A brief uptick in traffic-related air pollution may be enough to increase a person’s short-term risk of stroke, new research suggests.

An analysis of 10 years of data from a major Boston stroke center has found that strokes are more likely to occur immediately following 24-hour periods in which air quality drops into the range the Environmental Protection Agency (EPA) considers “moderate.”

“At levels that the EPA considers to be generally safe, we found an important effect of ambient air particles, which is one of many pollutants in the air, but an important one,” says study coauthor Gregory A. Wallenius, Sc.D., an assistant professor of community health at Brown University Medical School, in Providence, R.I. Wallenius collaborated with researchers at Beth Israel Deaconess Medical Center and the Harvard School of Public Health, both in Boston.

In their report, published Monday in the Archives of Internal Medicine, Wallenius and his colleagues compared 1,705 stroke cases in the Boston area with detailed data on the day-to-day levels of various airborne pollutants, including vehicle emissions such as particulate matter, black carbon, and nitrogen dioxide.
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After taking into account each patient’s medical history, the researchers concluded the odds of having a stroke were 34% higher following a day of “moderate” air quality than following a “good” air day.

Based on this finding, they estimate that a 20% reduction of levels of fine particulate matter would have prevented 6,100 of the 184,000 stroke hospitalizations that occurred in the northeastern United States in 2007.

The study doesn’t show that air pollution directly triggers strokes, although the researchers say that is biologically plausible. And because this is just one study in one location, Wallenius says, the findings don’t necessarily argue for tighter restrictions on U.S. air quality standards.

Still, he says, “if this was replicated in other parts of the country and in other populations, and similar findings came out, it would be a good idea to review the health warnings that come with certain particle levels.”

A second study, appearing this week in the Journal of the American Medical Association, provides additional evidence that air pollution may increase cardiovascular risk. In that study, a team of French researchers re-analyzed data from 34 previous studies conducted around the world. Most of the studies used methods similar to those of Wallenius and his team.

Higher levels of airborne pollutants—including particulate matter, carbon monoxide, nitrogen dioxide, and sulfur dioxide—were associated with a slight increase in the short-term risk of heart attack, the study found. As with Wallenius’ study, the researchers observed an uptick in heart risk even at pollution levels classified as safe by the World Health Organization.

The increase in heart-attack risk was small on the individual level, but it can have a substantial impact at the population level, says Hazrije Mustafic, M.D., the lead author of the analysis and a researcher at the University Paris Descartes.

“We must keep in mind that the entire population is exposed to air pollution in industrial countries, so the effect on public health is not negligible,” Mustafic says.

For instance, she says, although an increase in carbon monoxide levels of 10 micrograms per cubic meter raises an individual’s short-term heart-attack risk by just 5% or so, a change in air quality of that magnitude could be expected to account for 4.5% of all heart attacks in the exposed population.

The top sources of air pollution are the burning of fossil fuels (such as gas, oil, and coal) and industrial emissions. The pollutant Wallenius and his team focused on, fine particulate matter, consists of microscopic particles of metal, carbon, sulfates, and other materials. These specks of dust are about 30 times smaller than the thickness of a human hair, so they can be inhaled deep into the lungs.

Breathing fine particulate matter may harm the cardiovascular system in two ways, Wallenius says. If particles find their way into the bloodstream they can make blood vessels less elastic, and they may also boost activity of the sympathetic nervous system, which tends to increase heart rate and blood pressure and trigger the release of stress hormones.

“That would change how the blood flows through the body,” Wallenius says. “That could then take somebody [who's] susceptible to stroke and push them over the edge to actually having a stroke.”

In addition to affecting blood flow, air pollution appears to increase inflammation, an immune-system response that is believed to contribute to both heart disease and strokes.

If inhaled pollutants reach the alveoli—the tiny sacs in the lungs where oxygen and carbon dioxide pass into and out of the bloodstream, respectively—they trigger an inflammatory reaction, Mustafic says. “These pollutants can also spread through the blood stream and reach the heart,” she adds.

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Wallenius and Mustafic agree that people who are already at high risk of stroke or heart attack should consider taking steps to cut their exposure to very high levels of pollution.

But healthy people shouldn’t view the findings as a call to go out and buy face masks, says Robert D. Brook, M.D., a cardiologist at the University of Michigan, in Ann Arbor, who coauthored an editorial accompanying Wallenius’ study.

“The wrong thing to do is to get alarmed…that every time you’re in traffic or every time you’re exposed to a little bit of smog, you are going to get a heart attack or die,” Brook says. The best way to prevent a heart attack or stroke, he says, is to control personal factors such as blood pressure, cholesterol, smoking, and exercise.