GENEVA, June 29 (Reuters) - The chances of developing deep vein thrombosis double after travelling for four hours or more, the World Health Organisation (WHO) said on Friday in a study estimating one in 6,000 long-haul passengers is at risk.

Tall people whose legs are jammed in economy class and the very short whose feet do not touch the ground are particularly vulnerable to potentially dangerous blood clots linked to immobility during travel, the United Nations agency said.

The obese, women on birth control pills and those with blood clotting disorders are also more susceptible, as are frequent travellers and those who take very long journeys.

"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.

"What causes the risk is immobility," she told a news conference. "The risk is not only true for people flying."

Without regular muscle contractions, blood starts to pool in the legs and can create conditions for a clot, or thrombus, to develop in deep veins. Thrombosis can be symptom-free or trigger cramps, soreness and swelling in affected areas.

It is most dangerous in cases where blood clots travel through the body to the lung, blocking blood flow. Such pulmonary embolism, characterised by chest pain and breathing difficulties, can be fatal if untreated.

About 2 billion people travel by air each year and many more take overland trips where they sit still for prolonged periods.

Health experts said the overall prevalence of deep vein thrombosis is relatively low, noting that the 1 in 6,000 ratio includes those with small or asymptomatic clots.

Still, it means an average of one person is affected for every 20 long-haul flights carrying 300 passengers.

To reduce the chance of developing blood clots, WHO special advisor Shanthi Mendis said travellers should exercise their calf muscles with up-and-down movements of the feet and ankle joints, and leave their seats for a few minutes when possible.

Mendis cautioned air passengers against taking sleeping pills or drinking too much alcohol. People should also avoid wearing tight clothing that can constrain circulation, she said.