The type of stroke that results when a blood clot travels to the brain -- called an ischemic stroke -- is more likely to occur on days when the air contains a larger concentration of particulate matter, according to a study published online in Stroke: Journal of the American Heart Association.
Researchers at Beth Israel Deaconess Medical Center (BIDMC) and the Harvard School of Public Health (HSPH) examined air quality on a total of 37,000 days in nine cities. Risk of hospitalization for ischemic stroke was 1 percent higher on days with relatively high levels of air pollution, compared with low-air pollution days, reports lead author Gregory Wellenius, ScD, postdoctoral fellow in cardiology at BIDMC.
Third Cause of Death in US
"Although these effects sound relatively small," says Wellenius, "given the large number of people exposed to air pollution and the large number of people at risk for stroke ... the actual number of strokes could be significant."
Stroke is the third leading cause of death in the US, affecting more than 700,000 individuals each year.
A "consistent increased risk" for cardiac health problems associated with exposure to ambient air particles was established in earlier research by Wellenius and coauthors Murray Mittleman, MD, DrPH, of BIDMC's Cardiovascular Epidemiology Research Unit and Joel Schwartz, PhD, of HSPH.
"Air pollution has been shown to trigger heart attacks and to aggravate the conditions of patients with congestive heart failure," says Mittleman, who is also an associate professor of medicine at Harvard Medical School.
"These new findings, demonstrating that incidence of clot-based strokes also increase, [are] in keeping with our earlier data showing a relationship between air pollution and heart and lung disorders," he notes.
The researchers also looked at the incidence of hemorrhagic stroke, which is caused by bleeding in the brain, during the same "high pollution" days, notes Wellenius, but found no association between the two.
Reducing Exposure May Lower Risk
The air pollution in question -- particulate matter smaller than 10 micrometers in diameter -- includes particles from car and truck exhaust, power plants and refineries. The measurements were provided by the US Environmental Protection Agency from nine US cities: Birmingham, Ala., Chicago, New Haven, Conn., Cleveland, Detroit, Minneapolis, Pittsburgh, Salt Lake City and Seattle.
The authors analyzed hospital admissions among a group of Medicare patients with an average age of 79. Seventy-five percent of the patients were white, and 61 percent were female. Their findings showed that during the course of their study, there were 155,503 hospital admissions for ischemic stroke.
The final analysis demonstrated a 1.03 percent rise in ischemic stroke on the days with the highest pollution measures.
"We don't know exactly what mechanisms are involved that trigger these cardiac events," says Wellenius. "However, we do know that particulates in the air promote inflammation, which is a significant risk factor for cardiac events; that exposure to particulates can lead to changes in heart rate and blood pressure; and that pollution can cause changes in coaguable states (related to blood clotting abilities)."
The authors say that future research will focus on finding out which pollutants are most toxic, as well as which patients are at greatest risk for health problems stemming from air pollution.
"Taken together with previous work, these latest results support the idea that reducing exposure to particulate matter may reduce the risk of strokes and heart attacks," they conclude.