Air toxics come from many sources, including stationary and mobile sources.Air Toxics: Overview

The air we breathe can become contaminated with pollutants from a variety of natural and man-made sources.  Since federal legislation involving ambient air quality was enacted in 1970, air pollution control has focused on criteria air pollutants, including sulfur dioxide (SO2), carbon monoxide (CO), nitrogen oxides (NOx), ozone (O3) and particulate matter, which are primarily by-products of fossil fuel combustion.   However, due to increasing scientific evidence of their effects on human health,  the focus of ambient air quality has begun to shift to include not only "criteria" air pollutants, but also "toxic" air pollutants.  Toxic air pollutants are also referred to as air toxics or hazardous air pollutants (HAPs) and are defined as those pollutants that are suspected of causing cancer or other serious health effects in humans, or of having serious adverse effects on the environment.

In order to help protect the health of Maine residents and preserve our environment, the State of Maine Department of Environmental Protection, Bureau of Air Quality (DEP) developed this web page to help enlighten Maine's public as to the hazards associated with air toxics. In addition, the US EPA developed a program to control hazardous air pollutant emissions. This program is set forth in Section 112 of the 1990 Clean Air Act Amendments (CAAAs) .  It is designed to reduce the emissions and ambient air impact of a number of toxic air pollutants likely to be used by business and industry in the state.

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What are toxic air pollutants? Map of Maine

Toxic air pollutants are those pollutants that, at sufficient concentrations and exposure cause adverse health or environmental impacts, such as respiratory disease or cancer.  In general, the toxic air pollutants that are of greatest concern are those that are released to the air in large enough amounts to create a risk to human health and that have the potential to reach many people.

Title III of the CAAAs has identified 188 hazardous air pollutants (HAPs) . The 188 HAPs consist of toxic air pollutants identified by EPA which, when their emissions are not controlled through available technology, are most likely to have the greatest  impact on ambient air quality and human health.  The list of hazardous air pollutants regulated under Maine law (06-096 CMR Ch. 137) is slightly larger as it contains dioxin and other compounds not found on EPA's list.

Toxic air pollutants may exist as particulate matter or as vapors.  Examples of gaseous toxic air pollutants include benzene, toluene and xylenes which are found in gasoline; chloroform, which is released as a byproduct of pulp & paper production, perchloroethylene, which is used in the dry cleaning industry; and methylene chloride, which is used as a solvent by a number of industries, just to name a few.  Examples of air toxics typically associated with particulate matter include heavy metals such as cadmium, mercury, chromium, and lead compounds; and semivolatile organic compounds such as polycyclic aromatic hydrocarbons (PAHs) which are generally emitted from the combustion of wastes and fossil fuels.

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What are the human and environmental effects of toxic air pollutants?

human health effects of air toxicsThe emission of toxic pollutants into the air can have serious effects to human health and the environment.  Human exposure to these pollutants can include both short-term (acute) and long-term (chronic) complications. Many factors can affect how different toxic air pollutants may impact human health, including the quantity which a person is exposed to, the development stage of the person, the duration and frequency of the exposure, the toxicity level of the pollutant, and the person's overall health and level of resistance or susceptibility.  Short-term exposures can include effects such as eye irritation, nausea or difficulty in breathing.  Long-term exposures to many air toxics may result in damage to the respiratory or nervous systems, birth defects, and reproductive effects. 

In addition, toxic air pollutants can have indirect effects on human health through deposition onto soil or into lakes and streams, potentially affecting ecological systems and eventually human health through consumption of contaminated food.

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What are the sources of air toxic pollutants?

There are many man-made sources of toxic air pollutants in Maine. These sources may be divided into three categories.

  • point sources
  • area sources; and
  • mobile sources.

Each of these source categories are described below.

A point source (also known as a major source) is a stationary source that emits point source stackor has the potential to emit 10 tons or more per year of any one of the listed HAPs, or 25 tons or more per year of combined HAPs.  Examples of point sources can include chemical plants, paper mills, large printing and coating operations, power plants and waste incinerators. These sources can release air toxics through emissions from stacks and vents, fugitive process emissions, equipment leaks during material transfer and handling, or accidental releases.  EPA estimates that 26% of all man-made air toxic emissions come from point sources.

