacts on sensitive populations including children, the elderly, and minority and low-income communities.

For Further Information

EPA Air Toxics Website:

EPA Office of Air and Radiation:
(202) 564-7400

EPA Office of Transportation
and Air Quality:
(202) 564-1682

EPA Office of Chemical Emergency
Preparedness and Prevention:

(800) 424-9346

Summaries of EPA's Final Air Toxics MACT Rules
Summaries of Related Solid Waste Incineration Rules

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