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Study establishes link between air pollution, ischemic strokes

October 31, 2005

BOSTON - The risk of ischemic stroke - which results when a blood clot travels to the brain - increases with a rise in particulate air pollution, according to a study in Stroke: Journal of the American Heart Association. Led by researchers at Beth Israel Deaconess Medical Center (BIDMC) and the Harvard School of Public Health (HSPH), the findings are described in the October 27, 2005 on-line rapid access issue of the journal.

The study, which examined the air quality on a total of 37,000 days in nine separate cities, found that risk of hospitalization for ischemic stroke was one percent higher on days with relatively high levels of air pollution, compared with low-air pollution days, according to lead author Gregory Wellenius, Sc.D., postdoctoral fellow in cardiology at BIDMC.

"Although these effects sound relatively small, given the large number of people exposed to air pollution and the large number of people at risk for stroke, from a public-health perspective the actual number of strokes [we're talking about] could be significant," says Wellenius. Stroke is the third leading cause of death in the U.S., affecting more than 700,000 individuals each year.

Previous work by Wellenius and coauthors Murray Mittleman, MD, DrPH, of BIDMC's Cardiovascular Epidemiology Research Unit and Joel Schwartz, PhD, of HSPH had established a "consistent increased risk" for cardiac health problems associated with exposure to ambient air particles.

"Air pollution has been shown to trigger heart attacks and to aggravate the conditions of patients with congestive heart failure," says Mittleman, who is also Associate Professor of Medicine at Harvard Medical School. "These new findings, demonstrating that incidence of clot-based strokes also increase, is in keeping with our earlier data showing a relationship between air pollution and heart and lung disorders."

(The study also looked at the incidence of hemorrhagic stroke, which is caused by bleeding in the brain, during the same "high pollution" days, notes Wellenius, but found no association between the two.)

The air pollution in question - particulate matter smaller than 10 micrometer in diameter - includes particles from car and truck exhaust, power plants and refineries. The measurements were provided by the U.S. Environmental Protection Agency from nine U.S. cities including Birmingham, Ala., Chicago, New Haven, Conn., Cleveland, Detroit, Minneapolis, Pittsburgh, Salt Lake City and Seattle.

The authors analyzed hospital admissions among Medicare patients who represented an average age of 79. Seventy-five percent of the patients were white and 61 percent were female. Their findings showed that during the course of their study, there were 155,503 hospital admissions for ischemic stroke. The final analysis demonstrated a 1.03 percent rise in ischemic stroke on the days with the highest pollution measures.

"We don't know exactly what mechanisms are involved that trigger these cardiac events," says Wellenius. "However, we do know that particulates in the air promote inflammation, which is a significant risk factor for cardiac events; that exposure to particulates can lead to changes in heart rate and blood pressure; and that pollution can cause changes in coaguable states [blood clotting abilities]."

The authors say that future research will focus on finding out which pollutants are most toxic, and which patients are at greatest risk for health problems stemming from air pollution.

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

Beth Israel Deaconess Medical Center