Studies have shown that plane, train, bus or car passengers who remain seated for a journey of more than four hours are at higher risk for venous thromboembolism by causing venous blood to stagnate, allowing blood clots to form in the veins. In addition, passengers who take multiple flights in a short period of time are also at higher risk, because the risk of venous thromboembolism does not completely disappear after the end of a flight, but remains high for four weeks.

There are other factors that may increase the risk of venous thromboembolism during travel, the report suggests, including obesity, extremely high or low height (above 1.9m or below 1.6m), use of oral contraceptives and hereditary blood disease.

Experts suggest that the up and down movement of the ankle joint of the foot can exercise the calf muscles and promote blood flow in the veins of the calf muscles, thereby reducing blood stagnation. In addition, people should avoid wearing tight clothing while traveling, as such clothing may cause blood to stagnate.

In 2000, the death of a young British woman from a long-haul flight in Australia from a pulmonary embolism drew media and public attention to the risk of thrombosis in long-haul travellers. WHO launched the WHO Global Travel Hazards Project in 2001, with the goal of the first phase being to confirm whether travel increases the risk of venous thromboembolism and to determine the severity of the risk; after sufficient funding is obtained, the second A phased study will be initiated with the goal of identifying effective preventive measures.

According to WHO, the two most common manifestations of venous thromboembolism are deep vein thrombosis and pulmonary embolism. Deep vein thrombosis is a condition in which a blood clot or thrombus forms in a deep vein, usually in the lower leg. The symptoms of deep vein thrombosis are mainly pain, tenderness, and swelling in the affected area.

Thromboembolism occurs when a blood clot in the veins of the lower extremities (from deep vein thrombosis) breaks off and travels through the body to the lungs, where it deposits and blocks blood flow. This is called a pulmonary embolism. Symptoms include chest pain and breathing difficulty.

Venous thromboembolism can be detected through medical monitoring and treated, but if left untreated, it can be life-threatening, the WHO said.