The World Health Organization is recommending that passengers on long flights exercise their legs and avoid taking sleeping pills to reduce the risk of blood clots.
The U.N. agency said while the chances of developing a fatal deep vein thrombosis is small, it increases if people are immobile and in cramped conditions for long periods.
The WHO based the recommendations on research conducted after the death of a British woman seven years ago, on a flight from Australia.
She died from a pulmonary embolism which occurs when a blood clot in the arms and legs breaks off and travels to the lungs.
The report puts the risk of developing blood clots during a trip longer than four hours at one in 6,000 or about one for every 15 fully booked jumbo jets.