The number of people travelling to the high altitude regions, especially South America, Nepal, and India, has risen enormously in the past 10 years. Without special climbing ability these trekkers can be exposed to altitudes they will not have encountered in their home countries. For example,the height of Everest base camp is 5500 m whereas the top of Mount Blanc, the highest mountain in the Alps, is only 4800 m. The areas with the highest mountains are also the areas with the poorest facilities, especially for medical care. Trekkers must therefore understand the effects of altitude on their bodies (hypoxia, cold, and dehydration), the processes of acclimatisation, and prophylaxis against and treatment of altitude illness. Climbing Everest High altitude can also be a problem for people with cardiopulmonary disease, many of whom take long haul flights on commercial aircraft. They need to know how their condition can be affected by the cabin altitude of the aeroplane (typically 1800-2500 m). If there is any doubt they should be assessed before travel to determine whether their condition is likely to worsen significantly during flight. Relation between altitude and inspired oxygen pressure Although the percentage of oxygen in inspired air is constant at different altitudes, the fall in atmospheric pressure at higher altitude decreases the partial pressure of inspired oxygen and hence the driving pressure for gas exchange in the lungs. An ocean of air is present up to 9-10 000 m, where the troposphere ends and the stratosphere begins. The weight of air above us is responsible for the atmospheric pressure, which is normally about 100 kPa at sea level. This atmospheric pressure is the sum of the partial pressures of the constituent gases, oxygen and nitrogen, and also the partial pressure of water vapour (6.3 kPa at 37°C). As oxygen is …