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This airplane cabin simulator allowed scientists to test the link between high altitude and blood clots.

Feel Free to Move About the Cabin

Frequent fliers have one less thing to worry about. Although some reports have suggested that the lower oxygen levels and reduced air pressure inside an airplane cabin can contribute to painful blood clots known as deep vein thromboses, a new paper indicates that airplane passengers are at no greater risk from this condition than other long-haul travelers.

Blood begins to clot when it pools deep in major leg veins. Calf swelling and pain is common, and if the clots detach from the vein and travel to the lungs, they can cause a deadly pulmonary embolism. Past studies have linked the clots to long periods of sitting, such as might occur on a train ride. In addition, anecdotal evidence from Himalayan climbers suggests that the lower air pressure and oxygen levels associated with high altitudes increase the risk by revving up the body's clotting system.

Given this evidence, a long plane flight would seem like double trouble. On some flights, passengers can sit for 12 hours or more, and, although airline cabins are pressurized, atmospheric conditions inside can resemble those seen at 1500 to 2400 meters. But is the risk really any greater on an extended flight than it would be on a train?

To find out, cardiologist William Toff of Leicester University in the United Kingdom and colleagues placed 73 healthy volunteers in a flight simulator. The volunteers were divided into three groups: 49 people aged 18 to 40, 12 people aged 50 or older, and 12 women aged 18 to 40 who were taking contraceptive pills (which raise the risk of clots, as does age). The volunteers sat in a mock airline cabin for 8 hours, once at sea level conditions and once at conditions equivalent to 2400 meters of altitude. Blood samples taken before and after each session revealed no difference in blood clotting activity between altitudes or among the three groups, the team reports 17 May in The Journal of the American Medical Association.

If you're healthy, says Toff, sitting still for long periods is likely "the major cause" of deep vein thromboses, not atmospheric conditions. So, whether it be a plane or other extended form of transportation, travelers can take sensible precautions such as standing up every once in a while, he says.

Nevertheless, sports scientist Peter Bärtsch of the University of Heidelberg in Germany cautions that at-risk groups may still be susceptible to altitude-induced clots. There weren't enough volunteers in this study to draw definitive conclusions about these groups, he says, and future studies should also look at individuals with a genetic predisposition to the condition.

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Posted in Biology