Reduced air pressure and oxygen levels on planes do not increase the risk of blood clots in the legs, a study says.
DVT can be fatal
Previous research has suggested air passengers are at an increased risk of DVT because of the unique conditions.
But Leicester and Aberdeen university researchers said sitting for long periods was the main cause, and warned people about all forms of travel.
The study of 73 people was featured in the Journal of the American Medical Association.
One in 2,000 long-distance passengers will suffer a blood clot, which can be fatal if it reaches the lungs.
Experts have been uncertain about the exact causes of DVT, which stands for deep vein thrombosis, but some studies have suggested that flying can increase the risk.
During the study, the volunteers sat in chambers with reduced air pressure and oxygen for eight hours. They were allowed to move around for a couple of minutes each hour.
They were also tested in a chamber without changes in the atmosphere.
Blood samples were taken before and after each "flight" to check for factors involved in blood clotting.
For all these factors, no significant differences were seen between blood samples taken from volunteers on a simulated flight or exposed to normal air pressure.
Lead researcher Dr William Toff said: "Our study provides, for the first time, a carefully controlled assessment of the effects on blood clotting of the low air pressure and low oxygen level that might be found during a long-haul flight.
"We found no evidence that these conditions cause activation of the blood clotting mechanism."
But he said people undertaking long-haul travel should take sensible precautions such as regular leg exercises and perhaps wearing specially designed stockings.
And he added the study did not look at people with high risk of DVT, such as the elderly, those with a family history or cancer patients, and such people could still be affected by plane conditions.
Dr David Keeling, Royal College of Physicians advisor and expert in DVT at the Oxford Haemophilia Centre and Thrombosis Unit, said other studies had suggested flying could increase the risk for such patients.
"The sensible conclusion would be to accept that there is still some risk for certain people to fly. My advice to everyone would be to do regular exercises during travel."