[Effects of acute altitude exposure: which altitude can be tolerated?]

Exposures to natural and simulated altitudes are combined with a reduction of the available oxygen (hypoxia) and may provoke health problems or even emergencies. Although millions of people are living at high altitudes, are regularly or occasionally performing mountain sport activities, are transported by airplanes, and are increasingly frequently exposed to hypoxia at their workplace, e.g. with fire control systems, there is no consensus about the level of hypoxia which is well tolerated by most of human beings. Thus, the present mini review tries to define such a level. This review indicates that although humans show adaptive responses even to altitudes below 2000 m or corresponding normobaric hypoxia (FiO2: >16%) most of these subjects without severe pre-existing diseases well tolerate altitudes up to 3000 m (FiO2: 14.5%). If at all, symptoms of acute mountain sickness may develop in some persons after a minimum exposure time of about 6 hours. This also applies to children, elderly persons and pregnant women. Physical activity and unusual environmental conditions may increase the risk to get sick. Also patients with not severe cardiovascular, respiratory or metabolic diseases well tolerate such levels of altitude. However, individual differences in responses to hypoxia have to be considered and can at least partly be tested by simple hypoxia challenge tests. For safety reasons altitudes up to 2700 m or normobaric hypoxia up to FiO2 values of 15% are proposed to be well tolerated by most humans who are not severely diseased.