Leg and foot swelling during air travel is common and typically harmless. The most likely culprit is inactivity during a flight.
Sitting with your feet on the floor for a long period causes blood to pool in your leg veins. The position of your legs when you are seated also increases pressure in your leg veins. This contributes to foot swelling by causing fluid to leave the blood and move into the surrounding soft tissues.
To relieve foot swelling during a flight:
Foot swelling isn't a serious problem if it lasts only a short time. But excessive swelling that persists for several hours after you resume activity may be due to a more serious condition, such as a blood clot in the leg (deep vein thrombosis). If you have swelling in only one leg and also have leg pain, seek prompt medical care.
If you're at increased risk of blood clots — because you recently had major surgery or you take birth control pills, for example — talk with your doctor before flying. He or she may recommend wearing compression stockings during your flight. In some cases, your doctor may prescribe a blood-thinning medication to be taken before departure.