Washington ? The Environmental Protection Agency on Friday identified 225 counties in 20 states that don’t meet new clean air standards designed to protect against one of the tiniest but most harmful pollutants: microscopic soot.

The counties and the District of Columbia will have to move quickly to come into compliance. They have three years to devise a pollution-reduction plan for fine particles and then must meet federal standards by 2010.

Failure to comply could mean a county will have to limit development and its state could lose federal highway dollars.

EPA Administrator Mike Leavitt announced the list, which included 18 fewer counties than the agency identified in a preliminary report in June. He emphasized the agency was for the first time specifically regulating for fine particles, or soot, that are 2.5 micrometers in diameter — 1/30th the width of a human hair. Such pollution comes from power plants, car exhaust, diesel-burning trucks, wood-burning stoves and other sources.

EPA considers it potentially the most significant air quality health standard, because soot can penetrate deeply into the lungs.

“This is not a story about the air getting dirtier,” Leavitt told a news conference. “It is a story about higher, more stringent standards and healthier air.”

About 95 million people live in the 225 counties and the nation’s capital. EPA estimates the new standard, once met, will prevent at least 15,000 premature deaths, 95,000 cases of bronchitis and 10,000 hospital admissions for respiratory and cardiovascular diseases.

All but three of the states — Missouri, California and Montana — are east of the Mississippi River. The counties and states at issue might modify transportation plans, require new pollution controls when factories expand or impose stricter vehicle emission and inspection programs.