According to the World Health Organization (WHO) the chances of developing deep vein thrombosis ( DVT) double after traveling for four hours or more; this says WHO equates to one in 6,000 long-haul passengers at risk of potentially dangerous blood clots.
Particularly vulnerable are tall people whose legs are jammed in economy class and very short people whose feet do not touch the ground, and it is the immobility during travel which creates the danger.
Also at greater risk are the obese, women on birth control pills, those with blood clotting disorders, frequent travelers and those who take very long journeys.
One study within Phase I of the WHO Research Into Global Hazards of Travel (WRIGHT) project examined flights and found that those taking multiple flights over a short period of time are also at higher risk because the risk of venous thromboembolism (VTE) does not go away completely after a flight is over, and the risk remains elevated for about four weeks.
Catherine Le Gales-Camus, WHO assistant director-general for non-communicable disease and mental health says there is an increased risk of VTE during travel where the passenger is seated and immobile over four hours, whether in a plane, train, bus or car.
The two most common manifestations of VTE are deep vein thrombosis and pulmonary embolism.
Deep-vein thrombosis (DVT) is a condition in which a blood clot, or thrombus, develops in a deep vein - usually in the lower leg; symptoms of DVT are principally pain, tenderness and swelling of the affected part.
DVT can be detected through medical testing and can be treated but it can be life-threatening when associated with thromboembolism.
When there is no 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 particularly dangerous when blood clots travel through the body to the lung, blocking blood flow and such pulmonary embolism, characterized 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 travelers 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.
People are also advised to avoid wearing tight clothing during travel, as such garments may promote blood stagnation.
Air passengers are also warned against taking sleeping pills or drinking too much alcohol.
The WHO says travellers need to be given appropriate information regarding the risk of VTE by transport authorities, airlines, and medical professionals.
Phase II of the project, which requires additional funding before it can begin, will identify effective preventive measures.
Individuals with questions regarding prevention of VTE should consult their doctors before travelling.
The WRIGHT project came about in 2000 as a result of media and public attention being focused on the risk of thrombosis in long-haul travellers, following the death from pulmonary embolism of a young English woman who returned on a long-haul flight from Australia.
In the same year, a report from the Select Committee on Science and Technology from the British House of Lords recommended research into the risk of DVT and following a consultation of experts convened by WHO in March 2001, the WRIGHT project was initiated.
Phase 1 was funded by the UK Government's Department for Transport and Department of Health and the European Commission.
Phase I aimed to confirm whether the risk of VTE is increased by air travel and to determine the magnitude of risk.
Five studies were conducted under the auspices of WHO and performed by an international collaboration of researchers from the Universities of Leiden, Amsterdam, Leicester, Newcastle, Aberdeen and Lausanne.
They consisted of a population-based case control study to investigate the risk factors of VTE; two retrospective cohort studies among employees of international organizations and Dutch commercial pilots to investigate the actual risk of VTE related to air travel; and two pathophysiological studies to investigate the influence of immobility on VTE related to travel and the influence, if any, of low oxygen and low pressure in the cabin of aircraft on VTE related to travel.