Exposure to air pollution increases the risk for deep vein thrombosis, the blood clots that commonly occur in the leg veins. And the worse the air pollution, the higher the risk.

Researchers studied 871 D.V.T. patients in the Lombardy region of Italy, comparing them with 1,210 healthy people.

They tested levels of particulate air pollution  dust, soot and other tiny bits of matter suspended in the air  in areas where the patients lived, using monitors at 53 sites over a one-year period. The study was published May 12 in The Archives of Internal Medicine.

After adjusting for various health factors, the researchers found that for each increase of 10 micrograms per cubic meter in particulate matter, the risk for D.V.T increased by 70 percent. The effect of air pollution was smaller in women and not apparent at all in those using oral contraceptives or taking hormone therapy. Oral contraceptives themselves increase the risk for blood clots, but air pollution had no added effect.