A U.S. study found that patients who were already susceptible to heart attacks, such as diabetics, people with heart disease or seniors, experienced a 48% rise in their risk of a heart attack in the two hours after being exposed to severe air pollution containing particulates. The risk increased to 62% in 24 hours.

Particulates -- invisible pollutants that are less than 2.5 micrometres (2.5/100ths of a millimetre) in diameter -- are predominantly released into the atmosphere through automobile emissions, power plants and fireplaces.

The results stress the importance of heeding smog warnings for those at high risk for a heart attack, said Dr. Murray Mittleman, an assistant professor at Harvard University's medical school, who led the study.

"Try and spend more time indoors, preferably with the air conditioning on," he said. "These particles are so small they do penetrate into indoor air and air conditioning will filter them out."

The study did not address precisely how the particles can bring on heart attacks.

Past research, however, has shown that the pollutants are so small they can slip past the body's normal defence mechanisms and cause inflammation in the alveoli, the tiny air sacs of the lungs, causing inflammation and clotting. Such symptoms can increase the likelihood of a heart attack by blocking blood flowing to the heart.

Health Canada recently studied the effects of smog and air pollution on 11 Canadian cities and concluded that air pollution causes 5,000 premature deaths annually.

For the study, researchers interviewed 722 people who were admitted to Boston hospitals approximately four days after they suffered heart attacks.

The study, which appears in today's edition of Circulation, a research journal of the American Heart Association, is believed to be the first to examine whether air pollution has an immediate effect on the risk of heart attack.

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