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Tall smokestacks--stacks of 500 feet or higher, which are primarily used at coal power plants--release air pollutants such as sulfur dioxide (SO2) and nitrogen oxides (NOx) high into the atmosphere to help limit the impact of these emissions on local air quality. Tall stacks can also increase the distance these pollutants travel in the atmosphere and harm air quality and the environment in downwind communities. The 1977 amendments to the Clean Air Act encourage the use of pollution control equipment over dispersion techniques, such as tall stacks, to meet national air standards. Section 123 of the Act does not limit stack height, but prohibits sources of emissions from using the dispersion effects of stack heights in excess of a stack's good engineering practice (GEP) height to meet emissions limitations. GAO was asked to report on (1) the number and location of tall stacks of 500 feet or higher at coal power plants and when they began operating; (2) what is known about such stacks' contribution to the interstate transport of air pollution and the pollution controls installed at plants with these stacks; and (3) the number of stacks that were built above GEP height since 1988 and the reasons for this. GAO analyzed Energy Information Administration (EIA) data on power plants, surveyed states with tall stacks, and interviewed experts on the transport of air pollution. GAO is not making recommendations in this report. The Environmental Protection Agency and the Department of Energy stated they had no comments on this report

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