I have seen a lot of stories in the media about deep vein thrombosis. What is it?

Deep vein thrombosis (DVT) hit the headlines when Emma Christoffersen, 28, from Newport, south Wales died after flying back to London from Australia in October 2000. According to a government-funded study, more than 300 cases have been reported, providing 'circumstantial evidence of an association between air travel and DVT'.

Also dubbed 'economy class syndrome' (which is misleading, since it can affect anyone) and 'traveller's thrombosis', DVT is a medical condition that develops when people remain seated for long periods of time. Blood circulation slows down, causing clots to form in blood vessels deep in the legs. If these clots break off and get carried in the blood to the lungs, they can be fatal. Because DVT is linked with long periods of immobility, it can affect people travelling on aeroplanes as well as long-distance trains and buses.