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Air Trends

Basic Information

Under the Clean Air Act, EPA establishes primary air quality standards to protect public health, including the health of "sensitive" populations such as people with asthma, children, and older adults. EPA also sets secondary standards to protect public welfare. This includes protecting ecosystems, including plants and animals, from harm, as well as protecting against decreased visibility and damage to crops, vegetation, and buildings. Get more information regarding the national air quality standards.

EPA has set national air quality standards for six common air pollutants (also called the criteria pollutants):

Some of these pollutants (CO, SO2, and lead) are emitted directly from a variety of sources. Although some industrial sources release ozone directly into the environment, most ground-level ozone forms in the air from chemical reactions involving nitrogen oxides (NOx), volatile organic compounds (VOCs), and sunlight. NO2 is formed in the air through the oxidation of nitric oxide (NO). PM, also known as particle pollution, can be directly emitted, or it can be formed when emissions of NOx, sulfur oxides (SOx), ammonia, organic compounds, and other gases react in the atmosphere. Particle pollution is regulated as PM2.5, or “fine particles” with diameters less than or equal to 2.5 micrometers, and PM10, which includes all particles with diameters less than or equal to 10 micrometers.

Each year EPA looks at the levels of these pollutants in the air and the emissions from various sources to see how both have changed over time and to summarize the current status of air quality.

The Clean Air Act

The Clean Air Act provides the principal framework for national, state, tribal, and local efforts to protect air quality. Improvements in air quality are the result of effective implementation of clean air laws and regulations, as well as efficient industrial technologies. Under the Clean Air Act, EPA has a number of responsibilities, including:

  • Conducting periodic reviews of the NAAQS for the six common pollutants that are considered harmful to public health and the environment.
  • Ensuring that these air quality standards are met (in cooperation with the state, tribal, and local governments) through national standards and strategies to control air pollutant emissions from vehicles, factories, and other sources.
  • Reducing emissions of SO2 and NOx that cause acid rain.
  • Reducing air pollutants such as PM, SOx, and NOx, which can reduce visibility across large regional areas, including many of the nation's most treasured parks and wilderness areas.
  • Ensuring that sources of toxic air pollutants that may cause cancer and other adverse human health and environmental effects are well controlled and that the risks to public health and the environment are substantially reduced.
  • Limiting the use of chemicals that damage the stratospheric ozone layer in order to prevent increased levels of harmful ultraviolet radiation.

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