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.

Causes

The main cause is going too high, too quickly. The condition is caused by the body’s response to a reduction in oxygen supply to the muscles and brain, which in turn requires the heart and lungs to work harder causing breathing and pulse rates to increase. Red blood cells, which carry oxygen around the body, are made in greater numbers.

However, the body’s efforts to cope with low oxygen also results in a number of changes to blood acidity, lung pressure, electrolytes, fluid and salt balance. In a small number of cases, typically at above 3500m (11000ft), two potentially life-threatening complications occur: fluid accumulates in the lungs (high altitude pulmonary oedema); or in the brain (high altitude cerebral oedema).