Ninety-nine million Americans -- including residents of the District of Columbia and parts of Maryland and Virginia -- are breathing unhealthful air that can cause respiratory problems and even premature death, according to assessments released yesterday by the Environmental Protection Agency.

The agency identified 243 counties that fail to meet national air standards for fine-particle pollution -- mainly soot -- in response to state submissions that designated 141 counties. Once the rulemaking process is complete, state and local officials will have to devise plans to reduce the pollution. States now have three months to respond before the agency issues a final rule in November.

EPA Administrator Mike Leavitt said the announcement was "about getting our air cleaner and our standards getting tougher."

"This is a very good news story," he said.

Environmentalists praised the EPA's efforts to reduce emissions of sulfur dioxide and nitrogen oxide, which come from power plants, industrial boilers, boats and other sources, but they questioned whether the administration is moving fast enough. Fine particles of both pollutants infiltrate people's lungs to cause an array of illnesses.

"This is about defining areas in the country that have the problems and being able to set up the solutions," said John Balbus, who directs Environmental Defense's health program. "With particulates, there are many cases where the technology exists to solve the problem today, but the solutions are not being implemented until the next generation."

The EPA is calling for power plants to install pollution-control technology that would reduce emissions of sulfur dioxide and nitrogen oxide by 40 percent in 2010. Some environmental and health advocates have called for a 90 percent reduction by that time.

"There is always a desire to move faster, but the truth is, this is faster," Leavitt told reporters.

The administration is facing resistance from state and industry officials who fear the dirty-air designations will hinder economic development. States will have to develop air quality plans in response to the federal designations, though there are no specific penalties for failing to meet national goals.

Virginia, for example, identified no counties as failing to meet standards, while the EPA labeled Arlington, Fairfax, Loudoun and Prince William counties, along with the cities of Alexandria, Fairfax, Falls Church, Manassas and Manassas Park, as too sooty in its assessment of the Washington area. In Maryland, the EPA listed Anne Arundel, Baltimore, Carroll, Charles, Frederick, Harford, Howard, Montgomery, Prince George's and Washington counties along with Baltimore City. Also on the list was the District.

Under federal standards, levels of particulates must not exceed an annual average of 15 micrograms per cubic meter of air, or 65 micrograms per cubic meter on any given day. The particles are one-twenty-eighth the width of a human hair.

Industry officials and health advocates differ sharply on how much it will cost to reduce fine-particle pollution. Paul Billings, vice president of national policy advocacy for the American Lung Association, said the electric power industry has spent too little on reducing pollution. While it would cost millions of dollars per facility to install new controls, that could translate to an average cost of $2,000 per ton of reduced pollution at a given plant.

Jeffrey Marks, director of air quality for the National Association of Manufacturers, countered that cost estimates "to comply with both the new ozone and particulate matter standards range from about $50 billion to hundreds of billions of dollars."

Some areas, such as the District, face the problem that they are simply subject to dirty air blowing in from surrounding areas.

"Aside from a lot of hot air from K Street lobbyists, we don't have a lot of homegrown pollutants," said Tony Bullock, spokesman for Mayor Anthony A. Williams, adding that the city encourages residents to use the subway, rather than drive, as one countermeasure. "To a large degree, we're uniquely unable to do much, which is a sad way to be."