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The death zone is a term that refers to high altitudes, encountered by mountain climbers, where the amount of oxygen present cannot sustain human life. Specifically, it refers to altitudes above 8000 meters (26,250 feet)^[1]

The human body functions at its best at sea level, where the atmospheric pressure is measured at 1 atm. This is because the hemoglobin (the red pigment in red blood cells) is saturated with oxygen (nearly 100 %) at that air pressure.^[2] Oxygen is required for every bodily process.

As humans go higher, the air pressure drops and so does the amount of available oxygen. At 5,000 metres (16,400 feet, height of Mount Everest base camp), the amount of oxygen is only half that at sea level. At 8,848 metres (29,028 feet, summit of Mount Everest), only one third is available.^[3] When the partial pressure of oxygen drops, the human body tries to compensate by a process known as altitude acclimatization. Additional red blood cells are manufactured, the heart beats faster, non-essential body functions are temporarily shut down, and one breathes more deeply and more frequently. However, acclimatization cannot take place immediately – in fact, it takes place over a period of days or even weeks. Failure to acclimatize may result in altitude sickness, including high-altitude pulmonary edema (HAPE) or cerebral edema (HACE).^[4]

At extreme altitudes (above 7,500 metres, 24,600 feet), sleeping becomes very difficult, digesting food is impossible (as the body shuts the digestive system down),^{[citation needed]} and the risk of HAPE or HACE increases greatly.^{[citation needed]}

Finally, in the "death zone" at 8,000 metres (26,250 feet) and higher, no human body can acclimatize. Staying longer than necessary will result in deterioration of body functions, loss of consciousness and ultimately, death.^[1]

1. ^ ^{a b} Everest:The Death Zone. Nova. PBS (1998-02-24).
2. ^ Hypoxia, Oxygen, and Pulse Oximetry. FlightState Pulse Oximeter. Retrieved on 2006-12-29.
3. ^ Introduction to the Atmosphere. PhysicalGeography.net. Retrieved on 2006-12-29.
4. ^ Outdoor Action Guide to High Altitude: Acclimatization and Illnesses. Outdoor Action First Aid & Safety Training. Retrieved on 2006-12-29.