Passengers travelling on flights lasting more than four hours have a doubled risk of developing blood clots, according to a new series of studies by the World Health Organisation (WHO).

But despite the increased risk, Geneva-based WHO says the probability of developing a blood clot while seated and immobile remains relatively low at 1 in 6,000.

The conclusions have been drawn from first phase of WHO’s Research Into Global Hazards of Travel (WRIGHT) – the major study launched in 2002 which aimed to establish whether, and how much, air travel increases the risk of blood clots. The research, which did not cover preventative measures, comprised five studies performed by six universities.


WHO assistant director general for non-communicable disease and mental health Dr Catherine Le Gales-Camus says: “The study does confirm there is an increased risk of venous thromboembolism (VTE) during travel where the passenger is seated and immobile for over four hours, whether in a plane, train, bus or car.

“However, it is important to remember that the risk of developing VTE [a blood clot], when travelling remains relatively low.”

The study also established that passengers taking multiple flights over a short period of time are also at higher risk. “This is because the risk of VTE does not go away completely after a flight is over, and the risk remains elevated for about four weeks,” says WHO.

Other factors which place travellers at increased risk, according to WHO’s research, include obesity, being taller than 1.9m or shorter than 1.6m, use of oral contraceptives and inherited blood disorders.

Passengers are at the highest risk of developing a blood clot when they remain seated and immobile for more than four hours, as prolonged immobility can cause blood to stagnate in the veins, promoting clot formation.

VTE can take the form of either deep vein thrombosis (DVT), a blood clot which usually occurs in the lower leg, or a pulmonary embolism. If a blood clot forms in a vein and breaks off, it can travel to the lungs and block blood flow. This is known as a pulmonary embolism which, if left untreated, can be fatal.

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