Patients Are Asking: Does Flying Increase My Risk for a Clot

Dr. Susan Kahn


Home is where the heart is, though you might have to travel a long way to get there for the holidays. Long-distance travel may raise your stress level, but can it also raise your risk of getting a blood clot? Dr. Susan Kahn, a Professor of Medicine at McGill University and a Canada Research Chair in venous thromboembolism (VTE), weighs in. 

Blood clots affect about 1-2 in 1,000 U.S. adults per year, and the risk may double or triple after a flight that’s 4+ hours. One study estimates that 1 in 4,600 travelers will have a blood clot within 4 weeks of a long flight. According to Dr. Kahn, “the actual risk of a travel-related blood clot is quite small, and the average traveler doesn’t need to worry about it. Billions of people travel by plane every year, and most of them don’t get a blood clot.”

However, those who already have risk factors for a clot may have a higher risk of developing travel-related VTE. These risk factors include:

  • Obesity
  • Age over 40
  • Use of birth control pills or hormone therapy
  • Recent injury or surgery
  • Limited mobility
  • Pregnancy
  • A personal or family history of blood clots
  • Active cancer or recent cancer treatment


It’s unclear if long-distance air travel is any riskier than car or train travel, but there are factors unique to airplanes that can affect risk:

  • Air travelers sit in tight quarters with the back of the knee pressed against the seat. The vein behind the knee is a common area for clots to form.
  • Immobility can raise the risk of a clot, but your seat assignment, turbulence, etc. can make it difficult to move around the plane.
  • Your body takes in less oxygen when air pressure is lower, and some data suggest that lower oxygen levels can trigger the body’s clotting response.
  • Dehydration may also increase the risk of a clot and drinking coffee or alcohol on a flight can fast-track dehydration.
  • Many people sleep on flights – but if you’re sleeping, you aren’t moving or hydrating. 


The good news is that there are several commonsense methods that can help lower your risk:

  • Choose a bulkhead seat or see if your flight offers extra-legroom seating. If not, avoid putting a bag under the seat in front of you so you have more space for your legs and feet.
  • Try to move around the plane every 1-2 hours. Selecting an aisle seat can make it easier to get up and walk.
  • Avoid sleeping in awkward positions for long periods of time.
  • Try not to cross your legs.
  • Avoid wearing tight clothing.
  • Drink lots of water and avoid alcohol, caffeine, and sedatives.
  • Do some simple exercises in your seat to improve blood flow. (See picture at bottom.)



2012 American College of Chest Physicians (ACCP) Guidelines

Low-risk travelers should frequently move, perform calf exercises, and sit in an aisle seat if possible.

It’s suggested that high-risk travelers wear properly fitted, below-knee compression stockings while in flight.

Preventive blood thinners or aspirin are not recommended for long-distance travelers.

2018 American Society of Hematology (ASH) Guidelines

Compression stockings or anticoagulants/aspirin are not recommended for low-risk travelers.

“For the average healthy person taking a long flight, we don’t recommend anything other than commonsense measures, like walking around the plane and staying hydrated,” explains Dr. Kahn, who helped write the guideline.

ASH suggests that high-risk passengers use graduated compression stockings or a preventive dose of low-molecular-weight heparin (LMWH) for flights over 4 hours, If these measures aren’t practical, travelers can consider taking aspirin.

Because there’s still a limited amount of data on travel-related VTE, the ASH recommendations are conditional, meaning that the right course of action may be different for each patient.


The bottom line:

  • Travel-related blood clots are rare.
  • Moving around and staying hydrated are two ways to stay healthy – both in flight and on the ground.
  • Travel-related VTE is an area that needs more research. Clear-cut evidence is lacking.
  • Talk to your healthcare provider if you’re concerned about developing a blood clot while traveling. 



Blood Clots and Travel: What You Need to Know. Centers for Disease Control and Prevention. Updated February 1, 2019.

Kahn SR, et al. Chest. 2012;141(2):e195s-e226s.

Planes, Trains, and VTE. American Society of Hematology Clinical News. https://www. Updated March 1, 2019.

*Originally published in The Beat – December 2019. Read the full newsletter here.

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