A research in the United States has suggested that smog from traffic and factories can trigger the formation of potentially deadly blood clots.

The Archives of Internal Medicine report said for every 10 microgrammes per square metre increase in small particulates, the risk of developing deep vein thrombosis (DVT) went up by 70 per cent.

"Our study signals that we have to work a little harder," said Joel Schwartz, the lead author of the study from Harvard University. He said his research adds weight to calls for tighter pollution control laws.

The blood clot-pollution link was discovered following a study of some 2000 people by the Harvard School of Public Health in the region of Lombardy in Italy.

Researchers found that thrombosis risk was 70 per cent greater in areas where the annual concentration of pollutants, such as particles of soot from car exhausts, was around a quarter above average values seen in Milan, the largest city in the area, the New Scientist online said.

Deep vein thrombosis occurs when a blood clot forms in the lower leg or thigh. If a clot breaks loose, it can travel to the lungs and cause breathing difficulties and even death.

Numerous studies have linked the pollution, which comes from car exhausts and power plants, to increased risk of heart attack. Last year, a study in Boston suggested that the fumes could be delaying childrens mental development.

Linking pollution to climate change, scientists have even warned that it could soon slide out of control, with concentration of carbon dioxide in the atmosphere surging to a record high.