Air pollution, even at levels generally considered safe by federal regulations, increases the risk of stroke by 34 percent, Beth Israel Deaconess Medical Center researchers have found. Writing in the Feb. 14, 2012 issue of the Archives of Internal Medicine, researchers who studied more than 1,700 stroke patients in the Boston area over a 10-year period found exposure to ambient（周围的） fine particulate matter, generally from vehicle traffic, was associated with a significantly higher risk of ischemic（缺血性的） strokes on days when the EPA's air quality index for particulate matter was yellow instead of green.

Researchers focused on particles with a diameter of 2.5 millionths of a meter, referred to as PM2.5. These particles come from a variety of sources, including power plants, factories, trucks and automobiles and the burning of 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," says Murray A. Mittleman, MD, DrPH, the study's senior author, a physician in the CardioVascular Institute at Beth Israel Deaconess Medical Center and an Associate Professor of Medicine at Harvard Medical School. "Any proposed changes in regulated pollution levels must consider the impact of lower levels on public health."

"Considering that almost everyone is exposed to air pollution and is at risk for stroke, that's actually a pretty large effect," adds Gregory Wellenius, ScD, the study's lead author and an Assistant Professor of Community Health at Brown University.