Note: EPA no longer updates this information, but it may be useful as a reference or resource.
Please see www.epa.gov/airtrends for the latest information on Air Quality Trends.
National Air Quality
Air pollution causes a wide variety of health effects that range from eye irritation, heart and lung damage, and premature death. It can also impair visibility and reduce crop production, as well as damage ecosystems, national parks, wilderness areas, and water bodies.
Air pollution comes from many different sources. "Stationary sources" such as factories, power plants, and smelters, "area" sources which are smaller stationary sources, "mobile sources" including cars, buses, planes, trucks, and trains, and "natural sources" such as wildfires, windblown dust, and volcanic eruptions all contribute to air pollution. The Clean Air Act provides the principal framework for national, State, Tribal, and local efforts to protect air quality. Under the Clean Air Act, which was last amended in 1990, EPA has a number of responsibilities, including:
- Setting national ambient air quality standards for pollutants considered harmful to public health and the environment.
- Ensuring that these air quality standards are met, or attained, (in cooperation with the States, Tribal, and local governments) through national standards and strategies to control air pollutant emissions from automobiles, factories, and other sources.
- Reducing emissions of SO2 and NOx that cause acid rain.
- Limiting use of chemicals that damage the stratospheric ozone layer in order to prevent increased levels of harmful ultraviolet radiation.
- Setting national emission standards for toxic pollutants that are known or suspected to cause cancer or other serious health problems.
- Ensuring that sources of toxic air pollutants are well controlled based on the maximum available control technologies or pollution prevention practices (in cooperation with the States, Tribal, and local governments).
The Clean Air Act established two types of national air quality standards. "Primary" standards are designed to establish limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly. "Secondary" air quality standards set limits to protect public welfare, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings.
EPA has set national air quality standards for six principal pollutants (referred to as "criteria" pollutants): CO, Pb, NO2, ozone, SO2, and PM. [Note: The pollutant ozone is not emitted directly into the air, but is formed when sunlight acts on emissions of NOx and VOC.]
The Clean Air Act Amendments of 1990 also identified 188(1) "toxic" air pollutants for regulation. Air toxics are those pollutants that are known or suspected to cause cancer or other serious health effects (such as birth or developmental effects). The Clean Air Act contains requirements for reducing air toxic emissions from industrial factories, mobile sources, and other sources.
EPA's Maximum Achievable Control Technology (MACT) regulations are beginning to achieve significant reductions of toxic air pollutants as well as the six principal pollutants. MACT standards are based on the emission levels already achieved by the best-performing similar facilities. This straight forward, performance-based approach yields standards that are both reasonable and effective in reducing toxic emissions.
To determine whether "post-MACT" risks are acceptable, a human health risk and adverse environmental effects-based "needs test" was added. In this test, referred to as "residual risk," EPA is required to announce additional standards for those source categories that are emitting toxic air pollutants at levels that present an unacceptable risk to the public or the environment. Using this risk framework, EPA will determine whether technology-based emission standards sufficiently protect human health.
EPA is required to develop an urban strategy that will reduce air toxic emissions from area sources to address the associated health risk problems posed by the most highly toxic pollutants. In addition, the EPA must study the need for and feasibility of controlling emissions of toxic pollutants from motor vehicles and fuels. EPA is looking at an integrated approach that addresses the urban air toxic emissions from both stationary sources and mobile sources.
EPA also has responsibility for setting standards to reduce chemicals that destroy the stratospheric ozone layer and pollutants that cause acid rain and visibility impairment. This brochure provides an overview of trends in these air pollution problems, as well as global warming issues and the processes EPA has developed for controlling pollutants that contribute to global warming.