When compared with 1,210 others living in the same region who did not have the problem, they found that for every increase in particulate matter of 10 micrograms per square meter the previous year, the risk of deep vein thrombosis increased by 70 percent.

On top of that, the blood of those with higher levels of exposure to particulate matter was quicker to clot when tested at a clinic, they reported in the Archives of Internal Medicine.

Air pollution from automobiles and industry can contain tiny particles of carbon, nitrates, metals and other materials that have been linked over the years to a variety of health problems.

While lung diseases were an initial concern, later research has indicated it may cause heart disease and stroke, possibly because it increases the rate at which blood can coagulate, Baccarelli and colleagues said.

Until now particulate pollution had not been linked to blood clots in the veins. The mechanism that causes problems for some air travellers is related not to the blood itself but to impaired circulation when sitting in one place without exercise for long periods of time.

The findings introduce a new and common risk for deep vein thrombosis, the researchers said and "give further substance to the call for tighter standards and continued efforts aimed at reducing the impact of urban air pollutants on human health."

In a commentary, Dr. Robert Brook of the of the University of Michigan in Ann Arbor said if the findings are proven by additional research it may turn out that "the actual totality of the health burden posed by air pollution, already known to be tremendous, may be even greater than ever anticipated."