An expert from the World Health Organization is recommending that passengers on long flights exercise their legs and resist taking sleeping pills to reduce the risk of potentially fatal blood clots.
After reviewing research from scientists in Britain, Switzerland and the Netherlands, the WHO concludes that the danger of developing deep vein thrombosis is small, about 1 in 6000. But the risk approximately doubles after travel lasting four hours or more and people are immobile for long periods in cramped conditions.
The numbers suggest that an average of one person is affected for every 20 long-haul flights carrying 300 passengers.
And with thousands of people traveling by air every day, that translates into a highly significant public health issue.
Deep vein thrombosis can be life-threatening if a blood clot in a leg vein breaks off and travels through the body to the lungs where it becomes lodged and blocks blood flow, called pulmonary embolism.
The obese, women on birth control pills and those with blood clotting disorders are more susceptible to the potentially dangerous blood clots that can develop in the calves, as are frequent travelers and those who take very long journeys.
Tall people whose legs are cramped and the very short whose feet do not touch the ground are also particularly vulnerable, the WHO said in an announcement Friday.
The study showed that plane, train, bus or automobile passengers are at higher risk of clots when they remain seated and immobile for long periods, because of bloodflow in the legs can slow, leading to blood clots in the veins.
"There is an increased risk of venous thromboembolism during travel where the passenger is seated and immobile over four hours, whether in a plane, train, bus or car," said Catherine Le Gales-Camus, WHO assistant director-general for non-communicable disease and mental health.
"What causes the risk is immobility," she told a news conference. "The risk is not only true for people flying."
Shanthi Mendis, a WHO expert on the issue, says by getting up for a short walk, or doing exercises to contract the calf muscles every hour, passengers can greatly reduce the risk of blood clots.
Mendis cautioned air passengers against taking sleeping pills or drinking too much alcohol. People should also avoid wearing tight clothing that can constrain circulation, she said.
Symptoms of blood clots include pain or cramps in the calves, and swelling of the leg, Mendis said.
The WHO says there is a need for travellers to be given appropriate information about the risk of developing blood clots. The next phase of their study will look at identifying effective preventive measures.
