SEATTLE (MCT) — Air pollution has long been known to be bad for the lungs. But new University of Washington research, involving thousands of older women in dozens of cities nationwide, shows that it also raises the risk of women dying from heart disease or stroke.

The increased risk comes from tiny airborne particles typically found in engine exhaust. And the damage they cause to arteries in the heart and brain is worse than previously believed, the study found.

“It looks like it’s about three times as big as previously estimated. … That’s a surprise,” said Dr. Joel Kaufman, the UW professor of environmental sciences who directed the study.

The scientists found that the greater the level of the so-called “fine particulate” pollution, the greater the risk of cardiovascular disease and death. Even a relatively slight increase boosted the risk significantly.

“There is no reason to think it isn’t the same for men,” said Kaufman.

Kaufman said the research focused on women because there was a readily available group already enrolled in long-term health research coordinated by the Fred Hutchinson Cancer Research Center in Seattle. Data from the project, the 15-year Women’s Health Initiative (WHI), have produced other important research on heart disease, breast cancer, colorectal cancer and osteoporosis.

Results from the air-pollution study are reported in this week’s edition of the New England Journal of Medicine with Kristin Miller, a UW doctoral student, as the lead author. Other scientists from the UW, Hutchinson center and Harborview Medical Center co-authored the study, which was financed by the Environmental Protection Agency (EPA) and the National Institute of Environmental Health Sciences.

The scientists analyzed the medical records of nearly 66,000 postmenopausal women from 36 cities and followed them for an average of six years. At the start, none had cardiovascular disease. But the researchers took into account nonpollution factors in reporting their results: age; race; smoking; education; income; weight and the presence of diabetes, hypertension and high cholesterol.

Pollution exposure was measured using monitors near their homes that looked for airborne particles smaller than 2.5 microns, or 1 millionth of a meter, in diameter. About 30 to 40 of the particles would equal the diameter of a human hair. They are invisible except when spewed in large quantities from tailpipes or chimneys, or when they form haze over a city.

During the study, a total 1,816 of the women had heart attacks, strokes or were diagnosed with diseased arteries of the heart or brain. Of those, 261 died.

The risk of dying from a stroke or heart attack increased 76 percent for each 10-microgram increase in the particles in a cubic meter of air. The danger of nonfatal strokes and heart attacks from the same rate of pollution increase pushed the risk up 24 percent.

Overall, the women in the study were exposed to an annual average of 13.5 micrograms of the particles per cubic meter of air. In Seattle, the annual average is 11.3 micrograms per cubic meter. The EPA has set a limit of 15 micrograms before sanctions can be imposed on cities that don’t rein in their pollution.

Other pollutants measured in the study did not increase the risk of heart disease or stroke.

Previous studies have shown much lower risks of cardiovascular disease stemming from pollution. But those studies looked only at death certificates instead of detailed medical records.

No one knows exactly how the pollution does its dirty work to the cardiovascular system. Kaufman speculates that the particles cause inflammation in the lungs, which spreads to arteries, increasing both arterial disease and the likelihood that deadly blockages will form.

Kaufman and his colleagues already have another study under way to better understand the mechanisms of air pollution’s damage in 7,000 men and women of various races around the country. It will study how arteries harden and thicken over time when exposed to air pollution.

In a commentary in the New England Journal, Harvard scientists Douglas Dockery and Peter Stone said the current findings by Kaufman and his colleagues “strongly support the recommendation for tighter standards for long-term fine-particulate air pollution.”