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Summary of Air Quality and Emissions Trends

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.

Railway cars in a railway station.

EPA tracks two kinds of trends: air concentrations based on actual measurements of pollutant concentrations in the air at selected monitoring sites throughout the country, and emissions based on engineering estimates of the total tonnage of these pollutants released into the air annually. However, starting in 1994, under the Acid Rain Program, EPA began tracking emissions of SO2 and NOx based on data from continuous emission monitors for the electric utility industry.

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.

Each year, EPA gathers and analyzes air quality concentration data from more than 5000 monitoring stations around the country. Monitoring stations are operated by State, Tribal, and local government agencies as well as some Federal agencies, including EPA. Trends are derived by averaging direct measurements from these monitoring sites on a yearly basis. During the last 10 years (1987 through 1996), air quality has continued to improve.

The most notable improvements are a 75 percent decrease in Pb concentrations and a 37 percent decrease in both CO and SO2 concentrations. Improvements in measured concentrations are also noted for the other principal pollutants including NO2, ozone, and PM during this same time frame.

Although some areas of the U.S. are experiencing air pollutions problems, overall air pollution continues to decline, despite extensive national growth. Air Quality Concentrations
Vehicle Miles Traveled 1970 - 1996

EPA estimates nationwide emissions trends based on actual monitored readings or engineering calculations of the amounts and types of pollutants emitted by automobiles, factories, and other sources. Emission trends are based on many factors including the level of industrial activity, technology developments, fuel consumption, vehicle miles traveled, and other activities that cause air pollution. Emissions trends also reflect changes in air pollution regulations and installation of emissions controls. Over the last 10-year period, emissions have shown improvement (decreased emissions) for all principal air pollutants, except NOx.

The dramatic improvements in emissions and air quality occurred simultaneously with significant increases in economic growth and population. The improvements are a result of effective implementation of clean air laws and regulations, as well as improvements in the efficiency of industrial technologies.

Between 1970 and 1996, U.S. population increased 29 percent, vehicle miles traveled increased 121 percent, and gross domestic product increased 104 percent. At the same time, total emissions of the six principal air pollutants decreased 32 percent.

Despite great progress in air quality improvement, in 1996 there were still approximately 46 million people nationwide that lived in counties with monitored air quality levels above the primary standards.


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