But researchers have not understood how these microscopic particles actually kill people.

"We now know how the inflammation in the lungs caused by air pollutants leads to death from cardiovascular disease," said Dr. Gokhan Mutlu of Northwestern University in Chicago, who studied the effects of air pollution in mice.

Lungs inflamed by pollution secrete interleukin-6, an immune system compound that sparks inflammation and has been shown to make blood more likely to clot.

The research appears in the Journal of Clinical Investigation. It follows a study last week in the New England Journal of Medicine that found breathing diesel fumes interfered with heart attack survivors' ability to break down blood clots.

Mutlu got a clue about the clotting issue two years ago when he was studying the effects of air pollution on heart failure in mice. Mice who had been exposed to pollution bled significantly less.

"They were forming blood clots," he said in a telephone interview.

In the latest study, he and colleagues exposed mice to particles of air pollution collected by the US Environmental Protection Agency. These were mixed with a saline solution and injected into the lungs of mice.

Mice exposed to pollution showed a 15-fold increase in interleukin-6 just 24 hours later. That time frame is important because some studies have shown a spike in air pollution can boost heart attacks with 24 hours.

Mutlu and colleague Dr. Scott Budinger said they were able to prevent this clotting by suppressing immune cells in the lungs called macrophages that attack foreign substances and secrete interleukin-6.

Mice with suppressed immune responses did not show increased blood clotting. "This suggested that interleukin-6 was the driving force," Mutlu said.

He said most people understand that high levels of air pollution can make lung diseases such as asthma worse.

"The same thing is not known for patients with coronary artery disease or congestive heart failure," Mutlu said. "I think we need to increase the awareness of this link among those individuals."