CONDITIONS particular to travelling on a long-haul flight, such as low air pressure and reduced oxygen, do not increase the risk of potentially lethal blood clots, research indicates.

A study into the causes of deep vein thrombosis (DVT), which kills thousands of Britons every year, suggests that the clotting is not a result of circumstances unique to flying but is chiefly caused by sitting down for too long in a confined space.

The research highlights the danger of any travel that involves long periods of inactivity, including by rail and road.

A team of British scientists simulated cabin conditions, including low air pressure and reduced oxygen, on an eight-hour flight. They found no increase in blood clotting among healthy people.

Sitting for hours in seats with poor leg room reduces circulation, which may increase the chances of clots forming. But it has been suggested that other factors unique to air travel may also be important.

DVT, sometimes referred to as “economy-class syndrome”, is a condition in which a small, potentially fatal blood clot forms, often in the deep veins of the legs. It can become deadly if a part of the clot breaks off and blocks a blood vessel in the lungs.

About one in 2,000 long-distance passengers will suffer a blood clot. A recent Australian study put the risk of a dangerous incident at one in 40,000.

William Toff, of the University of Leicester, and colleagues, including Mike Greaves, of the University of Aberdeen, monitored the blood of 73 healthy people after they had spent eight hours seated in a hypobaria chamber that simulated the conditions of an air cabin.

Blood samples were taken before and after each “flight” to check for factors involved in blood clotting. Similar tests were carried out on volunteers, who spent eight hours seated at normal ground-level pressure.

The researchers measured blood coagulation markers, the activation of clot-stimulating platelets, the normal breakdown of small, naturally occurring clots and the behaviour of endothelial cells that line the walls of blood vessels.

No significant differences were discovered between the blood samples taken from volunteers on a simulated flight and from those exposed to normal air pressure.

“It is another piece in the jigsaw,” Dr Toff said of the findings, published this week in the Journal of the American Medical Association.

“Our study provides, for the first time, a carefully controlled assessment of the effects on blood clotting of the low air pressure and low oxygen level that might be found during a long-haul flight. We found no evidence that these conditions cause activation of the blood-clotting mechanism.”

He said that other studies had shown “quite clearly” that prolonged sitting in any situation, such as on long journeys by air, road or rail, increased the risk of DVT.

“People undertaking long-haul travel should take sensible precautions appropriate to their own inherent level of risk,” he said. “For everyone, that should include regular leg exercises and getting up to walk around from time to time when conveniently possible.”

The study also included groups thought to be particularly at risk, such as women on the contraceptive pill and people over the age of 50, but their results were no different.

WHO IS AT RISK . . . AND HOW TO AVOID IT

Some doctors advise people who are more susceptible to take heparin, a blood-thinning agent. Avoiding sedatives and excessive alcohol also helps