Long distance air travel can increase the risk of blood clots in the legs, a potentially fatal condition that has been known for more than half a century. Now British researchers report that the culprit is not airplane cabin air pressure, as some have speculated. They suggest that the condition is caused by lack of movement.
On a flight from Britain to Australia not long ago, Simon Snedden did what he always used to do on an airplane trip.
"The plan was always get in, get sat down, have a snooze, watch the film," he said.
Snedden will now alter that plan to include a little more leg movement. That is because he took part in a recent study looking into the air travel conditions that can lead to blood clots, or deep vein thrombosis.
The condition is dangerous for reasons explained by University of Leicester physician William Toff.
"Particularly looking at clots that form in the larger leg veins higher up, they may break away and travel through the heart to the lungs, where they block the blood flow and they in some cases prove fatal," said Toff.
Toff and colleagues conducted the study in which Simon Snedden and 72 other healthy volunteers took part. They wanted to know whether blood clots were the result of the low oxygen and air pressure levels in an airplane cabin.
So they put the volunteers in a cramped chamber for eight-hour shifts to copy the conditions of an actual flight. The researchers lowered the air pressure and oxygen levels in the chamber to match flight levels and even put a table in the middle to simulate the close quarters of the cabin. For comparison, they also had the volunteers spend eight hours in the chamber with normal oxygen and pressure levels.
There were some changes in the blood toward clotting, but they occurred at the same rate in both air pressure and oxygen conditions.
Toff says blood tests made before and after the simulated airline cabin conditions tell the story.
"We looked at a very wide range of blood markers, looking at all aspects of the body's clotting mechanism. We found no evidence that the low pressure, low oxygen were activating the body's blood clotting mechanism," he explained.
The researchers say the clotting changes were probably the result of sitting still for eight hours. So Toff advises travelers to stretch their legs, extend and flex the knees and ankles, and stand and walk whenever possible.
"Any form of long-haul travel, whether by air, rail, or road is associated with an increased risk of thrombosis. People should take sensible precautions according to their level of inherent risk," he added.
The study appears in the Journal of the American Medical Association.
In a commentary accompanying the research, University of Heidelberg sports physician Peter Bartsch notes that the study included small numbers of elderly people and women taking oral contraceptives. As a result, he says it is not possible to draw conclusions about these two groups.