Area sources are smaller air toxic sources, each releasing less than 10 tons per year of any individual HAP and less than 25 sector contributions to air toxicstons per year of combined HAPs.  Examples of stationary area sources include sources such as dry cleaners, gas stations, small print shops, auto body shops, electroplaters and furniture manufacturers.  Area sources also include consumer products used in homes and offices such as cleaners, pesticides, paints, glues, etc.  Even though emissions from each individual area source may be relatively small, their collective emissions can create significant health risks particularly where there are a large number of area sources located in a small area, or when they are located in populated areas. EPA has estimated that 24% of all man-made air toxic emissions are attributable to area sources.

Mobile sources include automobiles, trucks, buses and non-road vehicles such as boats, trains, farm equipment and recreational vehicles.  Despite major improvements in vehicle emissions over the past two decades, people are driving longer distances today. EPA estimates that 50% of all man-made air toxic emissions come from mobile sources.  Toxic air pollutants are generated by mobile sources through the incomplete combustion of fuel as well as through the evaporation of toxic components of the fuel. The US EPA web site on Air Toxics from Motor Vehicles is an interesting site to visit. For additional information on mobile sources in general, check out EPA's mobile sources web site.

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What is being done to control toxic air pollutant emissions in Maine?

Several programs have been implemented in Maine to control the emissions of toxic air pollutants.  A summary of each of these programs is presented below.

Major Stationary Sources
Major stationary sources of HAPs (and some area sources that are of particular concern) in Maine are subject to the National Emissions Standards for Hazardous Air Pollutants (NESHAPs) found in Title III of the Clean Air Act Amendments (CAAA) of 1977. Rather than setting ambient air quality standards for each pollutant, the 1990 CAAA directed EPA to set technology-based standards. The technology standards use maximum achievable control technologies (MACT) for 175 source categories that require reductions of HAP emissions. These MACT standards have been phased in over the past several years.  When fully implemented, these standards are expected to significantly reduce total annual HAP emissions to the ambient air. 

Mobile Sources
Motor vehicles are such an integral part of our society that virtually everyone is exposed to their emissions. EPA estimates that mobile source (car, truck, and bus) air toxics could cause up to 1,500 cases of cancer each year, or about half of the cancers caused by all outdoor sources of air toxics.  Maine has several programs in place which help to reduce emissions of air toxic compounds from mobile sources.

  • Vehicle Emissions Control Program - The control of air toxics emissions from motor vehicles has been addressed at the federal level through technological controls on motor vehicles such as catalytic converters.  Pre-1975 vehicles without catalytic converters, and even pre-1981 vehicles with simple catalysts, emit far more air toxics than newer vehicles. New cars today are capable of emitting 90% less hydrocarbons over their lifetimes than the uncontrolled models of the 1960s. Most air toxics are hydrocarbon compounds, so any hydrocarbon control program indirectly reduces air toxics emissions as well.
  • Stage I and Stage II Vapor Recovery Program - Distribution and marketing of gasoline is one of the largest combined sources of uncontrolled air toxic emissions in Maine.  Gasoline vapors contain benzene, a known carcinogen, as well as many other toxic substances.  Maine has implemented programs, known as Stage I and Stage II vapor recovery systems, to control emissions from these sources.  Stage I gasoline vapor recovery systems capture the vapors released from the underground storage tanks at gas stations when being refilled by tank trucks. Stage II systems capture gasoline vapors which would otherwise be vented during individual vehicle refueling at gas stations. In addition, all gasoline tank trucks operating in Maine must be equipped with gasoline vapor control equipment.

    Virtually all gasoline dispensing facilities statewide were required to install Stage I vapor recovery equipment by November 1, 1994. Stage II vapor recovery requirements apply to gasoline dispensing facilities in the three counties of York, Cumberland, and Sagadahoc with an annual throughput of 1,000,000 gallons of gasoline or more in a given year.
  • Congestion Mitigation and Air Quality Improvement Program - The Transportation Equity Act for the 21st Century (TEA-21) provides funding and flexibility for a variety of transportation purposes. One key component of TEA-21 is a program called the  Congestion Mitigation and Air Quality Improvement program (CMAQ) which was created specifically to fund projects to improve air quality. The CMAQ program assures a source of funds for reducing congestion and for developing transportation alternatives such as public transit, shared-ride programs, bicycle and pedestrian services, and employer trip reduction programs.
  • Alternative Fuels - A switch to fuels that are cleaner than today's gasolines offers gas/electric hybrid caranother promising strategy for reducing air toxics. There are several choices, including alcohols, natural gas, propane, and electricity. These fuels are inherently cleaner than conventional gasoline because they do not contain toxics such as benzene, and because they are simpler compounds that do not yield complex combustion by-products.  Clean fuels could reduce by 50% to 90% or more the number of cancers currently caused by vehicle air toxics emissions.
  • Clean Government Initiative – The Maine State Legislature and Governor have directed state agencies to, whenever possible, purchase alternative use vehicles, highly fuel-efficient vehicles, and vehicles with the lowest emissions possible. The clean government initiative also promotes alternatives to travel, such as teleconferencing. More information on Maine’s Clean Government Initiative is available from

