http://en.wikipedia.org/wiki/High-altitude_adaptation_in_humans wrote:
<<It is natural that the human species has been adapted to lowland environment where oxygen is generally abundant. When people from the general lowlands go to altitudes above 2,500 metres (8,200 ft), they experience mountain sickness, which is a type of hypoxia, a clinical syndrome of severe lack of oxygen. Complications include fatigue, dizziness, breathlessness, headaches, insomnia, malaise, nausea, vomiting, body pain, loss of appetite, ear-ringing, blistering and purpling and of the hands and feet, and dilated veins. The sickness is compounded by related symptoms such as cerebral oedema (swelling of brain) and pulmonary oedema (fluid accumulation in lungs). For several days, they breathe excessively and burn extra energy even when the body is relaxed. The heart rate then gradually decreases. Hypoxia, in fact, is one of the principal causes of death among mountaineers. In women, pregnancy can be severely affected, such as development of high blood pressure, called preeclampsia, which causes premature labour, low birth weight of babies, and often complicated with profuse bleeding, seizures, and death of the mother. There are distinctive characteristics of high-altitude environments, including low concentration of available oxygen (which is due to lower barometric pressure), increased solar radiation, greater daily temperature fluctuation, aridity, low biomass, and limitation on energy production. At elevations above 7,600 metres (24,900 ft), lack of oxygen becomes seriously lethal.>>
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