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Even at levels that comply with federal regulations set by the Environmental Protection Agency, air pollution may increase the risk of stroke, according to research led by Gregory Wellenius, assistant professor of epidemiology. Wellenius' study was published last month in Archives of Internal Medicine.

Wellenius and his team retrieved the medical records of 1,705 patients at Beth Israel Deaconess Medical Center in Boston hospitalized with ischemic stroke ­— a condition often caused by the blockage of an artery to the brain ­— and compared the timing of the onset of their symptoms with air pollution values.

Previous studies had used the date of hospital admission rather than timing the onset of symptoms, a methodology more "prone to error" as half of patients had their strokes on a different calendar day than the date of their hospitalization, Wellenius said. 

Boston was "always in compliance with federal standards" for air pollution during the study, Wellenius said, and yet the team found a 34 percent higher risk of stroke on days with "moderate" levels of pollution, as compared to "good" days. 

Pollution, as defined by the EPA, includes measurements of ozone, carbon monoxide, sulfur dioxide and nitrogen dioxide, as well as different size fractions of particles and lead, though this last component is not as prevalent since it ceased to be present in gasoline.

"At levels that the EPA considers to be generally safe, we're seeing a large increase in rate of stroke, which suggests that the EPA may not be adequately protecting the public," Wellenius said. "This really is a public health problem." Wellenius offered the tightening of regulations as a potential solution.

"I certainly can imagine where air quality will affect all sorts of things health-wise," said Elaine Jones, who specializes in stroke at Roger Williams Medical Center. "Why there should be a direct effect on strokes is a little surprising to me." 

Even though the study suggests a correlation between air quality and stroke risk, there may not be a direct cause-and-effect relationship between the two, Jones said.

"In science, you can have things that appear to be related but have nothing to do with each other," she continued. "On days of poor air quality, perhaps people will eat differently, you know?"

Jones added she would not change her patients' care or public policy based on Wellenius's study.

Wellenius' next goal is to replicate his findings in different cities across the country, as they may have different health risks. "Pollution mixtures change in different cities," he said. "So the pollution in (Los Angeles) is very different from the pollution in Boston."


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