Exposure to air pollution from traffic fumes raises risks of potentially fatal blood clots in the leg; it alters the blood’s coagulation properties and heightens the risk of deep vein thrombosis (DVT), a Harvard research study showed.

The Harvard School of Public Health published the results of the first study focused on air pollution’s effects on clotting in the veins in the May 12 issue of Archives of Internal Medicine.

Harvard researcher Andrea Baccarelli, MD, PhD, and her colleagues studied 870 patients diagnosed with blood clots in the leg from 1995 to 2005. They compared the particular air pollution exposure of people diagnosed with DVT in the year before their diagnosis in the Lombardy region of northern Italy to that of more than 1,000 people in a control group free of blood clots. Blood clots which form in the legs can travel to the lungs, where they can become lodged, triggering a potentially fatal pulmonary embolism.

The scientists measured the amount of small particles, tiny chemicals caused by burning fossil fuels, and liquid droplets in the Lombardy region of Italy in a study of 2,000 people.

Researchers found that those exposed to higher levels of small particles in the air before being diagnosed with DVT were more likely to develop blood clots. The risk of developing a DVT went up by 70% for every 10 microgrammes per square metre increase in small particles.

The association was weaker in women than in men because they used oral contraceptives or hormone therapy, which also had procoagulant effects.

“We surmise that prothrombotic mechanisms are already activated in [women] receiving hormone therapy so that they undergo less or no further induction after air particle exposure,” the authors said.

“Given the magnitude of the observed effects and the widespread diffusion of particulate pollutants, our findings introduce a novel and common risk factor into the pathogenesis of DVT and, at the same time, give further substance to the call for tighter standards and continued efforts aimed at reducing the impact of urban air pollutants on human health,” the authors concluded in the study.

“It's an exciting finding because air quality is something we can improve on through tightening air quality legislation.”