Sitting still for long periods cannot alone explain why air passengers are at higher risk of potentially deadly blood clots, research suggests. The risk of developing deep vein thrombosis is thought to be raised by air travel - particularly long flights. But a study by researchers found chemicals indicating clotting in 71 volunteers were higher during eight hours on a flight than in the cinema. It suggests low air pressure and oxygen levels on a flight may play a role. Deep vein thrombosis (DVT) occurs when blood passing through the deepest veins in the calf or thigh flows so slowly that a solid clot forms. DVTs themselves are not life-threatening but they are associated with complications which can be fatal. For instance, a piece of the clot can break off, and become lodged in the lungs, resulting in a potentially fatal pulmonary embolism. A team from Leiden University Medical Centre measured levels of chemicals indicating clotting activity in 71 healthy volunteers before, during and immediately after an eight-hour flight. They also compared the concentrations in the same individuals at the same time points during eight hours of sitting in a cinema and eight hours of regular daily activities. Four out of 10 participants carried a gene that put them at increased risk of thrombosis. The results showed increased concentrations of the chemicals during the flight compared to the other two situations - especially in volunteers with other risk factors for thrombosis.