Deep vein thrombosis (DVT) is a blood clot that forms in a vein deep inside the body. DVT occurs most commonly in the leg veins, but can occur in other parts of the body as well. If a clot becomes dislodged, it can travel to your lung. This condition is called a pulmonary embolism (PE) and can be fatal.

What causes DVT? Several factors contribute to its development. Among these, older age, obesity , and a history of venous disease are particularly common in people who develop DVT. But the factor that has received the most attention from the media in the past few years is air travel. Researchers believe that air travel may increase the risk of DVT through one or more of the following mechanisms:

  • Decreased air pressure and the release of nitric oxide in the plane’s cabin
  • Dehydration as a result of low humidity in the cabin and/or the consumption of alcohol and caffeine
  • Prolonged sitting and pressure on the calves by the passenger’s seat, which can lead to stagnant blood flow in the veins

A new study in the December 8/22 issue of the Archives of Internal Medicine found that people who traveled on flights that lasted eight hours or more had a 4.4-fold increased risk of developing DVT. The incidence of DVT was low, however, and most people who developed DVT had other risk factors.

About the Study

The researchers recruited 964 people who were planning to travel on a flight lasting eight hours or more (the traveling group) and 1,213 people who were not planning to fly in the next three months (the control group). The participants did not use compression stockings or take anticoagulation medications, which are both thought to reduce the risk of DVT.

One week before their outgoing flight and within 48 hours of their return flight, members of the traveling group were assessed for DVT risk factors, including age, body mass index (BMI, a measure of weight in relation to height), personal and family history of venous disease, use of oral contraceptives, and blood coagulation disorders. At both examinations, the participants were given a venous ultrasound to detect isolated calf muscle venous thrombosis (ICMVT, a precursor to DVT) and/or DVT. The control group also had these two examinations. Participants who had thrombosis at baseline were excluded.

The researchers followed both groups for an additional four weeks to determine if they developed a venous thrombotic event or died. Then they compared the risk of developing these conditions between the traveling and control group.

The Findings

Significantly more people in the traveling group were diagnosed with ICMVT. Specifically, of the 964 people who took long flights, 20 (2.1%) were diagnosed with ICMVT, compared with 10 (0.8%) in the control group.

The traveling group was also significantly more likely to develop DVT. Seven (0.7%) of the people who had taken long flights, compared with two (0.2%) who hadn’t, were diagnosed with DVT. Overall, the traveling group had a 2.8-fold increased risk of having a thrombotic event (4.4-fold increase for DVT and 2.5-fold for ICMVT). No passengers died during this study or developed thrombosis during the four-week follow-up period.

All of the people in the traveling group who developed DVT or ICMVT had at least one risk factor. Older age and elevated BMI were the most common of these risk factors; 21 of the 27 passengers were either over the age of 45 or had an elevated BMI.

After the diagnosis of ICMVT or DVT, passengers were treated with a 10-day course of an anticoagulant, which can help prevent blood clots from causing serious problems, such as PE. This may have led to the underestimation of the incidence of DVT in the follow-up period.

How Does This Affect You?

These findings suggest that flights lasting eight hours or more significantly increase the risk of blood clots, especially in people who have one or more risk factors. Another study in the same issue of the Archives adds to these findings. Researchers looked at the 16 passengers who arrived at the Madrid-Barajas Airport over a six-year period with PE. All of the patients with travel-associated PE had flight durations of more than six hours, and the incidence of PE per one million passengers increased with increasing flight durations.

While these findings may make you wary of prolonged air travel, it’s important to keep in mind that the chance you will develop a serious blood clot is very low, even if you travel on a long flight. In this study, only 27 of the 964 passengers on flights lasting eight hours or more developed ICMVT or DVT. The researchers may have even overestimated the prevalence of thrombosis in this study, since it is possible that people at increased risk for blood clots were more likely to enroll. Also, less than one in a million people arriving at Madrid-Barajas Airport were diagnosed with PE.

Nevertheless, you may wish to take precautionary measures, especially if you have one or more risk factors for DVT. An editorial that accompanies these studies points out that the increased risk of DVT may have more to do with prolonged sitting than prolonged flying. You can avoid prolonged sitting by getting up and walking frequently during long trips by airplane, car, or train. Studies have also shown that below-the-knee elastic stockings and certain medications may help reduce your risk of thrombosis, so talk to your doctor to find out if these options are right for you.

What are the risk factors for DVT? According to the National Heart, Lung, and Blood Institute, the following factors increase your risk of developing DVT:

  • An inherited condition that predisposes to blood clotting
  • Low blood flow in a deep vein, due to injury, surgery, or immobilization
  • Cancer and its treatment
  • Related medical conditions such as varicose veins
  • Pregnancy, especially the first six weeks after giving birth
  • Being over age 60
  • Being overweight
  • Taking birth control pills or postmenopausal hormone therapy