Low environmental levels of fine and ultrafine particulate matter, as well as carbon dioxide, increase the risk of stroke, but the heightened risk occurs only during warm weather months, Finnish researchers report.

Previous research has linked air pollution with a higher risk of fatal and nonfatal stroke, according to a report in the rapid access issue of the journal Stroke. However, the Finnish study is the first, to the authors' knowledge, to examine the impact on stroke risk of ultrafine particulate pollution, defined as molecules with a diameter of less than 2.5 microns.

Dr. Jaana Kettunen, from the National Public Health Institute in Kuopio, and colleagues compared air pollution levels from 1998 to 2004 with the number of stroke deaths among elderly subjects living in Helsinki, a city known to have relatively little air pollution.

A total of 1,304 stroke deaths in the warm season and 1,961 in the cold season were logged in during the study period.

During the warm season, every 6 microgram per cubic meter increase in current-day levels of fine particulate air pollution was associated with a 6.9 percent increase in deaths from stroke. The corresponding stroke death rate for previous-day fine particulate increases was 7.4 percent. However, particulate air pollution had no effect on stroke during the cold season.

Previous-day levels of ultrafine particles plus carbon monoxide were also linked to stroke mortality. However, Kettunen commented in a statement, "these associations were less robust" than those seen with fine particulate pollution. "Coarse particles were not statistically significantly associated with stroke deaths," she added.

"Our results suggest that the levels of combustion-originating particles rather than coarse particles explain the association between particulate matter and stroke," the authors conclude. "Thus, regulatory efforts should be focused on reducing emissions of combustion particles."