Why do I get swollen ankles on flights?

The puffy ankles at baggage reclaim are fluid physics, and the NHS names the cause directly: oedema - a build-up of fluid - is usually caused by standing or sitting in the same position for too long, and a long-haul seat is exactly that. Your calf muscles normally squeeze the leg veins with every step, pushing fluid back up toward the heart; strapped into a seat for hours, that pump stops, gravity wins, and fluid seeps into the tissues around your ankles and feet. Cabin pressure changes and salty plane snacks add a little more. Both ankles, mildly swollen, indenting briefly when pressed, easing overnight with your feet up - that is the harmless travel version, and it responds to prevention: walk the aisle regularly, do seated calf raises and ankle circles every half hour, drink water, go easy on the salty food, and wear graduated compression socks on long flights. There is one distinction worth knowing cold, because the same immobility that swells ankles can also seed a clot. The CDC warns that sitting still in a confined space for over 4 hours raises the risk of a deep vein thrombosis, and advises moving your legs frequently and walking every 1 to 2 hours - some travellers benefit from compression stockings. The clot pattern differs from ordinary puffiness: one leg only, swollen and often warm, tender, or reddish, sometimes with calf pain. One-sided swelling after a flight needs same-day medical advice, and sudden breathlessness or chest pain after travel is an emergency - that is the clot moving. Pymander is a free AI doctor, 24/7 by text, that can help you tell the two apart.