WHO Links Flights of Four Hours or Longer to Elevated Clot Risk

GENEVA, July 2 – Airline travel that lasts four hours or longer is associated with a one in 6,000 absolute risk of developing venous thromboembolism, according to the World Health Organization.

But multiple long trips taken within four weeks increased the risk, and the longer the trip, the greater the risk, said Catherine Le Gals-Camus, Ph.D., a WHO assistant director-general.

Moreover, those who travel by train, bus, or car don't escape risk. WHO said that any trip that requires passengers to sit for four or more hours increased the risk of either deep vein thrombosis or pulmonary embolism -- the two most common manifestations of venous thromboembolism.

Travel by car, bus, or train led to a high risk of thrombosis in individuals with factor V Leiden mutation (odds ratio 8.1, 95% CI 2.7 to 24.7), those who were more than 6'2'' tall (odds ratio 4.7, 95% CI 1.4 to 15.4) and those who used oral contraceptives (estimated odds ratio >20).

In addition to the tall and short travelers, obese people who fly on flights longer than four hours have an increased risk of venous thromboembolism as do women using oral contraceptives and any traveler with a predisposition to thrombophilia.

Those findings emerged from the first phase of the WRIGHT project (WHO Research Into Global Hazards of Travel) that included a series of epidemiological and pathophysiological studies.

The studies did not investigate measures to prevent clots from forming. But a WHO statement released with the report cited well-known preventive measures, including exercising the calf muscles with up-and-down movements of the feet at the ankle joints and avoidance of tight clothing while traveling.

Dr. Le Gals-Camus also cautioned that although the study confirmed the link between traveling and risk of venous thromboembolism, the risk "remains relatively low."

The WRIGHT project was a collaborative effort of WHO, the International Air Transport Association, the International Civil Aviation Organization, the European Commission, as well as representatives from airlines and various consumer groups.

Phase I analyzed data from a population-based case-control study, a retrospective cohort study among employees of international organizations, a retrospective cohort study of Dutch commercial pilots, a hypobaric hypoxia study, and a travel and non-travel immobility study.

The population-based case control study included 1,906 patients younger than 70 who were treated at anticoagulation clinics in Holland and 3,812 matched controls. In that study traveling for more than fours was associated with a two-fold increase in risk of venous thromboembolism (odds ratio 2.1, 95% CI, 1.5-3.0).

In the retrospective study that analyzed date from employees of international organizations data from 267,241 flights -- including 86,748 that were four hours or longer -- identified an absolute venous thromboembolism risk of one in 5,944 for persons traveling on flights that were at least four hours.

Conversely, the Dutch pilots study found no association between the number of hours flown and the venous thromboembolism risk.

Travelers on long flights experience reduced cabin pressure and hypoxia, factors that were thought to increase the risk of venous thromboembolism. But a small study from England that exposed healthy volunteers to hypobaric hypoxia similar to that experience during long flights did not support that hypothesis.

Yet a second study that compared travelers with non-travelers, who were studied in an immobile situation that aped that of the buckled-in airline passenger suggested that factors unique to the airline cabin-hypobaric hypoxia or the type of seat used by airlines-increased thrombin generation in some individuals.

WHO concluded that as the number of travelers on so-called long haul flights increases, "air travel-related venous thromboembolism is an important public health issue. There is a clear need for travelers to be given appropriate information regarding the risks and for further studies to identify effective preventive measures, which will comprise phase II of the WRIGHT project."