WHO on 29. June 2007 released results from Phase I of the WHO Research Into Global Hazards of Travel (WRIGHT) project. Findings indicate that the risk of developing venous thromboembolism (VTE) approximately doubles after travel lasting four hours or more. However, the study points out that even with this increased risk, the absolute risk of developing VTE, if seated and immobile for more than four hours, remains relatively low at about 1 in 6000.Thromboembolism occurs when a blood clot (from a deep vein thrombosis) in a leg vein breaks off and travels through the body to the lungs where it becomes lodged and blocks blood flow. This is known as pulmonary embolism, and symptoms include chest pain and breathing difficulties. VTE can be treated, but if it is not, it can lead to death.The study showed that plane, train, bus or automobile passengers are at higher risk of VTE when they remain seated and immobile on journeys of more than four hours. This is due to a stagnation of blood in the veins caused by prolonged immobility, which can promote blood clot formation in veins.One study within the project examining flights in particular found that those 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.Source: WHO; read full article here. Final report of Phase I of the study for download here.