Long-haul travelers who take oral contraceptives or have a genetic mutation common in northern Europe are at increased risk of potentially fatal blood clots, the World Health Organisation said Friday.

The U.N. health body issued a 24-page report warning that while the risk of developing deep vein thrombosis, normally in the form of a blood clot in the calves is small, it increases if people stay immobile for long periods in cramped conditions, or are predisposed to the condition for genetic or lifestyle reasons.

The recommendations, based on research carried out by scientists in Britain, Switzerland and the Netherlands, include taking a short walk or doing simple exercises to contract the calf muscles every hour. Certain groups including the obese, elderly or women who take the birth control pill are at greater risk than the average population.

Also at greater risk were people shorter than 1.6 meters (5 foot 4 inches) or taller than 1.9 meter (6 foot 4 inches), the study said. One study had shown that women who take the pill are 10 times more likely to develop blood clots during long-distance travel than the average person, WHO said.

Genetic factors, such as a the "factor 5 Leiden mutation" common in northern Europe, can also play a role, the report said. WHO said that, while the danger of blood clots is the same for most long-distance passengers traveling by train, car or plane, those in the high-risk group were more likely to develop clots during air travel.

"For people with high risk, some other factor in aircraft travel may be playing a role," Mendis said, adding that more research would need to be done to determine the reasons for this.

According to the International Air Transport Association, some 2.2 billion journeys are made by plane every year, though it was unable to provide details on how many of those are long-haul.

IATA spokesman Anthony Concil said many airlines now inform their passengers of the risk of blood clots.

"The advice that we give to passengers is that if you're in one of those at-risk groups you should speak to your physician before traveling," he said.