The World Health Organization (WHO) has said that the risk of developing venous thrombo-embolism (VTE) approximately doubles after travel of four hours or more.
In a research study of global hazards of travel, the WHO, however, says that the absolute risk of developing VTE, if seated and immobile for more than four hours remains relatively low at about 1 in 6000.
According to the study, the two most common manifestations of VTE are deep vein thrombosis (DVT) 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. DVT can be life threatening when associated with thrombo-embolism.
Thrombo-embolism occurs when a blood clot (from a deep vein thrombosis) in a leg vein breaks off and travels through the body to the lung 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 the higher risk of VTE when 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 risk remains elevated for about four weeks.
The report shows that a number of other factors increase the risk of VTE during travel, including obesity, being very tall or very short (taller than 1.9 meters or shorter than 1.6 meters), use of oral contraceptives, and inherited blood disorders leading to increased clotting tendency.
Dr Catherine Le Galès-Camus, WHO Assistant Director-General for Non-communicable Disease and Mental Health said that this study did not investigate effective preventive measures against DVT and VTE.
Experts said that blood circulation could be promoted by exercising the calf muscles with up-and-down movements of the feet at the ankle joints. Moving feet in this manner encourages blood flow in the calf muscle veins, thus reducing blood stagnation, they added.
People travelling have also been advised to avoid wearing tight clothing to prevent blood stagnation.
Phase I of the research project concludes that there is a need for travellers to be given appropriate information regarding the risk of VTE by transport authorities, airlines, and medical professionals.