People who travel for more than four hours by air, car, bus or train are doubling the risk of blood clots, according to new research.
Until now, the problem of deep vein thrombosis (DVT) has largely been associated with long-haul flights, but the new Dutch study found that it was a hazard of many different modes of transport.
DVT occurs when blood flow is restricted and clots form, usually in the legs. It can prove fatal if a clot breaks off and reaches the lungs.
Researchers looked at 2,000 people who had suffered a blood clot for the first time and found that 233 had travelled for more than four hours in the eight weeks preceding the clot.
Travelling in general doubled the chances of having a venous thrombosis, or blood clot in a vein – although the risk was low to start with.
The hazard was greatest in the first week after travelling, and the overall risk from flying was no worse than going by car, bus or train.
The researchers found that particular groups of people – especially women on the Pill – were more at risk than others.
Long-distance travel was associated with an eight-fold increased risk for those with a specific mutation in one of the genes, called factor V Leiden.
The risk was increased almost ten-fold for people who were obese, while women taking oral contraceptives were 20 times more at risk of having a thrombosis.
The scientists, led by Dr Suzanne Cannegieter, from Leiden University Medical Centre in the Netherlands, said: ‘It can be concluded that the risk of venous thrombosis is two-fold increased for all travellers and to the same extent for all modes of travel.
‘In individuals who use oral contraceptives, are carriers of the factor V Leiden mutation, or are particularly tall, short, or obese, the risk is considerably higher, to such an extent that studies into the efficacy of prophylactic measures are required.’
The study did not explain how travelling raised the risk of blood clots, but suggested that immobility was probably a key factor.