Blood Clot Risk on Long Flights: What You Need to Know

What is DVT, and why do long flights matter?

Deep vein thrombosis (DVT) occurs when a blood clot forms in a deep vein, typically in the calf or thigh. The main danger is not the clot itself, but the risk of a piece breaking off and traveling to the lungs — a condition called pulmonary embolism (PE), which can be life-threatening 1Ref 1Centers for Disease Control and Prevention (2023).CDC Yellow Book 2024: Health Information for International Travel.DVT risk from long-haul flights, risk factor identification, movement and hydration strategies, compression stockings evidence, and the lack of evidence for aspirin in this context. Long-haul flights promote clot formation primarily through immobility: when you sit still for many hours, blood pools in the lower legs rather than returning efficiently to the heart.

A systematic review and meta-analysis found that air travel roughly doubles to triples the relative risk of venous thrombosis compared with non-travel periods, with flights over 4–8 hours carrying progressively more risk 2Ref 2Adi Y, Bayliss S, Rouse A, Taylor RS (2004).The association between air travel and deep vein thrombosis: systematic review and meta-analysis.Air travel is associated with a roughly 2–3 fold relative-risk increase in venous thrombosis; absolute symptomatic event risk is approximately 1 per 4,600 flights over 4 hours. The absolute risk for a single flight still remains low for most healthy travelers — but the combination of flight duration and personal risk factors determines your individual picture.

Who is at higher risk for DVT on flights?

For most healthy adults, the DVT risk from a single long flight is low in absolute terms 1Ref 1Centers for Disease Control and Prevention (2023).CDC Yellow Book 2024: Health Information for International Travel.DVT risk from long-haul flights, risk factor identification, movement and hydration strategies, compression stockings evidence, and the lack of evidence for aspirin in this context. People at notably higher risk include:

• Those with a personal or family history of DVT or PE
• Those with known clotting disorders (Factor V Leiden, protein C or S deficiency, antiphospholipid syndrome)
• Pregnant people and those in the postpartum period
• People who have had major surgery or trauma in the past few weeks
• Those with active cancer
• Those using estrogen-containing hormonal contraception or hormone therapy
• People with significant obesity

If you have one or more of these factors, a pre-flight conversation with your clinician is worthwhile.

What actually helps: movement

Regular movement is the most straightforward risk-reduction strategy available to everyone 1Ref 1Centers for Disease Control and Prevention (2023).CDC Yellow Book 2024: Health Information for International Travel.DVT risk from long-haul flights, risk factor identification, movement and hydration strategies, compression stockings evidence, and the lack of evidence for aspirin in this context. Getting up and walking the aisle for a few minutes every 1–2 hours improves blood return from the legs. When you cannot stand, seated calf raises (repeatedly lifting and lowering heels), ankle circles, and flexing and extending your ankles all activate the calf muscle pump — the mechanism that pushes blood up the leg veins. Avoid sitting with legs crossed for long periods, and consider an aisle seat if your risk factors warrant it.

Does hydration really matter?

Dehydration thickens the blood slightly, which can increase clot risk. Staying well hydrated throughout a long flight is a simple, low-cost precaution 1Ref 1Centers for Disease Control and Prevention (2023).CDC Yellow Book 2024: Health Information for International Travel.DVT risk from long-haul flights, risk factor identification, movement and hydration strategies, compression stockings evidence, and the lack of evidence for aspirin in this context. Alcohol and excessive caffeine both increase fluid loss, so being intentional about water intake alongside them is a reasonable step — not eliminating them, but balancing them.

Do compression stockings help?

Graduated compression stockings apply gentle pressure to the lower leg that helps prevent blood from pooling. A Cochrane systematic review of 11 randomized trials — covering 2,906 passengers on flights over 5 hours — found that among those wearing compression stockings on both legs, only 3 of 2,637 participants developed asymptomatic DVT, compared with 47 who were not wearing stockings. The reviewers concluded there is high-quality evidence that compression stockings substantially reduce the incidence of asymptomatic DVT in airline passengers 3Ref 3Clarke MJ, Broderick C, Hopewell S, Juszczak E, Eisinga A (2016).Compression stockings for preventing deep vein thrombosis in airline passengers.High-quality evidence from 11 RCTs (n=2,906): only 3 of 2,637 stocking-wearers developed asymptomatic DVT vs 47 non-wearers; substantial reduction in incidence of asymptomatic DVT in airline passengers.

For general healthy travelers, the benefit is modest. For those with identified risk factors, they are a practical addition. Ask your clinician which compression level is appropriate — light compression (15–20 mmHg) is adequate for most travelers.

What about aspirin — does it help?

Aspirin is commonly thought of as a blood thinner, but the evidence does not support it for DVT prevention during air travel. It targets platelet aggregation, not the venous stasis and coagulation cascade activation that drive flight-related DVT 1Ref 1Centers for Disease Control and Prevention (2023).CDC Yellow Book 2024: Health Information for International Travel.DVT risk from long-haul flights, risk factor identification, movement and hydration strategies, compression stockings evidence, and the lack of evidence for aspirin in this context. If you are already taking aspirin for heart disease, continue your existing regimen — but do not add aspirin specifically for flight-related DVT prevention.

When is medication-based prevention needed?

For people at substantially elevated risk — such as a recent history of DVT, a known severe clotting disorder, or flying shortly after major surgery — a clinician may recommend a short course of blood-thinning medication around travel 1Ref 1Centers for Disease Control and Prevention (2023).CDC Yellow Book 2024: Health Information for International Travel.DVT risk from long-haul flights, risk factor identification, movement and hydration strategies, compression stockings evidence, and the lack of evidence for aspirin in this context. This is not a decision to make independently; anticoagulants carry bleeding risks and interact with other medications. Bring this up explicitly at your pre-travel appointment if you think you may be in this category.