Inactivity, not altitude, is probably behind blood clots.

Low cabin pressure isn't to blame for the rare but dangerous blood clots that some passengers get during long flights, new evidence suggests. The likely explanation for the phenomenon, sometimes called economy-class syndrome, is that long periods of sitting promote clots, particularly in susceptible people, investigators say.

Deep-vein thrombosis--a condition in which blood clots form in veins deep in the legs--can be lethal if a clot breaks away and travels to the lungs. Past studies suggested that the low air pressure on flights increases the tendency of blood to coagulate.

To test that possibility, William D. Toff of the University of Leicester in England and his colleagues simulated the atmospheric conditions of a daytime, long-haul flight. A few at a time, 73 healthy volunteers sat in an airtight chamber for 8 hours, as if they were in a cramped plane cabin.

Experimenters controlled the chamber's air pressure so that it was nearly 1 atmosphere, or sea level pressure, for some experiments and, for others, just 0.74 atmosphere, the least permitted on international flights.

Drawing blood samples from the volunteers before and after each test, the researchers measured changes in 21 substances that reflect activation of the blood's dotting mechanisms. They found some differences between the morning and the afternoon samples. But those fluctuations occurred regardless of the air pressure to which volunteers were exposed.

The team concludes that although low pressure doesn't contribute to blood clots in most healthy passengers, the study doesn't rule out a possible effect in people at higher risk of thrombosis or with genetic predispositions to it. The researchers report their findings in the May 17 Journal of the American Medical Association.--B.H.