Air pollution may be associated with blood clots in deep leg veins

Exposure to particulate air pollution--very small particles of solid and liquid chemicals that come from burning fossil fuels and other sources--has been linked to the increased risk of developing or dying from heart disease and stroke, according to background information in the article. Recent studies have suggested this relationship may result at least in part from the effects of particulate air pollution on blood clotting.

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).

Individuals with deep vein thrombosis tended to have a higher exposure to particulate air pollution than controls. After adjusting for 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, 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 association between particle exposure and blood clots was 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 write.

"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 conclude.

This work was supported by grants from the Environmental Protection Agency Particulate Matter Center; grants from the National Institute of Environmental Health Sciences; a grant from the MIUR Internationalization Program; and grants from the CARIPLO Foundation and Lombardy region.  

Editorial: Blood Clot Risk Could Increase Estimates of Death Toll from Pollution

Air pollution "has become so omnipresent over the past century as to be commonly perceived as a normal natural entity--'the lazy, hazy days of summer'," writes Robert D. Brook, M.D., of the University of Michigan, Ann Arbor, in an accompanying editorial.

"While we have learned to live within this haze without a second thought, air pollution is neither natural nor benign," he continues. "Even though the absolute cardiovascular risk posed to one individual at any single time point is small, owing to the ubiquitous and constant nature of exposure, particulate matter ranks as the 13th leading cause of global mortality (approximately 800,000 deaths annually)."

Dr. Baccarelli and colleagues have presented evidence of a new category of health risks associated with pollution, he writes. "If future studies corroborate their findings and address some of the limitations, it may be proven that the actual totality of the health burden posed by air pollution, already known to be tremendous, may be even greater than ever anticipated," Dr. Brook concludes.