The study, published in the Archives of Internal Medicine, finds for every air pollution increase as measured as particulate matter of 10 micrograms per square meter, the risk of deep vein thrombosis increased 70 percent.

Study leader Dr. Andrea Baccarelli of the Harvard School of Public Health and colleagues compared 870 patients diagnosed with deep vein thrombosis in Lombardy, Italy, between 1995 and 2005 to 1,210 controls who did not have deep vein thrombosis.

The researchers then used the average concentration of particulate matter for each area -- obtained by monitors located at 53 different sites -- to estimate the level of exposure over the year before diagnosis.

In addition -- using a test administered clinically -- the researchers found the blood of patients in both the study subjects and control group members showed a shorter clotting time with higher levels of exposure to particulate matter.

The association between particle exposure and blood clots was stronger in men than in women, but disappeared among women taking hormones -- such hormone therapies are independent risk factors for deep vein thrombosis, the study authors say.