Study Finds Airline Cabin Pressure Does Not Cause Blood Clots    

Long distance air travel can increase the risk of blood clots in the legs, a potentially fatal condition that has been known for more than half a century. Now British researchers report that the culprit is not airplane cabin air pressure, as some have speculated. They suggest that the condition is caused by lack of movement.

Deep Vein Thrombosis and Blood Clot Formation

 On a flight from Britain to Australia not long ago, Simon Snedden did what he always used to do on an airplane trip.

"The plan was always get in, get sat down, have a snooze, watch the film," he said.

Snedden will now alter that plan to include a little more leg movement. That is because he took part in a recent study looking into the air travel conditions that can lead to blood clots, or deep vein thrombosis.

The condition is dangerous for reasons explained by University of Leicester physician William Toff.

"Particularly looking at clots that form in the larger leg veins higher up, they may break away and travel through the heart to the lungs, where they block the blood flow and they in some cases prove fatal," said Toff.

Circulatory System

Toff and colleagues conducted the study in which Simon Snedden and 72 other healthy volunteers took part. They wanted to know whether blood clots were the result of the low oxygen and air pressure levels in an airplane cabin.

So they put the volunteers in a cramped chamber for eight-hour shifts to copy the conditions of an actual flight. The researchers lowered the air pressure and oxygen levels in the chamber to match flight levels and even put a table in the middle to simulate the close quarters of the cabin. For comparison, they also had the volunteers spend eight hours in the chamber with normal oxygen and pressure levels.

There were some changes in the blood toward clotting, but they occurred at the same rate in both air pressure and oxygen conditions.

Toff says blood tests made before and after the simulated airline cabin conditions tell the story.

"We looked at a very wide range of blood markers, looking at all aspects of the body's clotting mechanism. We found no evidence that the low pressure, low oxygen were activating the body's blood clotting mechanism," he explained.

The key is blood FLOW

 The researchers say the clotting changes were probably the result of sitting still for eight hours. So Toff advises travelers to stretch their legs, extend and flex the knees and ankles, and stand and walk whenever possible.

"Any form of long-haul travel, whether by air, rail, or road is associated with an increased risk of thrombosis. People should take sensible precautions according to their level of inherent risk," he added.

The study appears in the Journal of the American Medical Association.

In a commentary accompanying the research, University of Heidelberg sports physician Peter Bartsch notes that the study included small numbers of elderly people and women taking oral contraceptives. As a result, he says it is not possible to draw conclusions about these two groups.

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Deep Vein Thrombosis Overview

Interventional Radiology Clot Busting Treatment Prevents Permanent Leg Damage

Deep vein thrombosis (DVT) is the formation of a blood clot, known as a thrombus, in the deep leg vein. It is a very serious condition that can cause permanent damage to the leg, known as post-thrombotic syndrome, or a life-threating pulomnary embolism. In the United States alone, 600,000 new cases are diagnosed each year. One in every 100 people who develops DVT dies. Recently, it has been referred to as "Economy Class Syndrome" due to the occurrence after sitting on long flights.

The deep veins that lie near the center of the leg are surrounded by powerful muscles that contract and force deoxygenated blood back to the lungs and heart. One-way valves prevent the back-flow of blood between the contractions. (Blood is squeezed up the leg against gravity and the valves prevent it from flowing back to our feet.) When the circulation of the blood slows down due to illness, injury or inactivity, blood can accumulate or "pool" which provides an ideal setting for clot formation.

Risk Factors

  • Previous DVT or family history of DVT
  • Immobility, such as bed rest or sitting for long periods of time
  • Recent surgery
  • Above the age of 40
  • Hormone therapy or oral contraceptives
  • Pregnancy or post-partum
  • Previous or current cancer
  • Limb trauma and/or orthopedic procedures
  • Coagulation abnormalities
  • Obesity


  • Discoloration of the legs
  • Calf or leg pain or tenderness
  • Swelling of the leg or lower limb
  • Warm skin
  • Surface veins become more visible
  • Leg fatigue

Post-thrombotic Syndrome

Post-thrombotic syndrome is an under-recognized, but relatively common sequela, or aftereffect, of having DVT if treated with blood thinners (anticoagulation) alone, because the clot remains in the leg. Contrary to popular belief, anticoagulants do not actively dissolve the clot, they just prevent new clots from forming. The body will eventually dissolve a clot, but often the vein becomes damaged in the meantime. A significant proportion of these patients develop permanent irreversible damage in the affected leg veins and their valves, resulting in abnormal pooling of blood in the leg, chronic leg pain, fatigue, swelling, and, in extreme cases, severe skin ulcers. While this use to be considered an unusual, long-term sequela, it actually occurs frequently, in as many as 60-70 percent of people, and can develop within two months of developing DVT. There is increasing evidence that clot removal via interventional catheter-directed thrombolysis in selected cases of DVT can improve quality of life and prevent the debilitating sequela of post-thrombotic syndrome.

Pulmonary Embolism

Left untreated, a deep vein thrombosis (DVT) can break off and travel in the circulation, getting trapped in the lung, where it blocks the oxygen supply, causing heart failure. This is known as a pulmonary embolism, which can be fatal. With early treatment, people with DVT can reduce their chances of developing a life threatening pulmonary embolism to less than one percent. Blood thinners like heparin and coumadin are effective in preventing further clotting and can prevent a pulmonary embolism from occurring.
  • It is estimated that each year more than 600,000 patients suffer a pulmonary embolism.
  • PE causes or contributes to up to 200,000 deaths annually in the United States.
  • One in every 100 patients who develop DVT die due to pulmonary embolism.
  • A majority of pulmonary embolism are caused by DVT.
  • If pulmonary embolism can be diagnosed and appropriate therapy started, the mortality can be reduced from approximately 30 percent to less than ten percent.

Symptoms of Pulmonary Embolism

The symptoms are frequently nonspecific and can mimic many other cardiopulmonary events.

  • Shortness of breath
  • Rapid pulse
  • Sweating
  • Sharp chest pain
  • Bloody sputum (coughing up blood)
  • Fainting