Is there a risk of blood clots during flight?

By Dr. Guðmundur Danielsson.

Air travel and blood clots – is there any connection?

Many people are familiar with and have experienced swelling and discomfort in their legs after long flights. One expects to be tired and out of sorts after a long flight, particularly when crossing many time zones. The swelling we experience in our feet is not a sign of any serious illness but can be the first indication of blood clots in the veins. The frequency of blood clot formation that can be traced to air travel is somewhat uncertain. The end of the last century saw raised awareness about the issue in connection with the tragic death of a young woman who was returning home from the Sydney Olympic Games. She suffered a blood clot in her foot which then travelled to her lungs and led to her death shortly after having arrived at Heathrow Airport after an 11-hour flight. Following extensive debate in the media about the risk of blood clots forming in the legs after long flights, grants were allocated for research into this phenomenon. A number of studies were carried out and one such revealed that blood clots were quite common. The study was performed by taking an ultrasonography of the veins in the extremities of persons arriving after a long flight (more than 6 hours). The results indicated that the presence of clots was quite common, or around 4%. None of them had any symptoms, however, and the clots in question were minor and in muscle veins.  Subsequent studies have shown that the frequency is much lower.

Why do blood clots form during long flights?

Why do blood clots form during long flights, and can one do anything to lessen the likelihood of their formation? During flights, we tend to stay seated for extended periods, in seats that usually do not provide the option of moving much and where it is often difficult to stretch one’s legs. This was often linked with the lack of space, which was generally more clearly the case in economy class and called, therefore, “coach-class thrombosis”. There is no evidence, however, that the risk is greater in coach class than in seats with more space between them, so the name is misleading.  The body commonly experiences fluid imbalances during long flights; there is in fact a lack of fluids. It is also possible that the air pressure, which is lower in aircraft than on Earth, may have a negative impact on the body’s clotting mechanisms.

Good advice

There are several simple but effective measures one can take to combat this risk that flying involves.

One of the most important things one can do is to move regularly, i.e. go to the lavatory or take a walk down the aircraft’s aisle. Another good idea is to do some leg exercises while seated. Clenching your calf muscles, for instance, will help squeeze the blood from the legs up to the heart. Drinking water to prevent dehydration is recommended, while the consumption of alcohol and sleep medication is not. In addition, wearing elasticated support socks is probably important, as these squeeze the vein system and thereby lessen the volume of blood in the veins that can form clots.


Blood clots in legs are often symptom-free. The first and only symptom may be when they break loose and float up to the lungs, an extremely severe, life-threatening occurrence. Individuals who pose high risk of getting blood clots should be treated with anticoagulants before long flights. This includes those who have previously had repeated blood clots or have a defective clotting system that increases the risk of blood clots. This is done by administering anticoagulant medication under the skin. What is a long flight?  Flights to Europe from Iceland (approximately 3 hrs) are probably not very risky, while flights that last longer than 8 hours are more dangerous. Aircraft operators became considerably concerned in the wake of the extensive debate in the media, and some speculated as to whether they could be held liable with respect to their passengers. Aloha Airlines, flying from the west coast of the US to Hawaii, took the issue very seriously and paid grants into studies into the matter at the beginning of the 21st century. The results of the studies showed that there certainly is a risk but also that it is a small risk.


In short, one could say that the risk of the formation of blood clots in the legs in connection with flying is small. The risk is greater if the flying time is longer than 8 hours. It is a good idea to do leg exercises during the flight and to stand up from your seat every now and then. Dehydration should be avoided, and having regular drinks of water, therefore, is wise. The consumption of alcoholic beverages should be kept to a minimum, as these increase fluid elimination. Elasticated support socks are recommended for longer flights and for passengers who have strong risk factors for the formation of blood clots. The administration of anticoagulants by syringe just before take-off may be a good idea. This applies particularly to those individuals who have previously suffered from blood clots. The use of heart magnyl (antithrombotica) as a preventative measure is not recommended, as this does not lessen the likelihood of blood clot formation in the veins.

Guðmundur Daníelsson is a vascular surgeon and operates Reykjavík Venous Center, RVC.