DVT Risk When Flying:
What to Know.

Deep Vein Thrombosis is a blood clot in a deep vein — most commonly in the leg. Long-haul flights are a recognised risk factor. Here's what causes it, who is most at risk, and the straightforward steps that significantly reduce that risk.

What Is DVT?

Deep Vein Thrombosis occurs when a blood clot forms in one of the deep veins of the body — most commonly in the calf or thigh. The clot can cause pain, swelling, and redness in the affected leg, but in many cases it produces no obvious symptoms.

The serious risk with DVT is pulmonary embolism — when a clot breaks loose and travels to the lungs. This is a medical emergency. However, it's important to note that this outcome is relatively rare, and the absolute risk for healthy travellers is low. The concern is real but shouldn't cause disproportionate anxiety about flying.

Why Long-Haul Flights Increase Risk

The NHS identifies prolonged immobility as a primary DVT risk factor — and few situations create more prolonged immobility than a long-haul flight in economy class. Sitting with bent knees for 8-12 hours prevents the calf muscle pump from functioning effectively, causing blood to pool in the lower legs.

The reduced cabin pressure and low humidity of aircraft cabins contribute to mild dehydration and slight changes in blood viscosity. These are minor factors individually but combine with immobility to create the circulatory conditions associated with increased clot risk.

The risk increases significantly on flights over 4 hours, and most substantially on flights over 8 hours. The risk doesn't disappear between 4 and 8 hours — it begins to become clinically meaningful above 4 hours.

Who Is Most at Risk

Several factors increase individual DVT risk on flights beyond the baseline. Previous DVT or pulmonary embolism — significantly increases risk on all flights. Clotting disorders — inherited or acquired conditions that make blood more prone to clotting. Pregnancy — raises DVT risk 5-10 times above baseline. Recent surgery — particularly orthopaedic surgery. Active cancer or cancer treatment. Obesity. Oral contraceptive pill or HRT — oestrogen increases clotting tendency. Age over 60. Tall or short stature — both extremes are associated with increased risk in the confined space of aircraft seats.

How to Reduce Your Risk

Compression socks. The most evidence-backed intervention for healthy travellers. The NHS specifically recommends below-knee compression stockings of 15-30 mmHg for long-haul flights. They keep blood moving in the lower legs and significantly reduce both swelling and clot risk.

Stay hydrated. Drink water regularly throughout the flight. Avoid excessive alcohol, which is dehydrating. Dehydration slightly thickens blood and increases clot risk.

Move regularly. Walk the aisle every hour where possible. When seated, perform calf raises, ankle rotations, and knee lifts to activate the calf muscle pump.

Choose an aisle seat. Makes it easier to get up and move without disturbing other passengers — removing a practical barrier to regular movement.

Avoid sleeping pills. They increase immobility during the flight.

When to See a Doctor After Flying

If you develop calf pain, swelling, redness, or warmth in one leg in the days following a long-haul flight, seek medical attention. These are potential DVT symptoms and require investigation. Similarly, sudden chest pain, shortness of breath, or coughing blood after a flight are potential symptoms of pulmonary embolism and require emergency medical attention.