Most non-compliant states are clustered around the Midwest, which has the lion's share of US coal-fired generation, and in Northeast states between Washington, D.C. and New York, the EPA said. Other counties with unhealthy levels are in Southern California and near Atlanta, Georgia, it said.

Governors of those states must submit cleanup plans by February 2008, with attainment of safe levels of particulate pollution required by 2010, the EPA said.

Counties with the most severe problems -- like those around Los Angeles -- could get a five-year extension, delaying attainment until 2015, the EPA said.

There is no outright penalty for noncompliance, but states that fail to submit plans could lose federal transportation funds.

Outgoing EPA Administrator Mike Leavitt touted data that showed particulate levels in 2003 fell to their lowest mark since 1999, when the government monitoring began.

"The air is cleaner than it was a year ago," Leavitt said. "We're seeing a reduction in pollutants."

Some 90 percent of affected counties will meet federal standards between 2010 and 2020 because of new rules affecting emissions from diesel engines and coal-fired power plants, Leavitt said.

Environmental groups criticized the EPA for delaying finalizing rules to reduce power plant emissions until March 2005. The EPA had planned to finalize its Clean Air Interstate Rule by the end of the year, but delayed the action so that Congress could consider legislation early next year.

"The administration must end the foot-dragging and finally cut the harmful pollution from power plant smokestacks," said John Balbus at Environmental Defense, an environmental lobby.

State governors need the new federal rules to compel utilities in their states to act, Balbus said.

Tiny particles come from a wide array of sources ranging from cars and trucks to wood-burning stoves, forest fires, power plants and factories.