Oxygen and altitude

The effects of high altitude on humans are considerable. The percentage of  determines the content of  in blood. After the reaches around 2,100 m (7,000 feet) above sea level, the saturation of oxyhemoglobin begins to decrease rapidly. However, the human body has both short-term and to altitude that allow it to partially compensate for the lack of oxygen. There is a limit to the level of adaptation; mountaineers refer to the altitudes above 8000 m as the "death zone", where it is generally believed that no human body can.

The death zone, in, refers to s above a certain point where the amount of is insufficient to sustain  life for an extended time span. This point is generally tagged as 8000 m, less than 356 millibars of atmospheric pressure). All 14 summits in the death zone above 8000 m, called s, are located in the and  mountain ranges. The concept of the death zone (originally the lethal zone) was first conceived in 1953 by, a Swiss doctor, in an article about acclimatization published in the journal of the Swiss Foundation for Alpine Research.

Many deaths in high-altitude mountaineering have been caused by the effects of the death zone, either directly (loss of vital functions) or indirectly (wrong decisions made under stress, physical weakening leading to accidents). In the death zone, the human body cannot acclimatize. An extended stay in the zone without supplementary oxygen will result in deterioration of bodily functions, loss of consciousness, and, ultimately, death.

Scientists at the High Altitude Pathology Institute in Bolivia dispute the existence of a death zone, based on observation of extreme tolerance to in patients with  and normal fetuses in-utero, both of which present pO2 levels similar to those at the summit of Mount Everest.

Time of useful consciousness
Time of useful consciousness (TUC), also effective performance time (EPT), is defined as the amount of time an individual is able to perform duties efficiently in an environment of inadequate  supply. It is the period of time from the interruption of the oxygen supply or exposure to an oxygen-poor environment to the time when useful function is lost, and the individual is no longer capable of taking proper corrective and protective action. It is not the time to total. At the higher altitudes, the TUC becomes very short; considering this danger, the emphasis is on prevention rather than cure.

For orbital altitudes and above, that is, direct exposure to space, 6–9 seconds of consciousness is expected.

Medical analysis and variations
There are many individual variations of, even within the same person. Generally, tends to reduce the efficiency of the pulmonary system, and can cause the onset of hypoxia symptoms sooner. drastically reduces oxygen intake efficiency, and can have the effect of reducing tolerance by 1,000-2,000 m. Hypoxia can be produced in a. This can be useful for identifying individual symptoms of hypoxia, along with rough estimates of the altitude that causes problems for each person. Identifying symptoms is often helpful for self-diagnosis in order to realize when altitude should be reduced.

The table below shows average TUCs as documented by the ; a rapid ascent results in a lower TUC. The TUCs for any given individual may differ significantly from this. during the TUC period will reduce the TUCs considerably; so will exercise immediately prior to the TUC as this induces an prior to exposure.

Armstrong limit
The Armstrong limit or Armstrong's line is a measure of above which  is sufficiently low that water  at the  of the. Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to and  functions, and eventually death, unless pressure is restored within 60–90 seconds. On Earth, the limit is around 18-19 km above, above which atmospheric air pressure drops below 0.0618 (6.3 , 47 , or about 1 ).

A pressure suit is normally required at around 15,000 m for a well conditioned and experienced pilot to safely operate an aircraft in unpressurized cabins. In an unpressurized cockpit, at altitudes greater than 11,900 m above sea level, the physiological reaction, even when breathing pure oxygen is —inadequate oxygen level causing confusion and eventual loss of consciousness. contains 20.95% oxygen. At 11,900 m, breathing pure oxygen through an unsealed face mask, one is breathing the same of oxygen as one would experience with regular air at around 3600 m above sea level. At higher altitudes, oxygen must be delivered through a sealed mask with increased pressure, to maintain a physiologically adequate partial pressure of oxygen. If the user does not wear a pressure suit or a counter-pressure garment that restricts the movement of their chest, the high pressure air can cause damage to the lungs.