"The United States depends on a variety of fuels to generate electricity, including fossil fuels (coal, natural gas, and oil), nuclear power, and renewable sources. Power plants that burn fossil fuels provide about 70 percent of U.S. electricity, but they also produce substantial amounts of harmful air emissions. In particular, electricity generating units at fossil fuel power plants are among the largest emitters of sulfur dioxide and nitrogen oxides, which have been linked to respiratory illnesses and acid rain, as well as of carbon dioxide, the primary greenhouse gas contributing to climate change. Of the three fossil fuels, coal is the most widely used fuel in the United States, providing about 45 percent of electricity in 2010, followed by natural gas, which provided about 24 percent. Coal plays a critical role in the reliability of the electricity grid, especially in certain geographic areas, but coal-fired units also generally emit more air pollution than units burning natural gas or oil. Under the Clean Air Act, the Environmental Protection Agency (EPA) establishes national ambient air quality standards for six pollutants that states are primarily responsible for attaining. States attain these standards, in part, by regulating emissions of these pollutants from certain stationary sources, such as electricity generating units. Numerous Clean Air Act requirements apply to electricity generating units, including New Source Review (NSR), a permitting process established in 1977. Under NSR, owners of generating units must obtain a preconstruction permit that establishes emission limits and requires the use of certain pollution control technologies."
Government Accountability Office: http://www.gao.gov/