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Cutting air pollution has an instant impact on heart health, experts believe, after reviewing studies of the 2008 Beijing Olympics.

The work in the Journal of the American Medical Association involved medical tests on 125 volunteers living in one of the world's most polluted cities.

When pollution dipped during the Games, the researchers saw significant signs of better health among the volunteers.

They say this is "biological proof" that pollution can harm the heart.

The British Heart Foundation said the link between heart disease and pollution had been known for some time but it was still not clear why this relationship existed.

Factories closed

China took major steps to improve Beijing air quality for the 2008 Olympic Games after the International Olympic Committee had warned of some events being postponed.

China managed to clean up its air that summer by closing factories and allowing cars on the roads only every other day.

There were concerns at the time that air quality could prove hazardous to the health of athletes and spectators.

The team at the University of Southern California took blood samples from the healthy volunteers both before and after the Games - when pollution levels were high - as well as during the Games, when the levels were much lower. This was to see if changing levels of air pollution had any effect on heart risk.

Specifically, they measured blood pressure and looked for blood markers linked to clotting and inflammation - known risk factors for heart disease.

They saw big improvements in these measures when the pollution levels went down.

Writing in the Journal of the American Medical Association, the study' s lead author, Prof Junfeng Zhang, said: "We believe this is the first major study to clearly demonstrate that changes in air pollution exposure affect cardiovascular disease mechanisms in healthy young people."

Caroline Dilworth, from the National Institute of Environmental Health Science (NIEHS), which provided funding for the study, said: "When air pollution levels are lowered, health benefits can be immediate."

But the researchers stress that their work could not look at long-term consequences, such as actual heart attack or stroke risk.

Amy Thompson of the British Heart Foundation said more research was needed.

"This small study found that exposure to higher levels of air pollution made the blood more likely to clot. For someone who already has heart disease, this could possibly trigger a heart attack.

"If you have heart disease, try to avoid spending long periods in heavily polluted areas where possible. If you have any concerns about your condition, you should speak to your GP."