Air pollution reduces blood flow and interferes with the body's natural ability to break up blood clots, says a study.

Researchers say this may help explain why pollution can cause heart attacks.

And the study, published in the New England Journal of Medicine, also suggests that heart patients trying to shape up might do their exercising away from traffic.

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 Organisation 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.Reuters