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Long haul flights roughly triple travellers' baseline risk of a symptomatic venous thrombosis. But for most people the absolute risk remains low—less than one event for every 5000 flights, according to a study of international corporate employees.
The researchers used records of business travel kept by international companies, combined with a web survey of 8755 employees, to calculate the risks associated with flights longer than four hours.
As expected, the incidence of deep vein thrombosis or pulmonary embolus was three times higher during the eight weeks after a long haul flight than at other times (incidence rate ratio 3.2, 95% CI 1.8 to 5.6). The absolute risk of an event was one for every 4656 flights, but the authors found a clear dose response effect—the longer the flight, the higher the risk. Frequent flying was also associated with a greater likelihood of venous thrombosis. The risks were highest in women—especially those taking oral contraceptives—short people, and tall people. Perhaps airlines should be encouraged to fit adjustable seats, say the authors, so tall people are less cramped, and short people don't have to sit with their legs dangling off the floor. Both problems could conceivably put pressure on the poplitial vein.