Six northeastern states and five environmental groups yesterday sued the Environmental Protection Agency, accusing the agency of failing to control air pollutants that are reducing visibility in national parks and wilderness areas.

The suit, filed in U.S. District Court in Portland, Maine, contends that the Clean Air Act set a 1979 deadline for the EPA to issue regulations aimed at protecting scenic views. The EPA issued regulations in 1980, but the rules were limited to controlling "plume blight," an identifiable streak of pollutants across an otherwise clear sky.

Scientists from the EPA and the National Park Service acknowledge that visibility is far more impaired by a phenomenon known as "regional haze," which contains a mixture of pollutants from sources that may be hundreds of miles away. In the East, regional haze is believed to have reduced normal visibility by more than half in the last 40 years.

"Regional haze is the most serious form of visibility impairment in the eastern United States, often virtually eliminating scenic vistas in numerous parks and wildernesses such as Acadia National Park in Maine and Shenandoah National Park in Virginia," said Howard Fox, attorney for the Sierra Club, one of the plaintiffs in the case.

EPA spokesman Christian Rice said the agency did not address regional haze in its 1980 regulations because it "lacked sufficient knowledge" about the phenomenon. "The agency concluded that further work needed to be done in order to establish a framework for dealing with regional haze," he said. Rice said the EPA is still researching the problem.

The other plaintiffs are the states of Maine, Vermont, New York, Massachusetts, Connecticut and New Jersey, the Natural Resources Defense Council, the National Audubon Society, the Environmental Defense Fund and the Conservation Law Foundation of New England.

Most of the plaintiffs were parties to a similar lawsuit filed several years ago, seeking to force the EPA to control the pollutants that cause acid rain. A district court ruling in their favor was overturned on appeal, and the Supreme Court declined last month to hear the case.

The latest lawsuit is aimed at the same mark, however, because the pollutants that cause acid rain -- chiefly sulfur dioxide and nitrogen oxide -- are also believed to be the main causes of regional haze.