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	Country pages ---------------------- Andean Region Brasil Germany Italy Japan Mexico Spain U.S.A.   Like most people, you're probably glad to have your feet back on the ground after several hours in the air so that you can stretch your legs properly. Many passengers complain of mild to severe pain in the legs during long-haul flights. The pain is generally due to cramp in the calf muscles. But it can also be due to thrombosis in the deep veins of the legs, through which as much as 85 per cent of the blood in the body passes on its way back to the heart. DVT develops most commonly where the blood vessels are constricted because the knees are kept bent for several hours, preventing the blood from returning to the heart and causing congestion. The blood forms a clot (known as a thrombus) which impairs circulation in the legs even more - and this is what causes the pain. The thrombus generally dissolves again as soon as the passenger lands and can move around again. The pain disappears not long after the passenger starts walking. Occasionally, however, the thrombus doesn't dissolve, and it passes through the inferior vena cava (a blood vessel) into the right side of the heart and then into the lungs, where it blocks the branches of the pulmonary arteries. If it disintegrates into several fragments (emboli) on its way into the lung, it can even block several blood vessels there. The result is life-threatening pulmonary embolism. Many affected individuals display flulike symptoms, such as coughing and chest pain, which disappear as soon as the embolus dissolves. Others have severe chest pain which doctors tend to interpret as a heart attack, leading them to initiate appropriate therapy. If the pulmonary arteries remain blocked, then the patient is in real danger. A pulmonary infarction can develop - frequently with a fatal outcome. Apart from sitting in cramped conditions, there are other aspects of flying which can contribute to the development of DVT. The air in the cabin would be much too thin at an altitude of between 10,000 and 12,000 metres, so it is pressurised to create the same air pressure that would normally be found at around 2,500 metres. If you go rambling through the mountains at this altitude, you will soon run out of breath! When the air pressure is this low, your veins expand, your circulation slows down, and the risk of blood clots rises - ideal conditions for a thrombosis. In addition, the humidity of the air in the cabin is only about three per cent - even the air in the Sahara Desert contains more moisture! As a result, your body loses water, your blood becomes thicker - another risk factor for the development of a thrombus. Consuming large quantities of alcohol is another serious risk factor, because alcohol also dehydrates the body. The combination of these factors which occur only in the air is why, in the view of experts, the risk of thrombosis is greater during a flight than during a train or bus journey of the same duration.