Breathing dirty, polluted air may increase your risk of suffering the most common type of stroke, according to a new study.

Air pollution has already been linked to a higher risk of heart attack and other heart-related problems such as heart failure. But researchers say this is the first study to link air pollution with an increased risk of ischemic stroke. Ischemic strokes are caused by a blood clot that blocks blood flow to the brain.

Researchers found the number of ischemic strokes reported in nine major U.S. cities was 1percent higher on days that air pollution levels were high compared with days when air pollution levels were relatively low.

Although a 1percent increase may seem minor, researchers say the impact of air pollution on stroke rates may be substantial because air pollution is so widespread.

The researchers write that although the relative increase is small, given the large number of people simultaneously at risk for ischemic stroke and exposed to urban pollution, even a small risk may be of significant public health interest. “Especially,” notes researcher Gregory Wellenius, ScD, “when you realize that someone in the United States has a stroke every 45 seconds." Wellenius is a postdoctoral fellow in cardiology at Beth Israel Deaconess Medical Center in Boston.

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Air Pollution Linked to Stroke

Researchers say previous studies have shown that exposure to air pollution raises the risk of heart attack, heart failure, and irregular heartbeat. While the mechanism remains unclear, the researchers suggest that exposure to particulate air pollution may change blood vessel function, leading to increased blood clotting and blood flow.

Some studies have suggested that air pollution may increase the risk of stroke, but most of these studies did not distinguish between ischemic and hemorrhagic stokes, which are caused by a ruptured blood vessel.

In this study, published in Stroke: Journal of the American Heart Association, researchers looked at the association between daily air pollution levels and hospital admissions among Medicare recipients for both types of strokes in nine major U.S. cities (Birmingham, Ala., Chicago, Cleveland, New Haven, Conn., Detroit, Minneapolis, Minn., Pittsburgh, Salt Lake City, and Seattle).

Specifically, they looked at daily Environmental Protection Agency levels of particles in the air smaller than 10 micrometers in diameter, which includes particles released by motor vehicles, power plants, and other sources for more than 37,000 days in the nine cities.

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Hemorrhagic Strokes

More than 150,000 ischemic strokes and 19,000 hemorrhagic strokes were reported on those days.

The results showed that a change in the level of small particulate matter from a low level to a higher level was associated with a 1 percent increase in ischemic stroke hospital admissions.

No relationship was found between air pollution and hemorrhagic stroke rates, but researchers say that may be due in part to the low number of such strokes that occurred during the study period.

"Taken together with previous work, these results support the idea that reducing exposure to particulate matter may reduce the risk of strokes and heart attacks," says Wellenius.

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By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Wellenius, G. Stroke: Journal of the American Heart Association, Oct. 27, 2005; vol 36. News release, American Heart Association.