Researchers at Harvard School of Public Health, Boston, have found that long-term exposure to air pollution appears to be associated with an increased risk of deep vein thrombosis, blood clots in the thigh or legs.
Exposure to particulate air pollution has been associated with the increased risk of developing or dying from heart disease and stroke.
And recent studies have suggested that this relationship may result at least in part from the effects of particulate air pollution on blood clotting.
For the study, Andrea Baccarelli, M.D., Ph.D., of the Harvard School of Public Health, Boston, and colleagues assessed exposure to particulate matter smaller than 10 micrometers in diameter among 870 patients who had been diagnosed with deep vein thrombosis in Lombardy, Italy, between 1995 and 2005.
These patients, along with 1,210 controls who did not have deep vein thrombosis, were assigned to one of nine geographic regions based on where they lived at the time of the study.
The researchers then used the average concentration of particulate matter for each area, obtained by monitors located at 53 different sites throughout the region, to estimate the level of exposure over the year before diagnosis (for cases) or examination (for controls).
Researchers found that individuals with deep vein thrombosis tended to have a higher exposure to particulate air pollution than controls.
After taking into account other environmental and health factors, for every increase in particulate matter of 10 micrograms per square meter the previous year, the risk of deep vein thrombosis increased 70 percent.
In addition, researchers found that the blood of patients in both the case and control groups with higher levels of exposure to particulate matter took less time to clot, as measured by a test given in the clinic.
The link between particle exposure and blood clots was found to be stronger in men than in women, and disappeared among women taking oral contraceptives or hormone therapy.
"Such hormone therapies are independent risk factors for deep vein thrombosis, which is also confirmed in this study by the higher prevalence of oral contraceptive and hormone use in the cases compared with the controls," 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 deep vein thrombosis 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," they added.
The study is published in the May 12 issue of Archives of Internal Medicine, one of the JAMA/Archives journals.