So say experts including Jaana Kettunen, MSc, of Finland's National Public Health Institute.
Kettunen's team studied fatal stroke risk and air pollution among people 65 and older in Helsinki, Finland's capital.
The study, published in Stroke, shows a rise in fatal stroke risk among people 65 and older on warm days when the air was laden with fine particle pollution.
Based on the findings, Kettunen offers some advice in an American Stroke Association news release.
"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," says Kettunen.
"They should also avoid heavy outdoor exercise on high pollution days, and nursing homes, for example, should not be built along heavily trafficked roads, where particle concentrations are at their highest."
Kettunen and colleagues studied air pollution and fatal stroke in metropolitan Helsinki -- where about a million people live -- from 1998 to 2004.
During those years, 3,265 people 65 and older died of stroke in metropolitan Helsinki, the study shows.
Most of the deaths -- 1,961 -- occurred during the cold months of October to April. The other 1,304 deaths happened during Helsinki's warm season, May to September.
The researchers also checked Helsinki's outdoor air pollution levels during the months. Data came from pollution-measuring stations around Helsinki.
Kettunen's team paid special attention to fine particles. Fine particles mainly come from combustion engines and have been suggested to be "especially harmful," write the researchers.
Pollution and Fatal Stroke Risk
Although stroke deaths were most common during the cold season, the mix of warm weather and pollution may have upped fatal stroke risk.
When outdoor levels of fine particle pollution were high, fatal stroke risk rose -- but only during Helsinki's warm season.
High levels of fine particle pollution weren't linked to fatal stroke risk during Helsinki's cold months. Though fine particle pollution was actually a bit higher during the cold months, the study shows.
But the researchers note that people spend more time outdoors, and are thus exposed to more air pollution, during warm weather.
During Finland's frigid months, people tend to nestle indoors. That cuts their exposure to air pollution, possibly lowering their risk of suffering a fatal stroke.
Ultrafine particle pollution, carbon monoxide, and coarse particle pollution weren't strongly linked to fatal stroke risk.
The study doesn't prove that pollution causes stroke.
Many factors -- including age, heart disease, smoking, and diabetes -- may make people more vulnerable to stroke. Kettunen's study didn't track those risk factors among the people who died of stroke.
The study only included fatal strokes. Future studies should also look at nonfatal stroke risk, the researchers suggest.
SOURCES: Kettunen, J. Stroke, March 2007. News release, American Stroke Association.
By Miranda Hitti
Reviewed by Louise Chang