Jump to main content or area navigation.

Contact Us

Air Trends

Air Quality Summary Through 2005

Air Quality Summary
Percent Change in Air Quality
  1980-2005 1990-2005

O3 (1-hr)
-7 a
Percent Change in Emissions
  1980-2005 1990-2005

PM10 b
PM2.5 b

--- Trend data not available

a. Based on percentage change from 1999

b. Includes only directly emitted particles

Negative numbers indicate improvements in air quality or reductions in emissions.

Under the Clean Air Act, EPA establishes 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 limits 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. To get more information regarding the national standards for these pollutants, visit http://www.epa.gov/air/criteria.html

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

  • nitrogen dioxide (NO2),
  • ozone (O3),
  • sulfur dioxide (SO2),
  • particulate matter (PM),
  • carbon monoxide (CO), and
  • lead (Pb)
Four of these pollutants (CO, Pb, NO2, and SO2) are emitted directly from a variety of sources. Ozone is not directly emitted, but is formed when oxides of nitrogen (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight. PM 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.

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

Reporting Air Quality and Emission Trends

Each year, air quality trends are created using measurements from monitors located across the country. The table to the left shows that the air quality based on concentrations of the principal pollutants has improved nationally since 1980.

EPA estimates nationwide emissions of ambient air pollutants and the pollutants they are formed from (their precursors).These estimates are based on actual monitored readings or engineering calculations of the amounts and types of pollutants emitted by vehicles, factories, and other sources. Emission estimates are based on many factors, including levels of industrial activity, technological developments, fuel consumption, vehicle miles traveled, and other activities that cause air pollution.

Emissions of air pollutants continue to play an important role in a number of air quality issues. About 141 million tons of pollution are emitted into the atmosphere each year in the United States. These emissions mostly contribute to the formation of ozone and particles, the deposition of acids, and visibility impairment.

Despite great progress in air quality improvement, approximately 122 million people nationwide lived in counties with pollution levels above the NAAQS in 2005.

Number of People Living in Counties with Air Quality Concentrations Above the Level of the NAAQS in 2005 by pollutant, showing 0 persons for Carbon Monoxide, 200,000 persons for Lead, 0 persons for Nitrogen Dioxide, 30.8 million people for Ozone (based on the 1-hour standard), 98 million people for Ozone (based on the 8-hour standard), 9.4 million people for PM10, 66.9 million people for PM2.5, 0 persons for Sulfur Dioxide, and 121.5 million people when all pollutants are considered together. Multiple years of data are generally used to determine if an area attains the NAAQS.

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 principal 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.

Related Links | Acronyms | Hardcopies

Jump to main content.