Long flights increase your risk of deep vein thrombosis (DVT), a blood clot that forms in a deep vein, usually in your leg. The risk rises meaningfully on flights lasting four hours or more, and it climbs higher with each additional risk factor you carry. The good news: a few simple habits before and during your flight can significantly reduce that risk.
Why Sitting on a Plane Causes Clots
Blood clots form when blood pools and moves too slowly through your veins. When you’re seated in a cramped cabin with your knees bent and your calves compressed against the seat edge, the large veins in your legs get partially squeezed. Your calf muscles, which normally act as pumps to push blood back toward your heart, stay inactive. Add low cabin humidity and mild dehydration, and your blood thickens slightly. The combination of slow flow, pressure, and thickening creates the conditions for a clot to start building.
This isn’t unique to planes. Any prolonged sitting, whether in a car, train, or even at a desk, carries the same basic mechanics. But air travel bundles multiple factors together: confined space, limited ability to move, lower cabin pressure, and dry air.
Who Faces the Highest Risk
Most people who develop a travel-related blood clot already have at least one other risk factor. According to the CDC, those include:
- Age over 40 (risk increases steadily from there)
- Overweight or obesity
- Recent surgery or injury within the past 3 months
- Estrogen-containing contraceptives such as birth control pills, patches, or rings
- Hormone replacement therapy
- Pregnancy or the postpartum period (up to 3 months after childbirth)
- Previous blood clot or family history of clots
- An inherited clotting disorder
- Active cancer or recent cancer treatment
- Limited leg mobility, such as wearing a cast
- Varicose veins
The more of these that apply to you, the greater your overall risk. A healthy 30-year-old on a six-hour flight has a very low chance of developing a clot. A 55-year-old on hormonal therapy who had knee surgery two months ago faces a meaningfully higher one.
Move Your Legs Every Hour
The single most effective thing you can do is get up and walk the aisle at least once an hour. Even a 2- to 3-minute stroll reactivates your calf muscles and pushes pooled blood back into circulation. If you’re in a window seat or the seatbelt sign is on, seated exercises make a real difference.
The most effective seated exercise combines straightening your knee with pulling your toes up toward your shin (dorsiflexion). One biomechanics study found that this specific movement generated 25% more calf muscle engagement and greater blood velocity in the femoral vein compared to other commonly recommended in-flight exercises. Do 10 to 15 repetitions on each leg, and repeat every 30 minutes or so.
Simpler movements help too. Ankle circles, pressing the balls of your feet into the floor, and alternating between pointing your toes and flexing them all keep the calf pump working. The key is frequency. A single stretch after three hours of stillness does far less than brief movements every 20 to 30 minutes.
Wear Compression Socks
Graduated compression socks apply gentle pressure that’s strongest at the ankle and decreases toward the knee, helping blood move upward rather than pooling. For travel, look for socks rated at 15 to 20 mmHg, which is the moderate compression level designed to help prevent DVT and reduce swelling during flights. You can buy them at most pharmacies or online without a prescription.
Put them on before you leave for the airport, not after your legs have already started swelling. They should feel snug but not painful. If you have peripheral artery disease or any condition that affects circulation to your feet, check with your doctor before wearing them, as compression can worsen certain arterial problems.
Stay Hydrated, Skip Alcohol
Cabin air humidity sits around 10 to 20%, far drier than most indoor environments. You lose moisture through breathing and skin faster than you realize. Dehydration thickens your blood, making clots more likely to form. Drink water consistently throughout the flight rather than waiting until you feel thirsty.
Alcohol and caffeine both promote fluid loss. One glass of wine probably won’t tip the balance, but several drinks over a long flight will leave you more dehydrated than you started. If you want coffee, match it with an equal amount of water.
Choose Your Seat Strategically
An aisle seat makes it far easier to get up and walk without climbing over other passengers. If you’re on a flight longer than four hours and you have any of the risk factors listed above, this small choice removes a real barrier to movement. Bulkhead and exit row seats also offer more legroom, which reduces the angle of knee compression and gives you space for seated exercises.
Avoid crossing your legs while seated. It feels natural but further restricts blood flow in the veins behind your knee.
Skip the Aspirin
Many travelers assume taking an aspirin before a flight will thin their blood enough to prevent clots. The CDC specifically recommends against taking aspirin for this purpose. Aspirin works primarily on arterial clots (the kind involved in heart attacks and strokes), not venous clots like DVT. The mechanisms are different, and aspirin carries its own bleeding risks.
If you’re already taking aspirin for a heart condition, continue as prescribed. But don’t start taking it as a flight precaution on your own. For travelers with multiple risk factors, a doctor may prescribe an anticoagulant medication or recommend higher-grade compression stockings before a long trip. That conversation is worth having before you book a 12-hour flight, not at the gate.
Symptoms to Watch For
A DVT in the leg typically causes swelling in one calf or thigh, pain or tenderness that worsens when you stand or walk, skin that feels warm to the touch, and redness or discoloration. The symptoms usually affect only one leg, which helps distinguish a clot from general post-flight stiffness.
Clots don’t always show up during the flight itself. Symptoms can appear days or even a couple of weeks after you land. The more dangerous scenario occurs if a clot breaks loose and travels to the lungs, a condition called pulmonary embolism. Signs of that include sudden shortness of breath, chest pain that worsens with deep breathing, rapid heartbeat, and coughing up blood. That’s a medical emergency.
If you notice one-sided leg swelling or pain after a long flight, especially if you carry risk factors, get it evaluated promptly. An ultrasound can confirm or rule out a clot quickly, and early treatment with blood thinners is highly effective at preventing complications.