Background

Air pollution causes a wide variety of health effects that range from eye irritation, to heart and lung damage, to 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 -- "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 (NAAQS) for pollutants considered harmful to public health and the environment.
• Ensuring that these air quality standards are met or attained (in cooperation with States) through national standards and strategies to control air pollutant emissions from automobiles, factories, and other sources.
• Reducing emissions of sulfur dioxide and nitrogen oxides that cause acid rain.
• Limiting use of chemicals that damage the stratospheric ozone layer in order to prevent increased levels of harmful ultraviolet radiation.
• Ensuring that sources of toxic air pollutants are well controlled.

The Clean Air Act established two types of National Ambient Air Quality Standards. Primary standards set 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): carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO₂), ozone (O₃), particulate matter (PM-10), and sulfur dioxide (SO₂). [Note: The pollutant ozone is not emitted directly to the air, but is formed when sunlight acts on emissions of nitrogen oxides (NOx) and volatile organic compounds (VOC).]

The Clean Air Act Amendments of 1990 also identified 189 "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 defects). The Clean Air Act contains requirements for reducing air toxic emissions from industrial factories and other 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.