How do you prevent blood clots when flying?

The link between long-haul flying and deep vein thrombosis (DVT) has long been known, but how can you reduce your risk? Dr Jane Wilson-Howarth gets to the heart of the matter

2 mins

In 1946, a doctor flew between Boston and Venezuela, a journey that took 14 hours. He suffered a deep vein thrombosis and decided that the long flight was the cause. Around the same time, a higher than expected incidence of clots was noticed in people sleeping in deckchairs in the London Underground during the Blitz. So the phenomenon of clots forming during immobility – especially when immobile in the sitting position – and the link between long flights and thrombosis has long been recognised. Why is there this association?

Blood is designed to clot when it comes into contact with air but, even when it is inside the body, a range of factors can make it clottier – sometimes disastrously. The heart pumps blood out into thick-walled arteries under pressure so that it can permeate every organ and muscle, even at the extremities. It then trickles through the tissues and is collected in increasingly larger veins on its way back to the heart and lungs. Veins are thin-walled and contain non-return valves so that when muscles contract, the veins within are compressed and blood is squeezed along, the valves ensuring that blood flows towards the heart. 

Sitting on a plane for more long hours can increase the risk of clotting (Shutterstock)

Sitting on a plane for more long hours can increase the risk of clotting (Shutterstock)

This pumping action works well during walking and other normal activities but not when immobilised in a seat on an aircraft. When the body is in such a position, blood will not flow well in the veins; it stagnates and this increases the tendency to clot. Any flight of 5.5 hours or more, or multiple short flights within several days, increase the risk of clotting significantly. This is why it is good to do calf- and thigh-tensing exercises onboard the aircraft, especially if travelling long-haul.

Note, reports suggest that the AstraZeneca and Johnson & Johnson COVID-19 vaccines slightly increase the clot risk. It occurs at the rate of 14 per million first doses. Both vaccination and flying are safe, but being informed reduces even tiny risks.

Flight socks (Shutterstock)

Flight socks (Shutterstock)

How to reduce clot risk when flying

• Keep well hydrated; if you do drink alcohol, keep it to a minimum

• Move around often

• Do exercises that mimic walking including calf tensing

• Wear loose clothing…

• But consider wearing flight socks, especially if a relative has had a clot or you are over 50 years old

The risk of a blood clot increases with: 

• Immobility

• Age (over 50, and significantly over 80 years)

• Cancer

• Following major surgery or serious leg injury, leg fracture, hip or knee replacement, or varicose vein stripping in the previous 12 weeks

• Having had a clot before

• People with a close blood relative who has experienced a clot

• Severe obesity

• Height (both very tall or very short people are at increased risk as airline seats inhibit blood circulation)

• Taking the combined oral contraceptive pill or HRT

Dr Jane is at and @longdropdoc

All content is for informational purposes only and does not constitute medical advice, treatment or diagnosis. Consult a healthcare professional before taking action.

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