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Air Trends

Fact Sheet - 1997 Air Trends Report

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.


  • Since 1970, air quality has continued to improve for six major air pollutants, sometimes referred to as "criteria" pollutants. These include carbon monoxide, lead, nitrogen dioxide, ground-level ozone, particulate matter, and sulfur dioxide. Air quality is determined by measuring pollutant concentration levels with air quality monitors placed in urban and some other areas.
  • Nationally, while air quality has improved for all of the "criteria" pollutants, actual emissions of nitrogen oxides (NOx) between 1970 and 1997 increased 11 percent. (Emissions of all of the other "criteria" pollutants have decreased significantly since 1970.) Emissions of NOx contribute to the formation of ozone. In September, 1998 EPA issued a rule that will significantly reduce emissions of NOx in 22 eastern states, and, in turn, reduce the regional transport of ground-level ozone and acid rain formation.
Nationally, the 1997 air quality levels are the best on record for all six of the "criteria pollutants". In fact, all the years throughout the 1990s have had better air quality than any of the years in the 1980s, showing a steady trend of improvement.
  • EPA has issued standards to reduce the emissions of over 100 different toxic air pollutants. These pollutants are known to or suspected of causing cancer or other adverse health effects. When fully implemented, these standards will reduce air toxics emissions by about 1 million tons per year - almost ten times the reductions achieved prior to 1990.
  • EPA is also working to reduce acid rain. In 1996 the sulfate level in rainfall (an indicator of acidity) decreased by an estimated 10 - 25 percent, particularly in some of the most acid-sensitive regions of the United States. This is based on a comparison to what sulfates would have been had the 1983 - 1994 trend continued.

The improvements in air quality and economic prosperity that have occurred since EPA initiated air pollution control programs in the early 1970s illustrate that economic growth and environmental protection can go hand-in-hand. Since 1970, national total emissions of the six criteria pollutants declined 31 percent, while U.S. population increased 31 percent, gross domestic product increased 114 percent, and vehicle miles traveled increased 127 percent. Despite continued improvements in air quality, approximately 107 million people lived in counties with unhealthy air in 1997. In addition, some national parks have experienced high levels of some air pollutants as a result of pollutants being transported many miles from their original source. For example, ground-level ozone concentrations in remote locations of the Great Smoky Mountains National Park have increased nearly 20 percent over the last 10 years.


  • Under the Clean Air Act, EPA has a number of responsibilities to aid in the protection of public health and the environment from the adverse affects of air pollution. These responsibilities include setting air quality standards for the six criteria pollutants, reducing emissions of pollutants that cause acid rain, limiting use of chemicals that damage the stratospheric ozone layer, and ensuring that sources of toxic air pollutants are well-controlled.
For the past 25 years, EPA has evaluated the trends and status of our nation's air quality, and has presented the results of this evaluation in an annual publication known as the National Air Quality and Emissions Trends Report. The 1997 National Air Quality and Emissions Trends Report discusses the nation's progress in cleaning up air pollutants, focusing on the 10-year period between 1988 and 1997. In the report, EPA tracks trends in both concentrations (as measured by air quality monitors located in and around urban areas and other locations in the country) and emissions of air pollutants. Air emissions and pollutant concentration data is collected for the prior calendar year, and is extensively reviewed and quality-assured prior to the development of this report. This quality-assured process is often time and data intensive. It takes several months to generate the Trends report once States submit the air quality and emissions data to EPA for review and analysis. As a result, EPA is issuing the report for 1997 data in late 1998.
  • Air quality information is based on actual measurements of pollutant concentrations in the air from thousands of monitoring sites located throughout the nation. Air pollutant emission estimates are based largely on engineering calculations of the amounts and kinds of pollutants emitted by automobiles, factories, and other sources of air pollution. In addition, some emission estimates are based on measurements from continuous emissions monitors (CEMs) that have recently been installed at major electric utilities to measure actual emissions. The Trends report incorporates data from CEMs collected between 1994 and 1997 for NOx and SO2 emissions at major electric utilities.

Generally there are similarities between air quality trends and emission trends for any given pollutant. However, in some cases there are notable differences between the percent change in ambient concentrations and the percent change in emissions. These differences can mainly be attributed to the location of air quality monitors. Most monitors are positioned in urban, population-oriented locales which are more likely to indicate reductions in emissions that occur in urban areas (such as emissions from automobiles) rather than emissions that occur in rural areas (such as emissions from power plants). Thus, trends in air quality more closely track changes in urban emissions rather than changes in total national emissions.

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