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Air pollution, researchers report, can trigger a heart attack in vulnerable people within hours of exposure. In a study of 772 Boston-area heart attack patients, the investigators found that patients were more likely to suffer an attack within hours of air-pollution spikes in the city.

Previous research has suggested heavy-pollution days take a toll on the heart, increasing not only heart attack risk, but also sending up blood pressure, accelerating heart rate and causing problems with implanted heart defibrillators.

The current study revealed that exposure to high levels of tiny air pollutants, such as those spewed into the environment during rush-hour traffic, temporarily raised patients' heart attack risk by about 50% to 70%. A substantial spike in air pollution in the 2 to 24 hours before patients' symptoms set in appeared to elevate heart attack risk.

Experts speculate that when fine particles are deposited in the lungs, this sets off an inflammatory response that may make blood flow more sluggish and prone to clotting. In people with artery-clogging plaques, such cardiovascular stress may trigger a heart attack.

Past studies have uncovered ``very consistent effects'' of air pollution on cardiovascular health. Based in part on this body of research, the Environmental Protection Agency (EPA) in 1997 set standards for particulate matter measuring less than 2.5 micrometers--the type Mittleman and his colleagues studied. The agency determined that no one should be exposed to more than 65 micrograms of the pollutants per cubic meter of air over a 24-hour period.

The Circulation (2001;103) added that older people with risk factors for heart attack, such as high blood pressure or diabetes, should listen to the pollution alerts that many news outlets broadcast.