According to researchers from France, the number of miles you fly affects your risk of pulmonary embolism. There's increasing concern that long haul travel could increase the risk of blood clotting in the veins of the legs (deep vein thrombosis, or DVT). If the clot breaks off, it may travel to the lungs, forming a pulmonary embolism (PE) which is a potentially life-threatening condition.
Researchers in France and the US have monitored all passengers arriving at Charles de Gaulle Airport in Paris between 1993 and 2000. During this time, over 130 million passengers entered the airport from 141 different countries and there were 50 cases of PE confirmed on arrival. The incidence of PE was much greater at 1.4 cases per million among those travelling more than 5000 kilometres than among those taking shorter flights - where the incidence was just 0.01 per million. For people travelling more than 10,000 kilometres, the risk rose to 4.8 cases per million. The message is, if you're going on a really long flight, take medical advice on the potential risks of PE.