Pollution and blood clots

Air pollution dramatically boosts an individual’s risk of developing deep vein thrombosis, a new study indicates. People with DVT typically develop clots in the leg or thigh. If those clots break off and travel, especially to the lungs, organ damage or death may follow.

The new study focused on exposure to airborne particles 10 micrometers and smaller, known as PM10. Created largely as part of combustion exhaust, these particles are so small that they can be inhaled deeply into the lungs.

“We were surprised at the magnitude of the relative risk” associated with this pollution, says Joel Schwartz of the Harvard School of Public Health in Boston, an author of the new report. Although earlier studies have linked cardiovascular disease to PM10, including risk of heart attack and stroke, this pollution now appears to have a more potent effect on elevating risk of DVT, his team reports in the May 12 Archives of Internal Medicine.

For every 10 micrograms per cubic meter increase in exposure to the pollution in an Italian study area, the risk of DVT rose by 70 percent. Even had the risk gone up only 7 percent, “that would still have been noteworthy,” argues Robert D. Brook of the University of Michigan in Ann Arbor.

The pollutant concentrations linked to DVT vulnerability fell well within what’s allowed in U.S. air, suggesting that supposedly benign levels can actually be hazardous. Some pollution values recorded were high enough to be associated with DVT risks elevated five- or 10-fold, notes Brook. Such high PM10 levels exist in many U.S. cities, he writes in an editorial accompanying the new paper.

Brook adds that the new study, the first to link PM10 exposures to DVT, “moves us into thinking about what other health effects yearlong exposures to the pollution might cause.”

Indeed, not so long ago, “particulate pollution was assumed to pose a threat primarily to the lungs,” points out C. Arden Pope of Brigham Young University in Provo, Utah. “The surprise, over the last several years, is that air pollution seems to have almost as important effect on the cardiovascular system.”

For the new analysis, Harvard’s Andrea Baccarelli and his colleagues tapped data that had been collected in the greater Milan area comparing people with DVT to those without it. Although DVT is not common, this study turned up 870 patients and compared them with 1,210 friends and family members — all living in and around Milan.

Some 50 or so monitoring stations in the region have been tracking PM10 data. The researchers matched values from the closest station to each participant in the study. “And people with thrombosis had been, on average, exposed to higher concentrations of particles in the previous year,” notes Schwartz. This link held even after accounting for other potentially confounding DVT risk factors.

In a related study, published last year, Baccarelli’s group evaluated blood from control subjects in this study. The higher their average particulate exposures had been, the more rapidly their blood could coagulate.

Because the findings in the new study are “quite robust, it gives us pretty good confidence that this association is real,” Brook says. “It’s also intriguing how large [that risk] is because the size is comparable — if not greater — than many established, well-known risk factors for DVT,” which include genetic susceptibility, smoking and hormone-replacement therapy.
A number of recent studies have shown that as people inhale particles, the lungs unleash an inflammatory response that can change blood’s ability to coagulate. Such changes can translate into increased risk of heart attack and stroke, Pope’s studies and others have shown.

The DVT findings “are not only very interesting, they’re also likely to be very important,” says Pope, because they now show that “this effect of air pollution on inflammation and blood coagulability in the heart and brain now appears true for other parts of the body as well.”