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Report on the Environment

Outdoor Air Quality

What are the trends in outdoor air quality and their effects on human health and the environment?

Importance of Outdoor Air Quality

Outdoor air—the air outside buildings, from ground level to several miles above the Earth's surface—is a valuable resource for current and future generations because it provides essential gases to sustain life and it shields the Earth from harmful radiation. Air pollution can compromise human health and the environment in many ways. For example, outdoor air pollution:

  • Is associated with a number of human health effects including heart attacks, asthma attacks, bronchitis, hospital and emergency room visits, work and school days lost, restricted activity days, respiratory symptoms, and premature mortality.
  • Can contribute to "acid rain."
  • Can impair visibility and damage crops and surfaces of treasured buildings and monuments.
  • Can diminish the protective ozone layer in the upper atmosphere.

Maintaining clean air is a challenging but achievable task.

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Anthropogenic Focus

Outdoor air pollution contains numerous substances of both natural (e.g., pollen, mold spores, dust) and anthropogenic (human-caused) origin. Anthropogenic emissions are of particular interest because they can be decreased through regulatory and voluntary actions, leading to air quality improvements. The ROE therefore focuses on outdoor air quality issues caused (at least in part) by human activity and acknowledges and quantifies contributions from natural sources, as appropriate.

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Three Categories of Indicators

The ROE presents indicators for three different types of air pollutants: criteria pollutants; air toxics and other pollutants; and stratospheric ozone issues.

  • Criteria pollutants. These six pollutants (carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter of different size fractions, and sulfur dioxide) are common in outdoor air and can harm human health and the environment. They are referred to as “criteria pollutants” because EPA regulates them by developing human health-based or environmentally-based criteria (or science-based assessments) for setting permissible levels.

    Specifically, the Clean Air Act requires EPA to set National Ambient Air Quality Standards (NAAQS) for these pollutants. EPA is required to periodically review and update the NAAQS to reflect the latest scientific information on how outdoor air quality affects human health and the environment.

    Extensive data are available on criteria pollutant emissions (or emissions of the pollutants' precursors—substances that once released into the air are converted to criteria pollutants) and ambient concentrations. Emissions of criteria pollutants (or precursors) and air toxics are compiled and reported in EPA's National Emissions Inventory (NEI); measurements of outdoor air quality for criteria pollutants and toxics are collected and housed in the national database known as the Air Quality System (AQS).
  • Air toxics and other air pollutants. Air toxics, also known as hazardous air pollutants, are known or suspected to cause cancer and other serious health effects, such as reproductive effects or birth defects, or adverse environmental effects. Under the Clean Air Act, EPA regulates air toxics from stationary and mobile sources. In addition, EPA's Toxics Release Inventory contains information on releases to air, land, and water for over 650 toxic chemicals.
  • Stratospheric ozone issues. The stratospheric ozone layer lies between 6 and 20 miles above the Earth's surface and protects the Earth's biota from harmful effects of the sun's ultraviolet radiation. Past and ongoing releases of a number of synthetic chemicals from throughout the world have depleted the ozone layer, allowing more ultraviolet radiation to reach the Earth's surface. This can lead to increased incidence of skin cancer, cataracts, and other health problems.1 High levels of ultraviolet radiation can also cause detrimental ecological effects, such as stressing productivity of marine phytoplankton, which are essential components of the oceanic food web.2

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Pollutant Fate and Transport

Most outdoor air quality issues can be traced back to sources that release pollutants into the air, including point sources (e.g., power plants, industrial facilities); area sources (e.g., many small air pollution sources over a large area, such as gasoline stations and dry cleaners); mobile sources (e.g., cars, trucks, airplanes, boats and ships, off-road vehicles); and natural sources (e.g., wildfires, wind-blown dust, volcanoes, vegetation). After release, air pollutants are subject to a variety of chemical and physical forces that determine when, where, and how they move through the environment.

  • Fate and transport: Once pollutants are airborne, prevailing wind patterns carry and disperse them from their sources to other locations. Atmospheric chemical reactions may consume some airborne pollutants and create others. Depending on their chemical and physical properties, some pollutants deposit to the Earth's surface near their sources, while others remain airborne for hours, days, or years.
  • Deposition: Deposition, especially of persistent and bioaccumulative pollutants, can lead to accumulation of airborne contaminants in other media (e.g., water and land).

