THURSDAY, Feb. 15 (HealthDay News) -- Inhaling fine particle air pollution, at least during warm weather, may increase people's risk for stroke, Finnish researchers report.

Earlier research has linked air pollution with heart attack, but this is the first study to look at ultra-fine particle air pollution and stroke.

"We evaluated the effects of these and other particle sizes on stroke to help determine the most harmful particle sources in the air," study author Jaana Kettunen, a researcher for the National Public Health Institute in Kuopio, said in a prepared statement.

The report is published in the March issue of Stroke.

Fine particle pollution is made up of tiny particles of dust and soot, less than 2.5 microns, about 1/30th the width of a human hair. They mostly come from car exhaust, power plant emissions and burning of fossil fuels. Ultra-fine particles are less than 0.1 microns and are usually found in car exhaust.

In the study, Kettunen's team looked at air pollution data, as well as data on people aged 65 and older who died from stroke in Helsinki from 1998 to 2004. They analyzed both warm and cold months.

"We found that, during the warm season, there was a positive association between stroke mortality among the elderly and current-day level of fine particles," Kettunen said in a statement. "There was a 6.9 percent increase in stroke death for every 6 micrograms per cubic meter of air increase in fine particles. In addition, there was 7.4 percent increase in stroke mortality for every 6 micrograms per cubic meter of air increase of previous-day fine particle levels," she said.

This seemed to be a warm-weather phenomenon, however: There was no association in the cold season with stroke and fine particles, ultra-fine particles or carbon monoxide, the researchers reported.

Kettunen suggests that people might be more vulnerable to air pollution in warmer months, because they are outside and open their windows and doors more than in cold months and, therefore, their exposures are higher. In addition, differences in the makeup of air pollution between seasons may play a role.

"We suggest that on high pollution days, elderly people should avoid spending unnecessary time in traffic, whether in a vehicle or walking, especially if they suffer from cardiovascular diseases, to lower their exposure to pollutants," Kettunen said. "They should also avoid heavy outdoor exercise on high air pollution days. And nursing homes, for example, should not be built along heavily trafficked roads, where particle concentrations are at their highest," she added.

One expert believes the association between stroke and pollution is possible, just as it is with heart attack.

"The link between pollution and heart attacks has been established a few years ago," said Dr. Gottfried Schlaug, an associate professor of neurology at Beth Israel Deaconess Medical Center and Harvard Medical School. "Now, we see more reports coming out to suggest that there are links between pollution and stroke as well. Possible explanations are systemic inflammations, which could change blood clotting parameters or could lead to rupture of atherosclerotic plaques, as well as increases in plasma [blood] viscosity, which could increase the stroke risk."

Another expert thinks these findings need to be repeated in other studies before they are convincing.

"The findings are intriguing, but the authors tested a number of associations," said Dr. S. Claiborne Johnston, director of the Stroke Service at University of California, San Francisco. "The association only with ultra-fine pollution and only in the warm season seems quite strange, and could be due to chance. These results really need to be repeated."

More information

For more information on stroke, visit the American Stroke Association.

SOURCES: S. Claiborne Johnston, M.D., Ph.D., director, Stroke Service, University of California, San Francisco; Gottfried Schlaug, M.D., Ph.D., associate professor, neurology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston; March 2007 Stroke

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