Altitude has strange effects on the body, and it鈥檚 mostly down to the reduced pressure of oxygen in the air. Cells need oxygen in order to survive. At higher altitudes haemoglobin, the blood protein that transports oxygen from the lungs to cells, cannot absorb oxygen to its full capacity, creating a deficit in the body.
The brain is very sensitive to oxygen levels, which is why headaches and dizziness are the first signs of altitude sickness. With prolonged stays above 5000 metres, muscle mass deteriorates and the risk of fatal fluid accumulation in the lungs and brain increases. Go higher than around 7500 metres and the magnitude of the oxygen deficit can lead to loss of consciousness and, ultimately, death.
What about living at altitude? Well, almost half the population of Bolivia lives in the country鈥檚 Altiplano region, 4000 metres above sea level. But go much higher and problems kick in. Some Chilean mine workers spend several weeks at a time at 5800 metres, but they probably couldn鈥檛 reproduce there because altitude temporarily suppresses male fertility, says Mike Grocott of the University of Southampton in the UK, who studies the physiological effects of altitude.
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鈥淥nly a handful of people have made it to the summit of Everest without oxygen鈥�
Most people can adapt to altitude if they take time to acclimatise. A good rule of thumb is that the higher you go, the shorter you should stay. If you were suddenly transported to the summit of Everest (8848 metres) without acclimatising first, you would probably be dead in 2 minutes.
Only a handful of people have made it to the summit without supplemental oxygen. Babu Chiri Sherpa set the record for remaining at the top of Everest without oxygen for 21 hours in 1999. He was probably genetically primed to cope at altitude, says Grocott.
What is the limit beyond which unaided survival is impossible? As it happens, the summit of Everest is pretty close to it. There has been just one ascent without breathing apparatus in winter, when barometric pressure falls and therefore the amount of oxygen drops further than ever, says Grocott. 鈥淚鈥檇 guess the limit would be about 9000 metres.鈥�
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