Over 10 million people live at altitudes above 4000 m in a hypoxic environment throughout their lives, often without evident clinical effects. However, millions of lowlanders, i.e. sea level residents or those usually residing below 3000 m travel higher each year. Some do so for pleasure, others for employment, to engage in warfare or to go on trekking, skiing or mountaineering trips. They frequently develop various forms of high altitude illness, the clinical syndromes caused by chronic hypobaric hypoxia. Often separated into acute mountain sickness (AMS), pulmonary and cerebral oedema, these conditions have a common patho-physiological basis.1 The degree of hypoxia at various altitudes is illustrated in Fig. 1. Serious clinical problems are seldom seen below 3000 m.
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