Table of Contents
Key Takeaways:
Long-distance travel is associated with an increased risk of venous thromboembolism, although the absolute risk remains low for most healthy travelers.
Prolonged immobility is a contributor because sitting for hours reduces the normal movement of blood through the legs. Risk generally increases as travel duration increases and is particularly relevant when other VTE risk factors are present.
Moving regularly during a long trip is a simple precaution that may help reduce venous stasis. Graduated compression stockings may be appropriate for some higher-risk travelers.
Anticoagulants should only be used for travel-related clot prevention when recommended by a healthcare professional.
Symptoms of DVT or pulmonary embolism after traveling require prompt medical evaluation.
If you've ever stepped off a long-haul flight with stiff, heavy legs, you've probably wondered whether all that sitting is doing more than making you uncomfortable. You're not wrong to ask. Long flights keep you in a small seat with limited room to move. For most travelers, this results in little more than stiffness and fatigue, but prolonged immobility can increase the risk of venous thromboembolism (VTE). This includes deep vein thrombosis (DVT) and pulmonary embolism (PE).
The connection between blood clots and flying is well studied, though, and the absolute risk for most healthy travelers remains low. Research shows that longer travel is associated with increased VTE risk, particularly among people who already have other risk factors for developing blood clots. Understanding why prolonged travel can contribute to clot formation, recognizing warning signs, and knowing which preventive measures are evidence-based can help you make informed decisions before a long trip.
Why Can Long Flights Increase Blood Clot Risk?
Blood clots are an essential part of the body's response to injury. Problems occur when a clot forms unnecessarily inside a blood vessel, interfering with normal blood flow.
Deep vein thrombosis is a clot that forms in a deep vein, most commonly in the leg. If part of the clot breaks away, it can travel through the bloodstream to the lungs and produce a pulmonary embolism. One of the primary concerns during long-distance travel is venous stasis, or slower blood flow through the veins.
Movement of the calf muscles normally helps push venous blood back toward the heart. Sitting relatively still for several hours reduces this muscle-pump activity. Research reviews examining travel-associated thrombosis identify prolonged immobility and venous stasis as important contributors to travel-related VTE.
Air travel has sometimes been singled out as uniquely dangerous because of cabin pressure, oxygen levels, and dehydration. However, prolonged sitting appears to be an important risk factor, and VTE has also been associated with long trips by car, bus, and train.
How Much Does Flying Increase Blood Clot Risk?
The answer depends partly on how long you travel and whether you already have risk factors for VTE. A systematic review found that long-distance travel was associated with approximately a two- to fourfold increase in the risk of venous thrombosis. Because the baseline risk among the general population is low, the absolute risk remained small.
Researchers estimated approximately one symptomatic event within four weeks for every 4,600 flights lasting longer than four hours.
Another meta-analysis involving more than 4,000 VTE cases found a relationship between travel duration and risk. The researchers reported an approximately 18% increase in VTE risk for every additional two hours of travel overall and approximately 26% for every additional two hours of air travel.
The precise risk varies among studies, and not every analysis has found the same magnitude of association. What is reasonably consistent is that longer journeys matter more than short ones, especially when additional risk factors are present.
Who Is at Greater Risk of Blood Clots While Flying?
Most healthy travelers do not require medication or specialized treatment simply because they are taking a long flight. Risk becomes more important when prolonged travel is combined with existing VTE risk factors.
Travel-related thrombosis guidelines identify people with factors such as a history of VTE, recent major surgery, active cancer, or multiple VTE risk factors as being at greater risk. The same guidelines emphasize that risk should be assessed individually rather than assuming that all airline passengers face the same danger.
Other factors that may affect an individual's overall VTE risk include pregnancy and the postpartum period, older age, obesity, estrogen-containing medications, certain inherited or acquired clotting disorders, and prolonged immobility for other reasons. Having one of these factors does not mean that a person will develop a blood clot while traveling. It may, however, justify discussing an upcoming long-distance trip with a healthcare professional.
How To Reduce Blood Clot Risk on a Long Flight
Preventing prolonged venous stasis is one of the most practical approaches to reducing travel-related risk.
Get Up and Move
When possible, periodically stand and walk during a long flight. Even when you cannot leave your seat, moving your ankles and contracting the calf muscles can keep the legs active. Walk in place, right in front of your seat.
Guidelines on travel-related thrombosis note indirect evidence supporting mobility and describe maintaining mobility as a reasonable precaution for travelers on journeys lasting more than three hours. A medical review of commercial air travel recommends walking periodically during long trips and performing seated exercises to reduce DVT risk.
This doesn’t require a complicated exercise routine. Ankle circles, repeatedly raising and lowering the heels, flexing the ankles, and periodically walking through the cabin are simple ways to activate the leg muscles.
Consider Compression Stockings When Appropriate
Graduated compression stockings apply the greatest pressure around the ankle and gradually decrease pressure farther up the leg. They are intended to help support venous blood flow. That does not mean everyone boarding an airplane needs compression stockings. Travel guidelines generally recommend considering them primarily for people at increased VTE risk, rather than for all travelers.
Stay Reasonably Hydrated
Hydration is sensible for general comfort and health, but the specific claim that dehydration is a major cause of travel-related VTE is less firmly established. Travel-related thrombosis guidelines found insufficient evidence for a direct association between dehydration and travel-associated VTE.
The Bottom Line on Blood Clots and Long Flights
Long-distance travel is associated with an increased risk of venous thromboembolism, but the absolute risk remains low for most healthy people. Longer journeys and pre-existing VTE risk factors substantially influence an individual's risk.
Regular movement is one of the simplest precautions during prolonged travel, while graduated compression stockings can be appropriate for people at increased risk. Preventive anticoagulant medication is generally reserved for selected high-risk travelers under medical guidance.
Most importantly, travelers should recognize the difference between supporting healthy circulation and treating or preventing a potentially dangerous blood clot. Anyone at elevated risk should discuss long-distance travel with a healthcare professional and develop an appropriate prevention plan before departure.
FAQs
How long does a flight have to be to increase blood clot risk?
Research suggests the association becomes more relevant as travel extends beyond approximately four to six hours, with risk continuing to increase with longer journeys. Individual VTE risk factors are also important.
Should everyone wear compression socks on long flights?
Not necessarily. Research supports compression stockings as a preventive measure, particularly for travelers with elevated VTE risk. Healthy, low-risk travelers generally do not require specialized thromboprophylaxis. A healthcare professional can recommend the appropriate compression level when necessary.
Can I take nattokinase before a long flight to prevent a blood clot?
Nattokinase has demonstrated fibrinolytic activity, but there is not adequate clinical evidence showing that taking it before flying prevents travel-associated DVT or pulmonary embolism. It should not replace anticoagulants or other preventive measures recommended by a healthcare professional.
*The information provided in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives. This content has not been evaluated by the Food and Drug Administration and is not intended to diagnose, treat, cure, or prevent any disease.
Sources:
Travel and thrombosis | National Library of Medicine
Travel and venous thrombosis: a systematic review | PubMed
Meta-analysis: travel and risk for venous thromboembolism | PubMed
