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Report on the Environment

Outdoor Air

Other Air Topics

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

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 outdoor air quality in many ways. Outdoor air pollution, for instance, is associated with various adverse health effects including asthma attacks and cancer; outdoor air pollution can also contribute to “acid rain,” damage crops and surfaces of treasured buildings and monuments, and diminish the protective ozone layer in the upper atmosphere. Maintaining clean air is a challenging task, especially considering the growing stressors on outdoor air quality such as increased population growth, increased use of motor vehicles, and increased energy consumption.

Outdoor air pollution contains numerous substances of both natural and anthropogenic origin. While natural sources release some potentially harmful substances into the air (e.g., pollen, mold spores, dust), emissions sources of anthropogenic origin are of particular interest because regulatory and voluntary reductions can lead to decreased emissions and associated air quality improvements. Accordingly, this section focuses on outdoor air quality issues caused at least in part by human activity and acknowledges and quantifies contributions from natural sources, as appropriate.

Most outdoor air quality issues can be traced back to emissions sources that release pollutants into the air. Emissions sources are typically classified into different categories, such as point sources (e.g., power plants, industrial facilities), area sources (e.g., air pollution sources over a diffuse area, such as gasoline stations and dry cleaners), mobile sources (e.g., cars, trucks, airplanes, off-road vehicles), and natural sources (e.g., wildfires, wind-blown dust, volcanoes, vegetation). 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. As pollutants mix in the atmosphere, 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 of air pollutants, especially those that are persistent and bioaccumulative, can lead to accumulation of contaminants in other media. The levels of air pollution at a given location and at a given time are influenced by emissions from nearby and distant sources as well as by atmospheric factors, such as meteorology.

Human exposure to outdoor air pollution is a function of the composition and magnitude of air pollution, combined with human activity patterns. Ambient concentration data, while useful for characterizing outdoor air quality, ultimately do not quantify exposures, because ambient air monitoring equipment measures air quality at fixed outdoor locations, while people breathe air in multiple indoor and outdoor environs throughout a day. Whether people are harmed by poor air quality depends on the mixture of pollutants found 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 therefore typically sought on emissions sources, ambient air concentrations, exposures, and effects.

Outdoor air pollution can contain hundreds of different pollutants, which are typically grouped into various categories based on shared attributes. Some categories are defined by pollutants’ physical attributes (e.g., gases, particulate matter), while others by regulatory terminology (e.g., criteria pollutants, air toxics). The indicators used to answer the question regarding outdoor air quality are organized into the following three categories, which were selected based on the different parts of the atmosphere to which they pertain and the different types of information available to support indicator development:

  • Criteria pollutants.The following six common pollutants are referred to as criteria pollutants: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter of different size fractions, and sulfur dioxide. These pollutants are known as “criteria pollutants” because EPA regulates them by developing human health-based or environmentally based criteria (or science-based guidelines) for setting permissible levels. Specifically, the Clean Air Act requires EPA to set National Ambient Air Quality Standards (NAAQS) for these pollutants that are commonly found in outdoor air and can harm human health or the environment. The NAAQS have been modified and, in some cases, revoked since they were originally established. 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 pollutants’ emissions (or emissions of the pollutants’ precursors) and ambient concentrations.

  • Air toxics and other air pollutants. Air toxics, also known as hazardous air pollutants, are known or suspected to cause cancer and are associated with other serious health effects, such as reproductive effects or birth defects, or adverse environmental effects. The Clean Air Act specifically identifies 188 air toxics. Numerous other air pollutants exhibit toxicity even though they are not classified as air toxics; included among these other pollutants are several hundred chemicals whose emissions are tracked in EPA’s Toxics Release Inventory.

  • Stratospheric ozone issues. The ozone layer occurs in the stratosphere 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 Further, high levels of ultraviolet radiation can cause detrimental ecological effects, such as stressing productivity of marine phytoplankton, which are essential components of the oceanic food web.2

Air pollution is manifest over a range of spatial and temporal domains—an important factor to consider when evaluating trends for the three categories considered in this section. The spatial domains of air pollution issues vary widely. Air pollution can be local in nature. For instance, ambient concentrations of benzene tend to be greatest in the proximity of major sources (e.g., oil refineries, chemical production facilities) and in high-traffic areas; long-range transport is relatively unimportant due to benzene’s photochemical reactivity and the dilution that occurs over longer distances. Air pollution can also extend over regional and national scales. For example, emissions sources hundreds of miles away can contribute to airborne fine particulate matter at a given location.3 Finally, a few air pollution issues are global in nature, such as intercontinental transport of particles during dust storms. Stratospheric ozone depletion, as another example, is affected by releases of ozone-depleting substances from countries worldwide. The spatial domains ultimately determine the minimum spatial resolution of monitors needed to adequately characterize trends.

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, attention also must be paid to changes in emissions estimation techniques and advances in ambient air monitoring technologies. 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 in this section are generally consistent with those documented in other EPA publications, though readers should not expect to find perfect concordance among individual data points. This is because some publications address different spatial domains or time frames and may use less rigorous selection criteria when identifying and compiling data sets.

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