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Altitude sickness (also known as acute mountain sickness) can occur in its mildest form at around 2500m (8000ft) above sea level (common ski resort height), but is more likely to occur, and to be more severe, at higher altitudes of 2500m and above.
As you go higher the air still contains the normal amount of oxygen (21%), but atmospheric pressure decreases which results in each breath containing fewer molecules of oxygen. For example, at 5500m (18000ft), each breath will contain roughly half the normal amount of oxygen.
The onset and severity of symptoms, and the altitude at which they are experienced, vary according to the individual, the rate of ascent, and the amount of time spent at high altitude. In most cases symptoms develop 24-36 hours after arrival at altitude, and begin to ease within 48 hours as the body acclimatises (gets used to it).