Climbers on Mount Everest often experience<unk> .<unk>  can first occur at 1,500 metres, with the effects becoming severe at extreme<unk> s (greater than 5,500 metres). Only brief trips above 6,000 metres are possible and supplemental oxygen is needed to avert sickness. As<unk>  increases, the available amount of oxygen to sustain mental and physical alertness decreases with the overall air pressure, though the relative percentage of oxygen in air, at about 21%, remains practically unchanged up to . The RMS velocities of diatomic nitrogen and oxygen are very similar and thus no change occurs in the ratio of oxygen to nitrogen until stratospheric heights. Dehydration due to the higher rate of water vapor lost from the lungs at higher<unk> s may contribute to the symptoms of<unk> . The rate of ascent,<unk>  attained, amount of physical activity at high<unk> , as well as individual susceptibility, are contributing factors to the onset and severity of high-altitude illness.<unk>  usually occurs following a rapid ascent and can usually be prevented by ascending slowly. In most of these cases, the symptoms are temporary and usually abate as<unk>  acclimatization occurs. However, in extreme cases,<unk>  can be fatal. High<unk>  illness can be classified according to the<unk> : high (1500-3500m), very high (3500-5500m) and extreme (above 5500m). High<unk>  At high<unk> , , the onset of physiological effects of diminished inspiratory oxygen pressure (PiO2) includes decreased exercise performance and increased ventilation (lower arterial partial pressure of carbon dioxide: PCO2). While arterial oxygen transport may be only slightly impaired the arterial oxygen saturation (SaO2) generally stays above 90%.<unk>  is common between 2,400 and 4,000 m because of the large number of people who ascend rapidly to these<unk> s. Very high<unk>  At very high<unk> , , maximum SaO2 falls below 90% as the arterial PO2 falls below 60mmHg. Extreme hypoxemia may occur during exercise, during sleep, and in the presence of high<unk>  pulmonary edema or other acute lung conditions. Severe<unk>  illness occurs most commonly in this range. Extreme<unk>  Above , marked hypoxemia, hypocapnia, and alkalosis are characteristic of extreme<unk> s. Progressive deterioration of physiologic function eventually outstrips acclimatization. As a result, no permanent human habitation occurs above . A period of acclimatization is necessary when ascending to extreme<unk> ; abrupt ascent without supplemental oxygen for other than brief exposures invites severe<unk> .
Altitude sickness
Hangover
Sweating sickness
Dehydration
Motion sickness
Fatigue
Chronic mountain sickness
Carbon monoxide poisoning