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Scientists have found a link between long-term exposure to air pollution and the development of blood clots in the legs.

Researchers assessed almost 900 Italian study subjects' exposure to particulate air pollution in the year prior to diagnosis of a blood clot in the leg, known as deep vein thrombosis. Particulate air matter is comprised of the very small pieces of chemicals that are created by burning fossil fuels and other substances.

The scientists, led by Dr. Andrea Baccarelli of the Harvard School of Public Health, compared this data to 1,200 study subjects who had not been diagnosed with blood clots.

They found that those with deep vein thrombosis had a 70 per cent increased risk of developing the condition for every increase of particulate matter of 10 micrograms per square metre.

Risk factors for deep vein thrombosis include sitting for a long period of time, injury to the lower leg, obesity and taking birth control pills.

In their study, the authors concluded that their findings not only offer a new risk factor for deep vein thrombosis but, "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 study was published Monday in the Archives of Internal Medicine.

In previous studies, air pollution has been linked to heart disease and stroke, and is a known contributor to respiratory problems, especially during the smoggy summer months.

In an editorial accompanying the study, Dr. Robert D. Brook of the University of Michigan, Ann Arbor, wrote that particulate air pollution is ranked 13th among leading causes of death worldwide.

"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," Brook wrote.

Abstract:

Exposure to Particulate Air Pollution and Risk of Deep Vein Thrombosis

Andrea Baccarelli, MD, PhD; Ida Martinelli, MD; Antonella Zanobetti, PhD; Paolo Grillo, MD; Li-Fang Hou, PhD; Pier A. Bertazzi, MD; Pier Mannuccio Mannucci, MD; Joel Schwartz, PhD

Background: Particulate air pollution has been linked to heart disease and stroke, possibly resulting from enhanced coagulation and arterial thrombosis. Whether particulate air pollution exposure is related to venous thrombosis is unknown.

Methods: We examined the association of exposure to particulate matter of less than 10 �m in aerodynamic diameter (PM10) with deep vein thrombosis (DVT) risk in 870 patients and 1210 controls from the Lombardy region in Italy, who were examined between 1995 and 2005. We estimated exposure to PM10 in the year before DVT diagnosis (cases) or examination (controls) through area-specific mean levels obtained from ambient monitors.

Results: Higher mean PM10 level in the year before the examination was associated with shortened prothrombin time (PT) in DVT cases (standardized regression coefficient [�] = -0.12; 95% confidence interval [CI], -0.23 to 0.00) (P = .04) and controls (� = -0.06; 95% CI, -0.11 to 0.00) (P = .04). Each increase of 10 �g/m3 in PM10 was associated with a 70% increase in DVT risk (odds ratio [OR], 1.70; 95% CI, 1.30 to 2.23) (P < .001) in models adjusting for clinical and environmental covariates. The exposure-response relationship was approximately linear over the observed PM10 range. The association between PM10 level and DVT risk was weaker in women (OR, 1.40; 95% CI, 1.02 to 1.92) (P = .02 for the interaction between PM10 and sex), particularly in those using oral contraceptives or hormone therapy (OR, 0.97; 95% CI, 0.58 to 1.61) (P = .048 for the interaction between PM10 level and hormone use).

Conclusions: Long-term exposure to particulate air pollution is associated with altered coagulation function and DVT risk. Other risk factors for DVT may modulate the effect of particulate air pollution.