One report from Italy's University of Milano concluded that air travel is a "mild risk factor for venous thromboembolism" in general, with passengers who have risk factors such as
genetic predisposition, recent surgery or the use of birth control pills, are at greater danger.

Venous thromboembolism is the formation of blood clots in veins. The clots can form in the legs and travel to the lungs, causing severe problems and even death.
   
Genetic factors such as a  lack of certain proteins in the blood that normally help prevent or break up clots could contribute to the formation of blood clots. Other risk factors include recent surgery, cancer and pregnancy.

Excercise   

While the extent of the risk from air travel remains unknown, some airlines advise passengers to stretch or do in-seat exercises with their legs to keep blood circulating
properly.

And while the problem has been characterized as one involving cramped quarters in economy class seating, some experts say it can hit in any class of seat, if the passenger is too sedentary.
   
The Italian study, published in the Archives of Internal Medicine, found that the risk of such clots in patients with genetic or other risk factors and those who had travelled by air in the past month, was 16 times higher compared to patients without risk factors.

Women using oral contraceptives who travelled by air in the past month were 13 times as likely to develop the clotting problem, it said.
 
Long-haul  

A second study from Germany's University of Dresden Medical School looked at 964 passengers who had been on flights longer than eight hours and compared them with another group who had not flown.
   
Venous blood clots were found in 27, or 2.8%, of the air passengers and in 12, or 1%, of the non-flyers. "Long-haul flights of eight hours and longer, double the risk for isolated calf muscle venous thrombosis," the study said.

But it also said flight-associated blood clot cases "occurred exclusively in passengers with well-established risk factors" such as those mentioned in the Italian study.