Oxygen and altitude

The **effects of high altitude on humans** are considerable. The percentage oxygen saturation of hemoglobin determines the content of oxygen in blood. After the human body reaches around 2,100 m (7,000 feet) above sea level, the saturation of oxyhemoglobin begins to decrease rapidly. The human body has both short-term and long-term adaptations to altitude that allow it to partially compensate for the lack of oxygen. However, there is a limit to the level of adaptation.

## Death zone

Mountaineers refer to the altitudes above 8,000 metres (26,000 ft) as the "death zone", where it is generally believed that no human body can acclimatize.

The death zone, in mountaineering, refers to altitudes above a certain point where the amount of oxygen is insufficient to sustain human life for an extended time span. This point is generally tagged as 8,000 m (26,000 ft), less than 356 millibars of atmospheric pressure). All 14 summits in the death zone above 8000 m, called eight-thousanders, are located in the Himalaya and Karakoram mountain ranges.

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).

## 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 flying duties efficiently in an environment of inadequate oxygen 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 unconsciousness. 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.

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

## Armstrong limit

A pressure suit is normally required at around 15,000 m (49,000 ft)\* in unpressurized cabins.

At or above this limit (the **Armstrong limit**), exposed body fluids such as saliva, tears, urine, blood and the liquids wetting the alveoli within the lungs—but not vascular blood (blood within the circulatory system)—will boil away without a full-body pressure suit, and no amount of breathable oxygen delivered by any means will sustain life for more than a few minutes.

Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within 60–90 seconds.

On Earth, the Armstrong limit is around 18-19 km (11–12 mi; 59,000–62,000 ft) above sea level, above which atmospheric air pressure drops below 0.0618 atm (6.3 kPa, 47 mmHg, or about 1 psi).