Coagulability rises steadily during air travel. Blood flow slows down, especially in the lower legs, and blood viscosity rises in the lower legs, increasing the likelihood of clotting. Clots form, usually painlessly, but some cause pain and swelling.
The longer the flight, the greater the risk, although many of the cases in our registry arose from relatively short flights of three hours or so.
Clots in the arterial system cause heart attack and stroke. If you are being treated for any heart condition, hardening of the arteries, diabetic arteriopathy, or similar conditions, taking aspirin may reduce your risk. Your doctor may also recommend injection (in the stomach fat) of a low molecular weight heparin, LMWH, such as Lovenox.
Scientific studies have shown that the physiological effects of altitude are the same in nearly all air travelers, not just a few with some unusual susceptibility. (See bibliography.)
Bjorne Bendz’ study in Norway put twenty young men in a chamber simulating cabin altitude and measured coagulation factors hour by hour. He found significant increases in coagulation factors in all the the twenty young men.
In Japan, Hamada had twenty young men drink one cup per hour of water. All developed significantly increased blood viscosity in the lower legs and increased urine output. Another twenty young men drinking an electrolytic beverage did not develop increased viscosity or urine output. (See Hydration)