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What is Maine's Mercury Reduction Strategy?

Maine's Mercury Reduction Strategy is a multi-disciplinary approach to reduce mercury releases from sources in Maine. 

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Are Maine's toxic air pollutant control strategies working?

Toxic Release Inventory (TRI)
EPA maintains a program for reporting releases and transfers of toxic chemicals to the environment known as the Toxic Release Inventory (TRI). The TRI is a database of information about releases of more than 650 toxic chemicals from manufacturing facilities throughout the US. TRI data from affected Maine facilities using, manufacturing, treating or releasing any of the listed toxic chemicals are reported to EPA on an annual basis.  Although the TRI database represents only a subset of total toxic emissions, the information can be used to identify trends in the release of air toxics emissions on an annual basis.  Based upon reported TRI data, Maine has reduced its air toxics emissions over 251% since 1988.

trends in the Toxic Release Inventory


US EPA National scale Assessment
One tool that helps us evaluate potential human exposure to air toxics is the USEPA's National Scale Assessment (NSA). The backbone of the NSA is EPA's National Emissions Inventory (NEI) for HAPs. For documentation on the development of the 1996 and later NEI emissions estimates refer to the NEI web page. Since the NSA utilizes 1996 emissions data, it is not representative of current conditions in Maine, especially in light of the fact that the TRI data for Maine indicates an overall reduction in HAP emissions of over 251% between 1988 and 2001. However, EPA is currently working on the 2002 version of the NEI, and that will be released in early 2005.

Emissions Inventory Program
DEP has also developed an enhanced air toxics emissions inventory for all of the 188 HAPs identified in the CAAA, as well as an additional 29 pollutants that were not included in the CAAA. The purpose of this program is to establish a comprehensive database of speciated HAP emissions from stationary sources. The database is used in evaluating the effectiveness of emissions reduction programs as well as assessing the impact on human health, in Maine.

Ambient Air Toxics Measurement Program
TRI data and Maine's air toxics inventory indicate that toxic emissions have decreased significantly in recent years. HAP emissions from mobile sources and area sources have remained relatively stable, with some modest decreases (less than 15%). The NSA data (modeled) indicate that ambient concentrations of at least 10 HAPs exceeded health based benchmark concentrations in Maine in 1990. Maine's air toxics monitoring program in 1990 confirmed that basic conclusion (although Maine could not monitor all of the HAPs that the NSA identifies). Almost all of these HAPs are emitted by the transportation sector, or small commercial sources, or came to the state via long range transport. Data collected in the Rumford/Mexico area in 1991, 1993, 1995, 1996, 1998, 1999, 2000, 2001, and 2003 and Portland monitoring from 1999, 2000, 2001 and 2002 indicate that benzene and 1,3-butadiene (transportation sector pollutants) still remain at levels exceeding 1 in 100,000 cancer risk levels.

Current Evaluation and Next Steps
In light of the NSA data suggesting the Air Toxics still pose unacceptable risk to Maine Citizens, in 2003 Maine DEP launched a new initiative to:

  • evaluate the accuracy of the NSA data,
  • find out which Air Toxics currently pose a risk to Maine Citizens; and
  • and develop an action plan to reduce the risk.

This Maine Air Toxics Initiative (MATI) is a facilitated, stakeholder process. By working with all affected parties, Maine DEP hopes to develop the most effective reduction program at the least cost. More information on the Maine Air Toxics Initiative is available here.

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Who do I contact for more information?

For more information on Maine's programs to control toxic air pollutants, contact Richard T. Greves at 207(287-7030).

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