Passengers stuck in their seats for more than four hours are twice as likely to develop a potentially life threatening blood clot, according to a research published on Friday by the World Health Organisation (WHO).
The risk of venous thromboembolism, VTE, also rises for air travellers undertaking frequent flights over a short period, but the study stated the danger still remains relatively low at about one in 6,000 people.
The research was carried out following the death of a young English woman who died after flying back from Australia in 2000. She suffered a pulmonary embolism when a blood clot lodged in her lung.
Hours of immobility increased the risk of blood clotting, said the report, with the effect lasting up to four weeks after a journey. Obesity and height might also play a part during travel.
"The study does confirm that 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.
"However, it is important to remember that the risk of developing VTE when travelling remains relatively low," said Dr Catherine Le Gales-Camus, WHO Assistant Director-General for Non communicable Disease and Mental Health.