Air travel, particularly long-haul flights, can increase the odds of developing dangerous blood clots but researchers said on Friday they are not only due to being immobile for long periods.
Low pressure and low oxygen levels in aircraft may also contribute in some people to the risk of deep vein thrombosis (DVT).
DVT, often called "economy class syndrome" because of the cramped conditions on planes, usually develops in the legs and can be fatal if the clot dislodges and moves to the brain or lungs.
Professor Frits Rosendaal of Leiden University Hospital in the Netherlands and his team compared concentrations of markers for blood clotting in 71 healthy people before, during and immediately after an 8-hour flight.
They also looked at the markers in the volunteers after 8 hours in the cinema and after doing daily activities.
"Activation of coagulation (clotting) occurs in some individuals after an 8-hour flight, indicating an additional mechanism to immobilization underlying air travel related to thrombosis," Rosendaal said in the study published in The Lancet medical journal.
The researchers said concentrations of the clotting markers were higher after the flight than they were after sitting in the cinema or doing daily activities, particularly in patients who had other risk factors for DVT.
Rosendaal said about 40 percent of the people in the study had an increased risk of DVT because of a genetic mutation or were women taking oral contraceptives, which also carry a raised risk of DVT.
In a commentary on the research, Dr. Hans Stricker of the Ospedale Regionale in Locarno, Switzerland described the research as an "important piece of evidence to the mosaic of travel-induced thrombosis."
He advised air travelers to avoid taking sedatives or drinking too much alcohol during flights to reduce the risk of DVT. Compressive stockings that improve blood flow could also help.
But Stricker said medical preventions such as low-dose anti-clotting drugs should be limited to people who have an increased risk of DVT such as those who have had recent surgery to the lower extremities.