In 1999, the agency accepted the states' arguments and ordered hundreds of industrial polluters to cut emissions of nitrogen oxides, which cause smog. The polluters sued. Businesses and state officials in the upwind states had argued that the E.P.A. could not directly regulate the factories and that it should leave the work to the states. Upwind polluters fought that, too, but they lost that case last June. The upwind industries appealed to the Supreme Court, but it refused to hear the case.
Environmental groups say direct regulation through petition is surer and faster.
The upwind industries have consistently argued that they are not at fault for pollution in distant states. Officials at American Electric Power, a multistate utility based in Columbus, Ohio, that is one of the largest coal users in the United States, had earlier attributed the pollution in the Northeast to the region's failure to control its own emissions from vehicles and other sources. Although the decision appears to be a victory for Eastern states, the details of who lost are more complicated. Twelve states were found to contribute to pollution in states further downwind, but those include New York and Pennsylvania. The others are Delaware, Indiana, Kentucky, Maryland, Michigan, New Jersey, North Carolina, Ohio, Virginia and West Virginia.