More than two days a year are spent in traffic

Breathing in traffic fumes over a long period of time can dramatically raise the risk of potentially deadly blood clots, a study has found.

Higher levels of exposure to tiny pollutant particles in the air up the chances of suffering a deep vein thrombosis (DVT), or blood clots in the legs, researchers discovered.

As the density of particulate matter rose, so did the likelihood of having blood clots. For every 10 micrograms per square metre increase, the risk of developing a DVT went up by 70%.

Scientists also found that the blood of people most exposed to the particles clotted quickly. Particulates are very small grains or blobs of solid and liquid chemicals produced by burning fossil fuels. They are most abundant in the exhaust fumes of diesel vehicles, such as lorries and buses.

The particles are small enough to penetrate deep into the lungs. Those smaller than 10 micrometres across, known as PM10s, are thought to be especially damaging.

Exposure to particulate air pollution was already known to be associated with higher death rates from heart disease and stroke. The new findings suggest that blood clots may be one of the reasons for the link.

The researchers studied 870 patients in Lombardy, Italy, who were diagnosed with deep vein thrombosis between 1995 and 2005. They were assigned to one of nine geographical regions based on where they were living at the time of the study, as were 1,210 DVT-free individuals.

Air pollution levels in each region were checked by means of monitors placed at 53 different sites. The readings were used to assess average particulate matter concentration over the previous year.

Study participants were also tested for their blood's ability to clot. Those with high levels of exposure to particulates had blood which coagulated more quickly, the tests showed. After adjusting for other environmental and health factors, every 10 micrograms per square metre increase in particulate concentration was accompanied by a 70% increased risk of DVT.

The scientists reported their findings in the journal Archives of Internal Medicine.