What are toxic air pollutants?

Toxic air pollutants 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. Generally, the toxic air pollutants of greatest concern are those that are released to the air in amounts large enough to create a risk to human health, and may expose many people. Title III of the Clean Air Act Amendments identifies 188 hazardous air pollutants (HAP) likely to have the greatest impact on ambient air quality and human health.

What are some examples of air toxics?

Toxic air pollutants exist as particles or as a gas. Examples of gaseous toxic air pollutants include: benzene, toluene, and xylenes, which are found in gasoline; perchloroethylene, which is used in the dry cleaning industry; and methylene chloride, which is routinely used as an industrial solvent. Examples of air toxics typically associated with particles 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 during waste and fossil fuel combustion.

What are the effects of toxic air pollutants?

Toxic air pollutants can have serious effects on human health and the environment. Human exposure to these pollutants can include short-term and long-term effects. Many factors can affect how different toxic air pollutants affect human health, including the quantity to which a person is exposed, 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 some 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.

What was the Cumulative Exposure Project (CEP)?

The U.S. Environmental Protection Agency (EPA) modeled available emission estimates of 1990 air toxics emissions for 148 air toxics from a variety of sources to develop estimates of 1990 air toxics concentrations for every state at the census tract level in the continental United States. Census tracts are geographic areas that vary in size but contain approximately 4,000 residents each. These estimates were then compared to health-based benchmark concentrations for each pollutant.

What standards were used to assess air quality? What is a benchmark concentration?

The federal government has ambient air quality standards for six chemicals (ozone, oxides of nitrogen and sulfur, particulate matter, lead, and carbon monoxide). In addition, the federal government and some states have identified toxicity values for many other chemicals, including those referred to as "air toxics", that are used to assess air quality. These toxicity values represent the potential to cause various types of diseases ranging from reversible irritation to chronic lung diseases to cancer, and are often referred to as "benchmark" values.

What emission sources were considered in the EPA study?

Toxic air pollutants can be emitted from many sources, including large manufacturing facilities (chemical production plants), combustion facilities (waste incinerators), small commercial operations (dry cleaners), lawnmowers, jet skies, and on-road and non-road mobile sources. For the CEP analysis, sources of toxic air emissions were divided into three groups:

  • Point Sources
    • Manufacturing (e.g., chemical manufacturing, refineries, primary metals)
    • Non-manufacturing (e.g., electric utility generators, municipal waste combustors)
  • Area Sources
    • Manufacturing (e.g., wood products manufacturing, degreasing)
    • Non-manufacturing (e.g., dry cleaning, consumer products, small medical waste incinerators)
  • Mobile Sources
    • Highway (e.g., cars, buses, trucks)
    • Off-road (e.g., construction equipment, boats, lawn and garden equipment).
  • Background Sources

Background pollutant concentrations resulting from natural sources and persistent historic emissions were also considered. Background concentrations may be comprised of contribution from past emissions, global transport, and natural sources, e.g., volcanoes, weathering or rocks.

What sources of emissions data were used for the CEP modeling study?

  • Point Sources. EPA's 1990 Toxics Release Inventory (TRI), a database of estimated emissions for large manufacturing sources.
  • Area and Mobile Sources. EPA's 1990 base year national interim emissions Inventories for volatile organic compounds (VOCs) and particulate matter (PM) Inventories, which includes facility-specific data on point sources such as incinerators and county-level emissions totals for mobile sources and small industrial, commercial, consumer and agricultural sources. Hazardous air pollutant emissions estimates were derived from VOC and PM emission estimates by the applying speciation profiles, which are industry-specific and/or process-specific estimates of particular chemicals in a VOC or PM emissions stream. These profiles were obtained from an EPA database and from numerous journal articles and published reports.
  • Background Concentrations. These were estimated using measurements taken in remote "clean air locations" which were removed from the impact of local populations or man-made sources.
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How did EPA's model work?

EPA used the ASPEN model in this study, which is a long-term Gaussian dispersion model where a set of equations mathematically represents the physical processes of transport and dispersion of emissions from a source of air pollution. EPA and others use Gaussian models as the standard approach for estimating outdoor concentrations of air pollutants within 50 kilometers of an emissions source. Modeling provides a way to estimate combined impacts from one or more sources where ambient data is not available. Generally, the following type of information goes into a model: stack height; emissions rate of each contaminant; exit velocity of the contaminant; temperature of the contaminant; and meteorological data such as wind speed, direction, temperature, and atmospheric turbulence.

What information did the CEP provide?

  • Estimates of 1990 average airborne concentration of 148 air toxics for each census tract in the continental U.S.
  • An air toxics emissions inventory apportioned by sector
  • A dispersion model for characterizing average concentrations of airborne pollutants
  • A 1990 baseline against which to measure future progress
  • A prioritized list of air toxics to assist in developing targeted control strategies

How does the CEP data compare to monitoring data?

A comparison of EPA's modeled air toxics concentrations to the limited available monitoring data for 1990 showed that the study's modeled concentrations of air toxics have a general tendency to underestimate concentrations. The tendency to underestimate may be due, in part, to modeling limitations and underestimated emissions used as model inputs.

What were the limitations of the study?

Given the uncertainties involved in computer modeling, the results of the study were most reliable when analyzed on a national or State scale, and had increasing uncertainty at the county and census tract levels. Therefore, comparisons at the census tract level may be misleading.

Another limitation is that the data represent estimates of air toxics concentrations for 1990, and emissions of air toxics have changed since then. Since 1990, EPA has issued Maximum Achievable Control Technology (MACT) standards that are expected to reduce emissions of air toxics from stationary sources by over one million tons per year. These standards affect about 50 categories of major industries, such as chemical plants, oil refineries, aerospace manufacturers, and steel mills. EPA standards have also reduced air toxic emissions from motor vehicles and other mobile sources. In addition, ongoing programs to address air toxics at the state and local levels (outlined below) have achieved reductions in emissions since 1990 that are not reflected in EPA's analysis.

Benchmark concentrations rely on toxicological data for air toxics that are uncertain. Many of the studies used to determine benchmark concentrations are based on animal studies at high levels of exposure.

How can government environmental and public health agencies use the results of the CEP study?

Agencies can compare the modeled concentrations with established health effect benchmark concentrations to:

  • Identify potentially important emission sources and pollutants;
  • Prioritize future regulatory programs to address those pollutants and emission sources that may present the most significant threat to public health;
  • Evaluate the effect of local, state, and federal air toxic control programs since 1990 to reduce ambient pollutant concentrations by using the established CEP methodology to model more recent emissions data.
  • Use the results of the dispersion modeling analysis to target ambient monitoring and emissions inventory efforts.