Under natural conditions the rarefied air meets only in highlands. In such air because of big height there are not enough molecules of oxygen and nitrogen that considerably complicates breath.
The rarefied air in mountains
The amount of oxygen and nitrogen is sharply reduced with height. It is all about the difference of pressure between the top and lower layers of the atmosphere. The top layers strongly press on lower therefore in the last there is much more air and below its pressure. Climbers, rising by big height, experience some difficulties.
Everything depends on height at which there is a person. If it does not exceed 1 km, the difference is almost imperceptible, and no harm for an organism will exist. Height from 1 to 3 km also cannot do harm to the healthy person (the organism easily compensates a lack of oxygen). To sick people, especially that who has asthma it is not necessary to go to so dangerous travel.
At the height from 5 to 6 km the organism of the healthy person mobilizes all systems and forces them to work in the emergency mode because of a lack of oxygen. The prepared person copes with such height therefore here various research bases and observatories often are located. The healthy sleep and healthy nutrition help an organism of scientists to cope with a stressful situation. The places located at the height from 7 km and above are unsuitable for life of people. Oxygen here so is not enough that blood cannot deliver it to all bodies fully. They begin to experience oxygen starvation. The person feels fatigue, a headache, the general state worsens. At the height from 8 km and above people can spend no more than 3 days.
Life in the mountain area
Inhabitants of mountainous areas have much the best health and longer live, than inhabitants of plains. What is it explained by? Oxygen by the nature - strong oxidizer. Any oxidizer in an organism to a greater or lesser extent causes its aging. But also the person cannot live without oxygen. For strengthening of health it is necessary a little smaller content of oxygen in air, than on plains. Optimum height for comfortable life is about 1500 meters above sea level. The organism experiences small oxygen starvation that puts into operation all systems in the strengthened mode. Blood circulation and ventilation of lungs improves, hemoglobin level in blood increases. The American scientists noticed that guttural sounds in the speech are characteristic of the people living in mountains. At big height it is much easier to say similar sounds as for this purpose it is necessary to compress air in a throat. The easiest to make it in mountain areas as here the air which was more rarefied, than on plains.