As humans ascend higher into the atmosphere, air density declines. While air composition remains similar, the drop in air pressure, and in return, air volume, effectively makes the respiratory system work much harder in order to supply the same levels of oxygen to the blood stream. In cases where it is impossible to provide similar levels of oxygen to the body, a host of health complications arise, which can potentially result in death.
The Death Zone
Used to describe high danger altitude on the Mount Everest ascent, the "Death Zone" begins at about 8,000 meters or 26,246 feet. At this elevation, unassisted from compressed air or oxygen, severe altitude sickness sets in, debilitating the human body and eventually resulting in death. The low density of air at this altitude makes acclimation impossible. Cold temperatures and weather conditions at this altitude also affect the high mortality rate of climbers.
Also known as "mountain sickness," altitude sickness describes the two conditions of high altitude cerebral edema, or HACE, and high altitude pulmonary edema, or HAPE. HAPE describes a condition caused by a shortage of oxygen in the body, which eventually leads to a build-up of fluid in the lungs. HACE is a similar condition, except that the fluid builds up on the brain and causes it to swell. Hyperventilation, along with general fatigue and cyanosis accompany HAPE. HACE causes a host of problems such as headache, vomiting, fatigue, hallucination and other temporary neurological problems which can lead to death after prolonged exposure.
Altitude sickness is known to set in at heights as little as 2500 meters or 8,000 feet. However, many people still climb and hike through high mountains, participate in sports, or even live in very high elevations. People who live at sea-level do this through acclimatization. This is a process in which one slowly introduces themselves to higher altitudes in order to let their body adjust to the decreased air pressure and decreased oxygen intake.
While those born and raised at or near sea level have trouble performing and even adapting their bodies to high-altitude conditions, there are people who have lived their whole lives in such areas. The people of Nepal, Tibet, Peru and Bolivia for example, have adapted genetically to living in high-altitude conditions. Native Peruvians and Bolivians have adapted their bodies to produce more hemoglobin in their blood, effectively increasing the capacity of their lungs. The Tibetans and Nepalese have adapted to breathe quicker, and also have larger arteries and capillaries to carry blood throughout their body.