A small research study reveals that reduced pressure inside an aircraft is probably not a major factor in deep vein thrombosis.
Long haul air travel increases the risk of deep vein thrombosis (DVT), a condition where a clot develops in the veins of the leg. The danger here is if the clot breaks off and travels to the lungs, creating a potentially fatal pulmonary embolism. It's been suspected that the risk of DVT might have something to do with the reduced air pressure inside an aircraft.

To investigate this idea, researchers at the University of Oxford, England, exposed eight healthy human participants to hypoxia - that is, the reduced air pressure similar to that inside the plane - for eight hours. Then they were exposed to normal air for eight hours. At the end of each eight hour period, blood tests were taken to see whether markers of coagulation were raised during hypoxia.

There was no difference between the blood results from the two time periods. This suggests that reduced air pressure is not a major factor in DVT. But this doesn't mean that the reduced air pressure doesn't contribute in a minor way to DVT and it may also have a greater impact on people who have other risk factors, like a history of thrombosis. A larger study may help clarify the role that reduced cabin pressure plays in DVT.