Why do people develop deep vein thrombosis on long air trips?

Deep vein thrombosis is a well-known complication of long-distance air travel. In deep vein thrombosis a clot forms within a vein, often in the leg. The symptoms of deep vein thrombosis can include pain and swelling of the affected leg (which is often unusually warm and discoloured, but deep vein thrombosis is often asymptomatic).

(The prefix a- means without, or a lack of – anaemia is a lack of haemoglobin, and asymptomatic means without symptoms. The prefix an- has exactly the same meaning.)

The most serious complication of deep vein thrombosis is an embolus, which is a fragment of the clot that breaks free and travels through the circulation until it blocks an artery in the lungs (causing pulmonary thromboembolism) or brain (causing ischaemic stroke). A primary thromboembolism causes chest pain, difficulty in breathing, and coughing, and can be fatal if not treated appropriately.

Long-distance air travel increases the likelihood that clotting will occur in normal blood vessels. This is probably due to the effects of hypoxia that leads to activation of platelets and the induction of coagulation. Clotting will also be triggered when blood flow becomes very slow or ceases, which can occur when people are immobile for a long period of time. Contraction of the leg muscles will compress the veins and increase blood flow, as will the use of compression stockings. Dehydration may be another contributing factor because it makes the blood more viscous and therefore decreases blood flow through the veins. Factors that contribute to dehydration include low cabin humidity and possibly the consumption of caffeine and alcohol that increase water excretion from the body.