A new study has identified air travellers who are more susceptible to developing potentially fatal blood clots.
The World Health Organisation (WHO) report warns that while the risk of developing deep vein thrombosis (DVT) is small, certain groups are more at risk than others.
It identifies the elderly, people who are obese and women who take birth control pills, as those who at greater risk of developing DVT.
It also found that people shorter than 1.6m (5ft 4in) or taller than 1.9m (6ft 4in) are more likely to develop the potentially fatal clots.
A previous study suggested that women who take the pill are 10 times more likely to develop DVT during long-distance travel than the average person.
Genetic factors, such as one mutation that is common in northern Europe, can also play a role, the report concluded.
The WHO said that, while the danger of developing blood clots is the same for most long-distance passengers travelling by train, car or plane, those in high-risk groups are more likely to develop clots during air travel.
The British, Swiss and Dutch scientists who carried out the research said passengers should take short walks or do simple exercises as they fly to reduce the risk of developing DVT.