Car exhaust and other air pollution, even at levels considered safe by federal regulations, may substantially increase the risk of a stroke, a research team from Beth Israel Deaconess Medical Center has found.

After reviewing the medical records of more than 1,700 stroke patients in the Boston area over 10 years, the researchers found a 34 percent increase in the risk of ischemic strokes on days with moderate air quality compared with days when the air was rated good by the US Environmental Protection Agency. Ischemic strokes occur when a clot blocks blood flow to part of the brain.

On days with moderate air quality, levels of fine particulate matter is higher but within allowable limits.

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“This is a significant study because we have documentation that the risk of stroke can be elevated when the air quality is still within the guidelines set by the current EPA regulations,” said Dr. Murray A. Mittleman, an author of the study who teaches at Harvard Medical School and works in the CardioVascular Institute at Beth Israel Deaconess. “This implies that the current regulations can be strengthened further to prevent these catastrophic health events.”

The researchers said the EPA should also revise the language it uses to describe the health consequences of moderate air quality to reflect the apparent increased stroke risk.

The federally funded study, published today in the journal Archives of Internal Medicine, focused on particles with a diameter of less than 2.5 millionths of a meter, which come from sources including power plants, factories, automobiles, and burning wood. They can lodge deep in the lungs and have been linked to difficulty breathing in asthmastics and heart attacks.

The researchers matched the onset of stroke symptoms in patients to hourly measurements of air pollution taken at the Harvard School of Public Health’s environmental monitoring station, which was within 13 miles of 90 percent of the stroke patients’ homes. They estimated the hour the stroke symptoms occurred, rather than relying on when patients were admitted to the hospital. They also included only strokes confirmed by neurologists, rather than going by insurance billing records.

“We think that this study is novel in that it has high-quality data on both air pollution exposure and stroke diagnosis,” said Gregory Wellenius, another author of the study and an assistant professor of community health at Brown University.

The researchers calculated that the peak risk to patients from pollution exposure occurs 12 to 14 hours before a stroke. They also found heightened stroke risk was more closely tied to levels of two air pollutants from vehicles, black carbon and nitrogen dioxide, than to pollution from other sources.

Strokes are a leading cause of long-term disability and the third leading cause of death in the United States. An estimated 795,000 Americans suffer a new or recurrent stroke every year, resulting in more than 135,000 deaths and 829,000 hospital admissions annually.

Previous studies have shown an association between air pollution and stroke risk, but Dan Costa, interim national program director for air climate and energy research in the EPA’s Office of Research and Development Research, said, “This study shows that concentrations of particulate matter can be lower than previously thought for strokes, and that the duration of exposure can be shorter.”

He said the EPA will issue new recommendations this summer on what constitutes dangerous levels of particulate matter in the air. There will be a public comment period over the following year before any new regulations take effect.

But he said the conclusions of the new study, which his office helped fund, come too late to be taken into account in the drafting of the new rules. They will be considered the next time the EPA reviews its air quality standards, in five years.

“I think this study shows the urgency of looking at particulate matter at low concentrations and for durations for exposure that are shorter than we typically monitor,” Costa said. “The weight of evidence suggests this needs continued research to keep the particulate matter levels as low as we can.”

Doug Brugge, a professor of public health at Tufts University School of Medicine who studies air pollution and cardiovascular disease, said that the study’s methods appeared to be sound and that it reinforces similar research.

“This study fits into a much broader literature that convincingly shows there are risks below the current EPA standards for fine particulate matter,” he said. “Particulate matter is the biggest environmental risk we face.”

In another study published today in the same journal, researchers at Rush University Medical Center in Chicago found that prolonged exposure to coarse and fine particulate matter in the air is associated with cognitive decline in older women.

The Boston researchers estimated that reducing the particulate matter they studied by about 20 percent could have prevented 6,100 of the 184,000 stroke hospitalizations in the northeastern United States in 2007.

They noted that Boston has relatively clean air and that their findings suggest the consequences could be more costly in other cities.

“We still find that within these moderate levels the risk of stroke is higher on days with more particles in the air,” Mittleman said.

When asked how Boston residents should respond to the findings, Mittleman said those at higher risk of a stroke should closely monitor air quality advisories.

“They probably should avoid exercising outdoors when the air quality index is anything beyond satisfactory,” he said.