The Environmental Protection Agency said Tuesday that 243 counties -- mostly in the eastern third of the country -- will probably be found in violation of the federal air standard for soot.

State officials will have to develop plans aimed at cutting the pollution by 2010 or 2015, depending on the severity of the air quality problem, or face sanctions, including possible loss of highway funds.

Reducing microscopic soot, EPA Administrator Mike Leavitt told reporters, is "the single most important action we can take to make our air healthier."

But getting those reductions has been far from easy.

The EPA issued a new air quality standard for "fine particulates" -- particles less than one-seventh as wide as a human hair -- in 1997, only to have it challenged by industry. The legal fight went to the U.S. Supreme Court, and eventually the EPA prevailed.

But the agency has yet to implement the tougher standard. The action Tuesday was the first step to designate what areas have air so dirty that additional pollution controls will be needed.

Leavitt said the designations are preliminary, and some counties may be taken off the list after further discussions with state officials. A final designation of areas in noncompliance will be made in November.

The need to address the problem of sooty air is clear, Leavitt said. Airborne microscopic soot annually causes 15,000 premature deaths, 95,000 cases of chronic or acute bronchitis, and thousands of hospital admissions because of respiratory or cardiovascular illnesses, he said. Children, the elderly and people with asthma are most susceptible.

The 243 counties listed Tuesday are home to nearly 100 million people. In May, governors submitted a list of 141 counties they view as failing to meet the soot requirements. The EPA added another 102 counties -- many of them not because their air is too dirty, but because they have activities that cause pollution to drift into nearby areas that are in noncompliance.

Leavitt said rules being phased in to reduce sulfur in gasoline and diesel fuel, along with requirements for cleaner trucks and programs aimed at curbing interstate transfer of pollution from power plants, will go a long way toward meeting the standards.

But he acknowledged that the states may have to take additional steps, such as modifying transportation plans or requiring new pollution controls when factories expand. Other measures could include restricting wood stoves and open fires or retrofitting school buses with improved emission controls, he said.