Air Pollution May Heighten Risk for Deep-Vein Blood Clots

Long-term exposure to air pollution may increase the risk for developing blood clots in veins deep within the legs, a condition known as deep vein thrombosis. Although air pollution has previously been linked to heart disease and stroke, this is the first study to find a connection to the sometimes-dangerous blood clots that can form in the veins.

Deep vein thrombosis often arises after extended periods of inactivity, like after bed rest or sitting through a long plane ride. Birth-control pills and other hormone therapies can also raise the risk for deep-vein clots. The clots usually form in the legs, causing painful symptoms. If they break free and lodge in the blood vessels of the brain, heart or lungs, they can cause severe organ damage or even death.

The new study, led by Dr. Andrea Baccarelli of the Harvard School of Public Health, suggests that fine-particle air pollution may be another contributor to deep vein thrombosis. The international research group, including scientists at the Milan Policlinico Hopsital and the University of Milan in Italy, was supported by NIH’s National Institute of Environmental Health Sciences (NIEHS), the U.S. Environmental Protection Agency and Italian funding agencies. Their report appeared in the May 12, 2008, issue of the Archives of Internal Medicine.

The scientists focused on airborne particles less than 10 microns in diameter—a type of pollution that comes from burning coal, gasoline, diesel and other fossil fuels. Evidence from previous studies suggests that these fine particles can penetrate deep into the lungs or even enter the bloodstream, causing inflammation and affecting clotting-related proteins.

The researchers assessed exposure to particulate air pollution among 870 patients who’d been diagnosed with deep vein thrombosis between 1995 and 2005. All were residents of the Lombardy region of Italy, as were the 1,210 control group participants who did not develop deep vein thrombosis. About half of the participants were men, and half women. The researchers estimated each patient’s exposure to fine-particle pollution during the year before diagnosis by drawing on data from air pollution monitors located at 53 different sites across the region.

After accounting for other environmental and health factors, the researchers found that the risk for deep vein thrombosis increased by 70% for each increase in particulate matter of 10 micrograms per cubic meter. In addition, laboratory tests showed that blood from both patients and the control group tended to clot more quickly if they’d been exposed to higher levels of fine-particle pollution.

The linkage between air pollution and blood clots appeared to be stronger in men than in women. As expected, the researchers found that women taking birth control pills or hormone therapy seemed more likely to develop deep vein thrombosis. However, exposure to air pollution did not appear to add to their risk.

The investigators note that their study had a few limitations. For instance, pollution exposure was based on each person’s residence and didn’t account for time spent away from home, in traffic or other variables. Further studies will be needed to fully assess the risk for deep vein thrombosis and other potential health risks of fine-particle air pollution.

—by Vicki Contie