Air pollution has been linked to cardiovascular disease, and now researchers may be closer to understanding why: it increases atherosclerosis, or hardening of the arteries, a known risk for heart attack and stroke.

Researchers did ultrasound examinations on 5,362 men and women over 45 in six metropolitan areas, measuring the thickness of their right common carotid artery, one of two arteries that carry oxygenated blood to the neck and head. Then they followed them for an average of two and a half years, comparing the thickening of their arteries as observed by ultrasound with air pollution data measured by concentrations of particulate matter. The study appears in the April issue of PLoS Medicine.

They found that the greater the level of air pollution, the greater the thickening of the carotid artery. The association persisted even after controlling for race, education, smoking and other health and socioeconomic factors.

“I think this provides us with even more evidence that air pollution is a cause of heart disease,” said the lead author, Sara D. Adar, an assistant professor of epidemiology at the University of Michigan. “And it puts us one step closer to understanding how that happens.”