The World Health Organization warned passengers today that four hours or more of flight may cause serious blood clotting, that can lead to potentially fatal deep vein thrombosis (DVT).

The WHO issued a report titled “Research Into Global Hazards of Travel” on Friday, where it warns that long-haul flights may double the risk of developing potentially fatal blood clots.

DVT, deep vein thrombosis, can be brought on by prolonged immobility. A blood clot forms in a deep vein along the calf or tight and then travels to the lungs, where it blocks blood flow, or the heart or brain, causing a heart attack or stroke.

“There is an increased risk of venous thromboembolism during travel where the passenger is seated and immobile over four hours, whether in a plane, train, bus or car,” said Catherine Le Gales-Camus, WHO assistant director-general for non-communicable disease and mental health.

While the risk of developing blood clots from traveling for long periods at a time is remote, one in 6,000, DVT is nevertheless a health issue because so many people travel, the WHO said.

The organization emphasized that the risks posed by prolonged immobility apply to airline passengers as well as to those traveling by car, bus or train.

Passengers were advised to get up for short walks during the flight, avoid taking sedatives that would immobilize them for long periods of time, exercise their calf muscles and avoid wear tight clotting that restricts blood circulation.