Exhaust fumes contain small particulates
Breathing in air pollution from traffic fumes can raise the risk of potentially deadly blood clots, a US study says.
Exposure to small particulates - tiny chemicals caused by burning fossil fuels - is known to increase the chances of heart disease and stroke.
But the Harvard School of Public Health found it also affected development of deep vein thrombosis - blood clots in the legs - in a study of 2,000 people.
Researchers said the pollution made the blood more sticky and likely to clot.
The team looked at people living in Italy - nearly 900 of whom developed DVT.
Blood clots which form in the legs can travel to the lungs, where they can become lodged, triggering a potentially fatal pulmonary embolism.
The risk of DVT is known to be increased by long periods of immobility. In particular, passengers on long-haul flights have been shown to be vulnerable, but so are people who spend long periods of time sitting at their office desk without exercising, or walking around.
Researchers obtained pollution readings from the areas they lived and found those exposed to higher levels of small particulates in the year before diagnosis were more likely to develop blood clots.
The Archives of Internal Medicine report said for every 10 microgrammes per square metre increase in small particulates, the risk of developing a DVT went up by 70%.
Air quality guidelines generally state that small particulate concentrations should not exceed 50 microgrammes.
Lead researcher Dr Andrea Baccarelli said: "Given the magnitude of the effects, our findings introduce a novel and common risk factor into the development of DVT.
"And, at the same time, they give further substance to the call for tighter standards and continued efforts aimed at reducing the impact of urban air pollutants on human health."
Dr Beverley Hunt, medical director of the DVT charity Lifeblood, said: "We have known for some time that air pollution has been associated with increased risk of heart attack and stroke.
"This study shows for the very first time that air pollution also increases the risk of clots in the veins and tells us why.
"It's an exciting finding because air quality is something we can improve on through tightening air quality legislation."