Some air travelers have a higher risk than others of developing blood clots during long flights, a phenomenon sometimes called "economy class syndrome," according to studies published on Monday.
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 at greater danger.
Genetic factors can include a lack of certain proteins in the blood that normally help prevent or break up clots. Other risk factors include recent surgery, cancer and pregnancy. Oral contraceptives have long been known to increase the risk of blood clots in general.
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.
While the extent of the risk from air travel remains controversial, 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 who had traveled by air in the past month was 16 times higher compared to patients without risk factors. Women using oral contraceptives who traveled by air in the past month were 13 times as likely to develop the clotting problem, it said.
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 percent, of the air passengers and in 12, or 1 percent, 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 found "occurred exclusively in passengers with well-established risk factors" such as those mentioned in the Italian study.