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Exposure to Outdoor Air Pollution

Human exposure to outdoor air pollution is a function of the composition and magnitude of air pollution, combined with human activity patterns. Determining exposures to outdoor air pollution requires a complex assessment that factors in the amount and types of air pollution, the number of people exposed, and the exposure duration. Therefore, ambient concentration data—while useful for characterizing outdoor air quality—ultimately do not fully represent human exposures, because ambient air monitoring equipment measures air quality at fixed outdoor locations, while people breathe air in several indoor and outdoor environs throughout a typical day.

Whether people are harmed by poor air quality depends on the level and mixture of pollutants in the air, exposure doses and durations, individuals' susceptibilities to diseases, and other factors. Similarly, air pollutants' interactions with ecosystems determine whether air pollution causes harmful environmental effects.

For a complete understanding of a given air pollution issue, information is typically sought on emissions sources, ambient air concentrations, exposures, and effects on exposed populations. The question on Diseases and Conditions in Human Exposure and Health presents several indicators of diseases and conditions for which outdoor air is a risk factor, including Cancer, Asthma, Cardiovascular Disease, and Chronic Obstructive Pulmonary Disease. However, since there are many other risk factors for these diseases, the particular contribution of outdoor air to these trends cannot be determined.

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Other Considerations

  • Air pollution manifests across a range of area (spatial domain) and time (temporal domain) — important factors to consider when evaluating trends in outdoor air quality.
    • Area: The spatial domains of air pollution ultimately determine the minimum spatial resolution of monitors needed to adequately characterize trends. They vary widely from local to global in scale:

      Air pollution can be local. For instance, ambient concentrations of benzene tend to be greatest near major sources (e.g., oil refineries, chemical production facilities) and in high-traffic areas.

      Air pollution can also be regional or national in nature. For example, emissions sources hundreds of miles away can contribute to airborne fine particulate matter at a given location.3

      A few air pollution issues are global in nature, such as intercontinental transport of particles during dust storms. Stratospheric ozone depletion, also, is affected by releases of ozone-depleting substances from countries worldwide.
    • Time: Temporal scales also vary among pollutants and typically reflect some combination of changes in emissions and fluctuations in weather. Ambient air concentrations of some air pollutants, like ground-level ozone, have considerable diurnal and seasonal variations.4 However, temporal variations are far less pronounced for pollutants that are long-lived in the atmosphere, including many ozone-depleting substances.

      Temporal variations largely determine the appropriate monitoring frequency for quantifying trends and the most meaningful statistic (or averaging time) used to report ambient air concentrations. When quantifying and interpreting long-term trends in outdoor air quality, changes in emissions estimation techniques and advances in ambient air monitoring technologies are also relevant.

      Unless otherwise noted, the outdoor air quality indicators only come from data sets generated using consistent methodologies over the entire time frame of interest.
  • The nationwide air quality trends presented here are generally consistent with those documented in other EPA publications particularly the National Air Quality Trends Report, which has been published for over 30 years. Specific data points used may differ from study to study because some publications address different geographic areas or time periods and may reflect different baselines or analytical scenarios. Different data selection criteria may also be used when identifying and compiling data sets.

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ROE Indicators

The outdoor air quality indicators presented in the ROE track emissions, ambient concentrations, pollution-related effects, or energy use for differing geographic and time periods, depending on the availability of underlying data. The most extensive temporal coverage of these indicators tracks trends from 1949 to the present.

  • Criteria pollutants: By virtue of national monitoring and reporting requirements, criteria pollutants have some of the most extensive data available to support national indicators for ambient air concentrations and emissions.

