DVT on long-haul flights and who actually needs compression socks

For a healthy traveller, the risk of a blood clot on a long-haul flight is small: about 1 in 6,000 for a journey of more than four hours, according to the World Health Organization. The risk is real but concentrated. The US Centers for Disease Control and Prevention says 75–99% of people who develop a travel-related clot already had at least one other risk factor, such as a previous clot, recent surgery, pregnancy, oestrogen-containing medicine or obesity. For everyone else, the useful habits are free: get up and walk, work your calf muscles, and take an aisle seat if you can.

How big the risk actually is

The chance of a clot after one long flight is low, and most of it sits with people who were already at risk. The WHO’s WRIGHT research project found that the risk of venous thromboembolism — a deep vein thrombosis (DVT) in the leg, or a pulmonary embolism (PE) in the lung — roughly doubles after four or more hours of seated travel, but that the absolute risk stays at about 1 in 6,000.

The cabin itself is not the main culprit. The CDC Yellow Book notes a similar rise in risk on long bus, car and train journeys, which points to hours of sitting still rather than cabin pressure.

The journey to Kathmandu from the US, the UK or Australia means at least one long-haul leg, usually with a connection. That puts most visitors to Nepal inside the group the research describes — which does not make a clot likely.

Who is actually at risk

Most travel-related clots happen to people with at least one existing risk factor; the CDC puts the share at 75–99%. One study it cites found that 20% had five or more. The CDC and the NHS list broadly the same factors:

• a previous clot, or a family history of clots or an inherited clotting disorder;
• surgery or injury in the past three months, or a leg in a cast;
• pregnancy, or having given birth recently (the CDC says up to three months; the NHS says six weeks);
• the combined contraceptive pill, other oestrogen-containing contraception, or hormone replacement therapy;
• active cancer or recent cancer treatment, or obesity;
• older age — the CDC says risk rises after 40; the NHS lists being over 60.

Height is the factor few people know. The WHO and the CDC both report higher risk for air travellers shorter than 1.6 metres, whose legs may press on the seat edge, or taller than 1.9 metres, who have less room to move.

What works on the plane

Walking the aisle, calf exercises and an aisle seat are the measures every agency recommends; stockings and medicine are for people at increased risk only. The evidence is thinner than the confident advice suggests, and the CDC Yellow Book says so plainly.

The CDC’s routine is simple: stretch your legs out and pull your toes towards you, or pull each knee to your chest and hold it for 15 seconds, up to ten times. Walk whenever the seatbelt sign is off.

On stockings, the two US guideline bodies the CDC summarises agree. The American College of Chest Physicians recommends fitted below-knee stockings of 15–30 mmHg at the ankle for travellers at increased risk on journeys over six hours, and not for anyone else. The American Society of Hematology advises against stockings, injected blood thinners and aspirin for travellers without risk factors.

Hydration deserves an honest footnote. The NHS recommends plenty of water and little alcohol, and there is no harm in either, but the CDC Yellow Book finds no evidence that dehydration is linked to travel clots. Drink water because a long flight dries you out, not as a clot plan.

Warning signs after you land

A clot can appear days or weeks after the flight, so a painful swollen leg or sudden breathlessness in the following month needs urgent attention. The CDC says most appear within one to two weeks, and that about half of people with a DVT have no symptoms at all.

Why this matters on a Nepal trek

The weeks after a long flight overlap with a trek, so breathlessness or a swollen calf on the trail should never be written off as altitude or a hard day. Most Annapurna and Everest itineraries start within days of landing in Kathmandu, inside the window when travel-related clots appear.

Altitude brings its own breathlessness, and the altitude sickness guide explains why breathlessness at rest is already a reason to stop climbing. Nobody should try to diagnose on the trail. Tell the guide about new calf pain, one-sided swelling or chest pain at once; both a clot and serious altitude illness need medical care.

Note any clot history or blood-thinning medicine in the trek first aid kit and on the insurance paperwork. Higher-risk travellers should book the pre-travel appointment early, as the Nepal vaccinations guide already advises.

The verdict

If you are healthy and have no risk factors, you need no stockings and no medicine — just move, choose the aisle and know the warning signs. If you have had a clot, recent surgery, a pregnancy, cancer treatment or take oestrogen, talk to your doctor before the trip, because that is the group the research says carries most of the risk.

Nobody should buy aspirin at the airport “to be safe”: the guideline bodies advise against it for low-risk travellers. The correct plan here is also the free one.

Keep reading

Sources: World Health Organization, “Study results released on travel and blood clots” (WRIGHT project, Phase I); US Centers for Disease Control and Prevention, CDC Yellow Book 2026, “Deep Vein Thrombosis and Pulmonary Embolism”, for absolute risk, risk factors, timing, compression stockings, hydration, seat position and the ACCP and ASH guideline summaries; CDC, “Understanding Your Risk for Blood Clots with Travel”, for symptoms and in-seat exercises; NHS, “DVT (deep vein thrombosis)”, reviewed 30 April 2026, for symptoms, risk factors and when to seek urgent help; all read on 27 September 2026. Trek timing and altitude context from Travel Himalaya Nepal field reporting. This is general information, not medical advice; speak to a doctor or pharmacist about your own situation.

Photos via Pexels: Pew Nguyen (cover), Natã Romualdo, Lisanne W.