SITTING still for a long time is the main cause of life-threatening blood clots, scientists said yesterday after carrying out research that showed lower oxygen levels and pressure on long-haul flights did not have an effect on healthy people.
Experts from Aberdeen and Leicester universities warned that any mode of travel - cars and trains as well as planes - and also spending hours in front of a computer could lead to a deep-vein thrombosis (DVT) in the leg, which can prove fatal if it tr
avels to the lungs.
Earlier this month, a computer programmer told how he had collapsed at his desk while working at home. A clot that formed in his leg was later found lodged in his lung.
However, it is not thought that other forms of prolonged immobility, such as going to the cinema to watch a particularly long film, represents any kind of significant risk. The risk of DVT is thought to start after about three hours of immobility.
Professor Mike Greaves, the head of Aberdeen University's school of medicine and one of the authors of a report in today's issue of the Journal of the American Medical Association, said they had studied 73 healthy people who spent eight hours sitting in a chamber where air conditions were similar to those experienced in planes.
Air pressure was reduced to that found at an altitude of 8,000ft and oxygen levels were also reduced. The volunteers' blood was then tested to see if it had become more prone to clotting than previously, and "no significant difference" was found.
Professor Greaves said: "Our study provides reassurance regarding previous concerns about the effect of low air pressure and oxygen on clotting in healthy people.
"The carefully conducted study closely simulated the relevant conditions experienced during lengthy air travel and the results indicate that blood clotting is not adversely affected.
"It is known that immobility is an important risk factor for deep vein thrombosis, due to the reliance of blood flow in the veins of the legs on muscle contraction and relaxation.
"It is likely, therefore, that in healthy people it is the sedentary state imposed by long-distance mechanised travel, including air travel, which is a major contributor to the risk of deep vein thrombosis."
The overall risk of DVT on a long-haul flight - one of four hours or more - is estimated at one passenger in 2,000. The risk of a fatal clot is much lower.
For people with known risk factors, the threat may be higher, but for those without other risk factors it is likely to be very low and "should be kept in perspective", the scientists said.
Anyone concerned about taking a long-haul flight was advised to take "sensible precautions", such as regular leg exercises and walking around.
Professor Greaves said sitting at a computer for a long time without moving was potentially a risk, but other activities, such as watching a film, were not thought to pose a threat.
"The differences between a cinema and a plane are the amount of leg room and the amount of time a person may sleep," Prof Greaves said.
"A lot of people [on planes] have a drink and then go to sleep with their legs in very cramped conditions. I wouldn't suggest people going to the cinema are at significant risk."
The scientists also cautioned that their study had investigated the effects of low oxygen and pressure on healthy people, saying those in higher risk groups may be affected differently by the same conditions.
And they added that other factors associated with flying - such as the type of seating and increased stress - may make planes more dangerous.