    Nationwide, air emissions of every criteria pollutant (or the corresponding precursors) have decreased between 1990 and 2011. Consistent with the emissions trends, every criteria pollutant showed decreasing ambient air concentrations based on aggregate measurements from the nation's ambient air monitoring system, which measures levels of air pollution primarily in urban and suburban areas, where the majority of the U.S. population resides and works. Air quality improvements occurred during a time of nationwide increases in energy use, population, motor vehicle use, and gross domestic product.
  • Acid rain: Long-term monitoring data show that wet deposition of acidic sulfates and nitrates decreased between 1989 and 2013, consistent with decreased emissions of sulfur dioxide and nitrogen oxides over roughly the same time frame (Acid Deposition). As a result, many surface waters throughout the Adirondack Mountains, the Northern Appalachian region, and New England show signs of recovery from acidification (Acidity in Lakes and Streams).
  • Air toxics: Between 1990 and 2011, nationwide emissions aggregated across 187 air toxics decreased by 50 percent (Air Toxics Emissions). Corresponding reductions were observed in the ambient air concentrations of several air toxics (Air Toxics Concentrations).
  • Stratospheric ozone: Since 1990, the United States phased out most production and import of ozone-depleting substances. Consequently, exposure to ozone-depleting substances in the United States decreased during this last decade, along with globally representative ambient air concentrations of ozone-depleting substances in the lower atmosphere (Ozone-Depleting Substances Concentrations).

    While such decreases are expected to help restore the stratospheric ozone layer, stratospheric ozone levels over North America actually decreased slightly since the 1980s, though they have remained largely unchanged in the last decade (Ozone Levels over North America). This trend is due to various factors, including ongoing use of ozone-depleting substances worldwide and the fact that ozone-depleting substances are extremely long-lived in the atmosphere.

There are several limitations associated with the ROE indicators for outdoor air, as well as challenges in developing additional indicators to characterize outdoor air quality trends.

While EPA's source emission and ambient air monitoring measurements provide comprehensive data for regulatory and compliance purposes, there are limitations in using these data as outdoor air indicators for this report. Opportunities to improve the quality of ROE outdoor air indicators include the following:

  • Emissions: Additional emissions measurements at more types of sources would improve the ROE emission-based air quality indicators. Presently, emissions for some sources are estimated rather than measured directly. Although these estimates have inherent uncertainties, the emissions inventory data are of high quality overall. EPA routinely updates emission estimates to remain consistent with the current scientific understanding of emissions from different source categories.
  • Concentrations: The current national ambient air quality monitoring network is comprised of thousands of measurement sites located across the country. This network is developed through state and local air quality agencies and is based on well-established network design and monitor siting criteria. The primary purpose of the national air monitoring network is to provide the data necessary to assess protection of public health and the environment.

    While a denser monitoring network (i.e., more monitoring sites) might improve concentration-based air quality indicators in the ROE, the nation's air quality monitoring network primarily focuses on and accurately captures concentrations in areas expected to have the highest concentrations and areas where exposure to elevated air pollutant levels is most likely.

    While the national trends for criteria pollutants and air toxics unequivocally reflect improved air quality at the national level, ambient air quality trends can vary on a local scale. For example, concentrations of some pollutants may be improving less rapidly than national trends in some areas (e.g., those experiencing rapid population growth or near newly constructed point sources) while decreasing in other areas at a faster rate than the national average.

Several challenges complicate efforts to develop national-level indicators of outdoor air quality exposure and effects. For example, ambient concentration data do not quantify individual exposures, because ambient air monitoring equipment measures air quality at fixed outdoor locations, while people breathe air in multiple indoor and outdoor settings during a typical day.

Actual human exposure to air pollution can be measured with personal monitoring devices, which sample the air that individuals breathe as they move through different microenvironments. However, conducting such studies on a national scale over an extended time frame would be extremely resource-intensive.

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[1] World Meteorological Organization. 2007. Scientific assessment of ozone depletion: 2006. Geneva, Switzerland

[2] DeMora, S., S. Demers, and M. Vernet. 2000. The effects of UV radiation in the marine environment. Cambridge, United Kingdom: Cambridge University Press.

[3] U.S. Environmental Protection Agency. 2004. The particle pollution report: Current understanding of air quality and emissions through 2003. EPA/454/R-04/002. Research Triangle Park, NC.

[4] U.S. Environmental Protection Agency. 2004. The ozone report: Measuring progress through 2003. EPA/454/K-04/001. Research Triangle Park, NC.

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