Leg and foot swelling during air travel is common. It's usually harmless. The most likely reason for it is sitting for a long time without moving during a flight.

Sitting with the feet on the floor for a long time causes blood to pool in the leg veins. The position of the legs while seated also increases pressure in the leg veins. This plays a role in swelling by causing fluid to leave the blood and move into the surrounding soft tissues.

A dangerous blood clot called deep vein thrombosis (DVT) sometimes causes leg swelling. But the risk of getting DVT on an airplane is very low for healthy people, especially on flights that last under four hours. In general, the chance of getting DVT starts to rise on flights over 12 hours.

You can reduce foot and leg swelling, and lower your risk of blood clots, by wearing compression stockings on a long flight. The stockings apply pressure to the lower legs.

If you notice swelling in one leg that doesn't go away or starts within two weeks of a long flight, get a health care checkup right away. The swelling might be a symptom of DVT or another condition that needs treatment. If you have a higher risk of blood clots, talk with a member of your health care team before you fly.