Economy class syndrome: The formation of blood clots in veins deep within the legs -- deep vein thrombosis -- occurring during (or just after) a long airplane flight, especially in economy class (tourist class) where there is the least space allotted per passenger and ones legs tend especially to be immobilized for lack of leg room.
The economy class syndrome is directly related to immobility for long periods during which blood pools in the legs, raising the risk of clot formation. The tendency to immobility is often compounded by the fasten-seat belt sign, the presence of carts in the aisles, etc.
Other risk factors contributing to the syndrome include lower oxygen pressure and dehydration. Changes in oxygen pressure in the cabin tend to decrease the oxygen level in the blood. The air in the cabin lacks the normal degree of humidity which contributes to dehydration. The serving of coffee, tea, and alcoholic beverages (all of which are diuretics) further causes passengers to become dehydrated.
The economy class syndrome is not confined to that class on a plane. It is recommended that all persons traveling on air flights, irrespective of which class they are in, drink lots of water and move their legs by walking whenever possible and by periodically flexing and extending their ankles, knees, and hips to minimize the risk of economy class syndrome. Deep vein thrombosis is potentially serious and can even have fatal consequences.