Officials from many of the world's leading airlines have agreed at a meeting in Geneva to cooperate in further research to probe the link between air travel and potentially fatal deep-vein thrombosis (DVT).
There is growing concern that cramped seats and a lack of mobility on long-haul flight can cause clots to form in the lower limbs. These clots can prove fatal if they travel to the vital organs. It has become known as "economy class syndrome".
In a significant breakthrough, medical officers from 16 airlines acknowledged that "there probably exists an association between venous thrombosis and travel in general".
The medical experts went further, saying that "a link probably exists between air travel and venous thrombosis". However, they said that such a link was likely to be small and mainly affected passengers with additional risk factors, such as a history of DVT, cancer, obesity, taking oral contraceptives or hormone replacement therapy or recent surgery.
The meeting was called by the World Health Organisation's in response to an apparently growing number of deaths of people who had recently made long-haul flights. The industry representatives and medical experts reviewed all existing studies into the condition, but they proved inconclusive.
"There is a gap in knowledge. We need get the right kind of evidence to fill that gap," says Dr Shanthi Mendis, of the WHO's cardiovascular diseases division.
"We simply don't know the risks or the incidence because we have no reliable data. There appears to be a small risk for certain kinds of passengers, and this needs to be explored," she told swissinfo.
To this end, the WHO is to launch a major study, to be funded by national governments. The outcome of the research should be known in two years' time. As many as 100,000 passengers will need to be tested.
The airlines have pledged to cooperate fully: "We can't launch this kind of study without their cooperation, and they have committed themselves to collaborate with us," says Dr Mendis.
"In addition, we would expect them to help us in implementing protocols once they've been drawn up," she adds.
Medical experts were at pains to point out that this is not an "economy-class syndrome": "Anyone with additional risk factors can be affected, regardless of where they sit," says Dr John Scurr, a British DVT expert.
He said it was also very important that the study would be international and carried out under the auspices of the WHO: "It must not be seen as a problem specific to one country, because that would put the national carrier in difficulty. It must be done in a multinational way," he says.
Airlines have been accused of not doing enough to consider the safety of their passengers, and are worried that they could face class action lawsuits. Campaigners have called for the air companies to completely review cabin design, in particular by introducing better seating, more legroom and space for passengers to move around
Some airlines have already started showing videos to passengers instructing them how to reduce the risk of deep-vein thrombosis (DVT). These measures include reducing alcohol intake, rehydrating regularly, wearing loose clothing and doing simple foot and leg exercises while seated.
"While there's no medical evidence to prove that these will prevent DVT, it's common sense to improve the comfort of passengers," Scurr says.
DVT refers to the formation of a blood clot within a deep vein, commonly in the thigh or calf. This clot can partially or completely block the flow of blood in the vein. It is a common complaint among elderly patients in hospital who suffer from poor blood circulation as a result of inactivity, and among pregnant women.
The most serious complication of DVT is a pulmonary embolism, where the blood clot breaks free from a vein wall and travels to the lung.
It is estimated that two million Americans suffer from DVT every year, and 60,000 die of a pulmonary embolism. It is also now the most common cause of death in childbirth.
by Roy Probert