As many as 1% of long-haul travellers may develop potentially-dangerous blood clots as a result of their trip, say New Zealand researchers.
Long-haul travel may increase the risk
Scientists tested almost 900 passengers before and after they undertook lengthy flights, and identified just nine with deep vein thrombosis.
However, the Lancet journal reported that two-thirds of these had medical problems which made these more likely.
Other studies have suggested a higher rate linked to long-haul travel.
The research was carried out at the Medical Research Institute of New Zealand, which recruited hundreds of frequent long-haul travellers for the study.
Deep vein thromboses happen when a clot forms in the veins of the lower legs, and are often blamed on poor blood flow in these limbs during long periods of sitting still.
All travelled for at least 10 hours, and on average flew for 39 hours over a six week period.
Blood tests were used to measure changes in a chemical called D-dimer, which is associated with blood clot risk.
Those who showed increases in D-dimer concentrations - 112 of 878 - were given ultrasound tests to see if they had a clot.
However, firm evidence of a clot emerged in only nine cases.
Four cases of pulmonary embolism - in which a piece of a clot breaks off and travels to the lung, causing potentially fatal complications - were recorded.
Six of the nine patients had pre-existing risk factors that made them more likely to have clots, such as heart disease, and two travelled extensively in business class, scotching the suggestion that cramped conditions in economy class might contribute to the problem.
In addition, four of the nine had been wearing compression stockings to improve blood flow in their lower limbs, a method often recommended as a way of preventing DVTs.
Dr Richard Beasley, who led the study, said: "Our results suggest an association between multiple long distance air flights and venous thromboembolism, even in individuals at low to moderate risk.
"The role of traditional risk factors and prophylactic measures in air travel related venous thromboembolism needs further investigation.
"The term 'economy class syndrome' is now redundant, with a better term being 'air travellers' thrombosis'."
However, even that term may be inappropriate, as anecdotal reports suggest that any mode of transport that involves sitting still for hours on end may involve a risk of DVT.