The researchers tested 20 male volunteers, all of them heart attack survivors, who pedaled an exercise bike while breathing diluted fumes from the exhaust of an idling Volvo diesel engine.

The exposure was comparable to the pollution levels found while driving in traffic.

Doctors already know that long-term exposure to air pollution increases the risk of heart problems. The World Health Organization has estimated that it causes 800,000 premature deaths worldwide each year.

The new study looked at one particularly suspect element of air pollution and how it affected people over the short term.

Nicholas Mills of Britain's Edinburgh University and his colleagues found that when the volunteers breathed diesel fumes, their hearts were far more likely to be starved of oxygen than when they were breathing clean air.

And when they tested the blood of the men, they found that the fumes inhibited the body's natural system of breaking down the clots that can spark a heart attack or stroke.

That may explain the results of population-based studies showing that air pollution increases heart problems, they said.

It is not known exactly why the hearts became starved of oxygen or which substance in the exhaust was responsible for the effects.

"The study was specific in evaluating the effects of dilute diesel exhaust, an extremely complex mixture of particles and gases; it is not possible to glean which constituents of diesel exhaust were responsible for the observed effects," Dr. Murray Mittleman, of Boston's Beth Israel Deaconess Medical Center, wrote in a commentary.

Although the study was only done on men with a previous heart attack, "these findings may represent the tip of an iceberg" and apply to anyone at risk for a heart attack, he said.

Exercise is already known to be beneficial and it especially decreases the risk that a person will have a heart attack while exerting themselves, Mittleman said.