But he vigorously denies that his rulings were in any way dictated by the White House.

"Every one of my decisions has been my decision. If they were not my decisions, I would not be here," he said.

George Gray, EPA science adviser and assistant administrator for research, said his boss skillfully navigates the difficult task of incorporating sound science with effective public policy.

"He has a real challenge reminding people that science doesn't provide answers. It provides input and you have to use that in some kind of a broader process to inform decisions that you need to make."

That process has been corrupted, says U.S. Sen. Barbara Boxer, a California Democrat who chairs the Senate committee with EPA oversight.

"Almost every time he acts," Boxer said, "it's against his scientists."

Unseen invaders

The danger of a short-term spike in pollution particles was illustrated by the 1948 deaths in Donora, and even more alarmingly four years later, when a smoky "fog" killed thousands in London.

Scientists at the time had only crude ways to measure this emerging threat, known as particulate matter. And in the ensuing decades, as some experts began to explore the possibility that such microscopic particles were harmful over the long term, many had their doubts.

"A lot of people were very skeptical," said Henderson, the toxicologist. "It's not intuitive."

In 1993, a Harvard study of more than 8,000 people in six cities began to convince the skeptics. Then came papers in 1995 and 2002, using data from several hundred thousand people, collected by the American Cancer Society.

The clearest culprit seemed to be "fine" particles - those measuring roughly 1/40 the thickness of a sheet of office paper.

For every 10-microgram increase of fine particles per cubic meter of air, according to the 2002 study, there was an 8 percent increase in heart attacks and other cardiopulmonary deaths. It was a modest but statistically significant impact, and there was evidence of harm even in relatively clean cities.

The findings held up even when other risk factors were taken into account: smoking, diet, occupation, exposure to other kinds of pollution.

The six-city study, meanwhile, found that the dangers from fine particles were even greater. And new experiments suggested just how these unseen invaders might be doing their damage.

Studies on animals, for example, indicated that long-term inhalation of such particles causes inflammation, which in turn can lead to plaque buildup in the arteries.

Still unclear was whether particles emitted by certain sources were more harmful than others - say, cement dust vs. diesel soot - but collectively there was evidence of harm.

The EPA was slow to act - so slow that in 2003 the American Lung Association and others sued it to speed the review.

As the law mandates, the agency weighed the evidence with the help of an in-depth review by a panel of independent scientists: the Clean Air Scientific Advisory Committee.

Armed with the science panel's advice and detailed research by EPA staff, the administrator is charged with setting standards that are "requisite to protect the public health" with an "adequate margin of safety."