The Sierra Club filed a federal lawsuit against the Environmental Protection Agency yesterday in a bid to make the agency enforce stringent clean air standards in the Washington area.

The suit seeks a court order that would require the EPA to impose the standards of the Clean Air Act that the D.C. area was supposed to meet by 1999. The agency had proposed allowing the region until 2005 to lower the amount of ozone-forming particles in the air, but a federal appellate court struck that down in July as illegal.

Since then, the agency has not moved to enforce the federal standards, according to the suit, filed in U.S. District Court in Washington.

"We want the EPA to reclassify the Washington area as 'severe,' from its current designation as 'serious,' " said David Baron, an attorney for Earthjustice Legal Defense Fund, a nonprofit agency handling the suit for the Sierra Club. "That will kick in stronger enforcement of emission standards for factories, power plants and motor vehicles."

Judy Katz, director of the EPA's air protection division for the mid-Atlantic region, said the agency has been moving forward. She said the EPA gave notice of a proposed change of classification in yesterday's edition of the Federal Register. That process, after a 30-day period for public comment, should produce a new classification by late January, she said.

"I haven't seen the suit, but this sounds responsive to most of their concerns," Katz said.

Baron said that the Sierra Club was aware of the published notice and that the group intended to continue with the suit, which marks the latest chapter in a long-running dispute about the capital region's air quality.

The Clean Air Act gave all cities until 1999 to meet standards for curtailing the amount of ozone particles, or smog, which aggravate asthma and other respiratory problems. The Washington region, including the District and 10 counties in Maryland and Virginia, had an average ozone level of 0.132 from 1997 to 1999. The federal limit is 0.12.

Areas that are found to be in severe violation of the EPA standards must reduce ozone-forming emissions -- from smokestacks, factories, car exhaust pipes and other sources -- by at least 3 percent a year until they come into compliance. The EPA had given Washington and other areas -- including cities such as Atlanta and St. Louis -- until 2005 to meet the standards, but a three-judge panel of the U.S. Court of Appeals for the D.C. Circuit struck that policy in July as "arbitrary and capricious."

Once the reclassification takes effect, officials said, the region will have to write a new clean air plan and impose stricter standards for factories, plants and perhaps automobiles. The precise effects are unclear, they said.

Metropolitan Washington recorded its worst levels of air quality in a decade this summer, with two Code Purple days, which means the air is "very" unhealthful; nine Code Red days, the next-highest level of alert for dangerous ozone; and 19 Code Orange days, which means the air is unhealthful for sensitive groups.