KYOTO, Japan -- Short term exposure to high levels of air pollution in warm weather more than doubles the risk of hemorrhagic stroke among older patients, found researchers here.
KYOTO, Japan, Sept. 21 -- Short term exposure to high levels of air pollution more than doubles the risk of hemorrhagic stroke among older patients, according to researchers here.
A comparison of death records from 13 Japanese cities with reports on hour-by-hour air pollution concentrations showed that fatal hemorrhagic strokes among those 65 or older were associated with a spike in air pollution levels about two hours earlier, found Shin Yamazaki, M.D., of the University of Kyoto, and colleagues.
Their findings suggest that "air quality standards or guidelines for particulate matter should be based not only on 24-hour mean concentrations, but also on hourly data," they reported online in Occupational and Environmental Medicine, a BMJ publication.
The effect was seen from April to September, during hours when the concentration of particulate matter was higher than the threshold 200 ?g per cubic meter. Pollution did not appear to affect the risk for ischemic strokes.
"Air pollution is associated with cardiovascular mortality and morbidity, and its effects can be seen very soon after exposure," the investigators wrote. "Inhaled particles can be detected in blood as early as one minute after they are inhaled, can remain at their maximum level in blood for up to 60 minutes, and may decrease vagal tone."
Because few studies have looked at the cardiovascular or cerebrovascular health effects of hourly rather than daily fluctuations in pollution levels, the authors undertook the research.
They collected mortality data for five years (January 1990 through December 1994) from a government database, focusing only on people from 13 urban metropolitan areas who were 65 or older at the time of death and died from either intracerebral hemorrhage or ischemic stroke.
The authors also gathered statistics on hourly mean concentrations of particulate matter of at least 7 ug in diameter (PM7), nitrogen dioxide (NO2), and photochemical oxidants recorded by air-quality monitoring stations in the 13 areas.
They performed a time-stratified case-crossover analysis to explore the possible relation of hourly pollution levels with stroke mortality.
The investigators pointed out the benefits of this type of analysis. "Case-crossover analyses require exposure data for cases only," they wrote. "They can be regarded as a special type of case-control study in which each case serves as his or her own control. This design has the advantage of controlling for potential confounding caused by fixed individual characteristics, such as sex, race, diet, and age."
There were a total of 63,724 stroke deaths: 17,354 attributed to intracerebral hemorrhage, and 46,370 pinned on ischemic stroke.
When the deaths were compared with pollution levels, the authors found that the one-hour mean concentration of PM7 > 200 ?g/m3 , measured about two hours before death from April to September, was associated with a more than two-fold mortality risk from intracerebral hemorrhage (odds ratio 2.40, 95% confidence interval, 1.48-3.89).
The higher risk was independent of the 24-hour mean concentration of PM7, and hourly exposures were not associated with deaths from ischemic stroke.
"The finding that exposure to PM7 is associated with the risk of death due to intracerebral hemorrhage but not ischemic stroke is noteworthy," the investigators wrote. "One possibility is that the interval from stroke onset to death is much longer, or much more variable, for ischemic stroke than for intracerebral hemorrhage."
Inhaled particles may also cause a rise in blood pressure in people with hypertension, who are already at elevated risk for hemorrhagic stroke, they added.
But the authors also acknowledged that results of other studies looking at a possible pollution-stroke link have been mixed.
"We believe that the association between hemorrhagic stroke and ambient particulate matter remains controversial and that further epidemiological studies are needed to confirm the association," they wrote.
Nonetheless, they recommended that to help prevent deaths from stroke related to air pollution, air quality standards for particulate matter should be formulated based on both hourly and 24-hour data on mean concentrations.