New air travel clot risk theory

Long-haul travel may increase the risk

Sitting still for long periods cannot alone explain why air passengers are at higher risk of potentially deadly blood clots, research suggests.

The risk of developing deep vein thrombosis is thought to be raised by air travel - particularly long flights.

But a Lancet study by Dutch researchers found chemicals indicating clotting in 71 volunteers were higher during eight hours on a flight than in the cinema.

It suggests low air pressure and oxygen levels on a flight may play a role.

Our study suggests it is not just immobilisation, there is something else that adds to the risk when you are in the air Professor Frits Rosendaal

Deep vein thrombosis (DVT) occurs when blood passing through the deepest veins in the calf or thigh flows so slowly that a solid clot forms.

DVTs themselves are not life-threatening but they are associated with complications which can be fatal.

For instance, a piece of the clot can break off, and become lodged in the lungs, resulting in a potentially fatal pulmonary embolism.

A team from Leiden University Medical Centre measured levels of chemicals indicating clotting activity in 71 healthy volunteers before, during and immediately after an eight-hour flight.

They also compared the concentrations in the same individuals at the same time points during eight hours of sitting in a cinema and eight hours of regular daily activities.

Four out of 10 participants carried a gene that put them at increased risk of thrombosis.

The results showed increased concentrations of the chemicals during the flight compared to the other two situations - especially in volunteers with other risk factors for thrombosis.

Other factors

Lead researcher Professor Frits Rosendaal told the BBC News Website that immobilisation during a long flight was still likely to be the most significant risk factor.

But he said: "Our study suggests it is not just immobilisation, there is something else that adds to the risk when you are in the air that does not exist when you are sitting down for a long time while on the ground."

Professor Rosendaal said low air pressure, and oxygen levels were the most likely candidates, but he said it was impossible to rule out other potential factors, such as stress or air pollution.

Mr John Scurr, a consultant general and vascular surgeon at the Lister Hospital in London, said previous studies into the possibe impact of low oxygen and air pressure levels on the likelihood of developing clots had produced mixed results.

"Currently the only evidence we have is that immobility is the most important factor, but a number of us have suspected for a long time that there must be other factors having an effect."

Mr Scurr said a much larger study was required to provide definitive answers.

However, he stressed the risk to the travelling public was low - and that there were common sense measures that could minimise that risk still further.