Hypoxia is a condition in which an organism's access to O2 is less than normal (normoxia). For many atmospheric applications, such as human respiration, "normal" may be taken to be a pressure of 1 atmosphere (1013 mbar) at room temperature composed of 21% O2 by volume.

The availability of O2 to the body depends on the inspired partial pressure of O2. This is not simply the partial pressure of the ambient atmosphere because the air is humidified as it passes into the lungs and some of the gas displaced by water vapor. For a relatively dry atmosphere the water vapor already in the ambient air can be considered negligible to that supplied by the body in the respiratory tract, giving the equation:

PIO2 = inspired O2 partial pressure
FO2 = fraction of ambient atmosphere that is O2
Patm = pressure of ambient atmosphere
PH2O = water vapor pressure in the body; for a normal body temperature of 37°C this is 63 mbar (47 mm Hg)

For normal atmospheric conditions, this equation produces PIO2 = 200 mbar (150 mm Hg). Any air mixture and pressure which produces a PIO2 below this may be considered a hypoxic atmosphere.

Given time, the human body can often acclimate to a hypoxic atmosphere and maintain normal function. However, there is a limit to this ability. In mountaineering any altitude above 8000 m is considered to be in the "Death Zone" because acclimation is impossible at the low pressures encountered there. At this altitude Patm= 356 mbar, so we can calculate that PIO2 62 mbar is required for sustained human respiration.

For pure O2 atmospheres FO2 = 100%. Using the above equation and previous calculations, we find that a pure O2 atmosphere of 263 mbar results in normoxic PIO2 and one of 124 mbar produces PIO2 at the limit required to sustain human life.