Although fossil fuels--coal, natural gas, and oil--account for more than two thirds of the nation's electricity, generating units that burn these fuels are major sources of airborne emissions that pose health and environmental risks. To limit emissions and protect air quality, the Environmental Protection Agency regulates emissions of sulfur dioxide and nitrogen oxides from a variety of sources including electricity generating units that burn fossil fuels, other industrial sources, and automobiles. Older electricity generating units--those that began operating before 1972--emit 59 percent of the sulfur dioxide, 47 percent of the nitrogen oxides, and 42 percent of all electricity produced by fossil-fuel units. Units that began operating in or after 1972 are responsible for the remainder of the emissions and electricity production. For equal quantities of electricity generated, older units, in the aggregate, emitted twice as much sulfur dioxide and 25 percent more nitrogen oxides than newer units which must meet the new source standards for these substances. Older and newer units emitted about the same amount of carbon dioxide for equal quantities of electricity generated. Of the older units, those in the Mid-Atlantic, Midwest, and Southeast produced the majority of the emissions, and in disproportionate quantities for the amount of electricity they generated compared with units located in other parts of the country. Older units that burned coal released a disproportionate share of emissions for the electricity they produced compared with units burning natural gas and oil. Thirty-six percent of older units, in 2000, emitted sulfur dioxide at levels above the new source standards applicable to newer units, and 73 percent emitted nitrogen oxides at levels above the standards. These "additional" emissions--those above the standards for newer units--accounted for 34 percent of the sulfur dioxide and 60 percent of the nitrogen oxides produced by older units. Coal-burning units emitted 99 percent of the additional sulfur dioxide and 91 percent of the additional nitrogen oxides, while other fossil fuel-burning units accounted for the remainder.
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