Taking a long-haul flight - one of more than four hours - doubles the risk of developing blood clots that can lead to potentially fatal deep-vein thrombosis, the World Health Organization (WHO) warns.

DVT occurs when a clot forms in leg veins during periods of relative immobility, such as long-haul travel. The clot can then migrate to the lungs - a pulmonary embolism - or to the heart or brain, causing a heart attack or stroke.

The overall phenomenon is known as venous thromboembolism, the WHO said in its Research Into Global Hazards of Travel project.

Following five separate research studies as part of first phase of the project, the WHO said that "the risk of developing venous thromboembolism approximately doubles after travel lasting four hours or more." The absolute risk of developing VTE if seated and immobile for more than four hours remains "relatively low" at about one in 6,000, the WHO said.

But the chances are greater for people deemed at risk, including those who regularly take long-haul flights, those who are obese, tall (above 1.9 meters) or short (below 1.6 meters), women who take oral contraception, or people with inherited blood disorders.

"There is a public health issue," said Shanthi Mendis, a WHO senior adviser on cardiovascular diseases, noting that some two billion people are estimated to fly every year.

The WHO stressed that the risks posed by immobility do not just apply to airline passengers but also those who undertake long journeys by car, bus or train. "We don't move enough when we travel," said Catherine Le Gales-Camus, a WHO assistant director general.

Travelers can take "common-sense" measures to reduce the risks of contracting VTE by regularly exercising their calf and ankle muscles during a journey, or simply getting up and moving around a bit, the WHO said.

Passengers should also avoid taking sedatives or too much alcohol ahead of a voyage.

But the WHO made no recommendations about taking blood-thinning medications or wearing elastic stockings designed to prevent clotting, saying more research was needed.