Boston, Massachusetts – Air pollution, primarily from vehicle exhaust, increases the risk of stroke by 34 per cent even at levels generally considered safe by U.S. federal regulations, according to researchers at the Beth Israel Deaconess Medical Center in Boston.

Writing in the Archives of Internal Medicine, researchers who who studied more than 1,700 stroke patients in the Boston area over a ten-year period found that exposure to ambient fine particulate matter, generally from vehicle traffic, was associated with a significantly higher risk of ischemic strokes on days when the EPA’s air quality index for particulate matter was in the yellow warning zone instead of green.

The researchers focused on particles with a diameter of 2.5 millionths of a meter, known as PM2.5. These come from a variety of sources, including trucks, automobiles, power plants, factories, and burning wood. They can travel deeply into the lungs and have been associated in other studies with increased numbers of hospital visits for cardiovascular diseases such as heart attacks.

“The link between increased stroke risk and these particulates can be observed within hours of exposure and are most strongly associated with pollution from local or transported traffic emissions,” said Dr. Murray Mittleman, the study’s senior author. “Any proposed changes in regulated pollution levels must consider the impact of lower levels on public health.”

The researchers used the patients’ medical records and matched the onset of stroke symptoms in each patient to hourly measurements of particulate air pollution taken at the nearby Harvard School of Public Health’s environmental monitoring station. The team was able to estimate the hour the stroke systems first occurred, rather than when patients were admitted to the hospital.

The team was able to calculate that the peak risk to patients from pollution exposure occurs 12 to 14 hours before a stroke. They also found that black carbon and nitrogen dioxide, two pollutants associated with vehicle traffic, were closely linked with stroke risk, suggesting that pollution from cars and trucks may be particularly important.

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