Partial Pressure


Oxygen, Nitrogen and the other trace gases all contribute to the total atmospheric pressure. The term Partial Pressure refers to the pressure each gas contributes to the total atmospheric pressure at any given altitude. At sea level, the partial pressure of Oxygen, or “pO2”, is about 3 psi (calculated by 14.7 psi x 20% = 2.94).


At 18,000 ft MSL, pO2 is about half that at sea level, 1.5 psi; and, at 34,000 ft MSL it’s a quarter of that at sea level, .75 psi.


The percentage of oxygen in the is constant throughout Earth’s atmosphere. However as altitude increases, pressure decreases, and the partial, or usable pressure, of O2 decreases proportionately.


This is why humans require either supplemental (additional) oxygen and/or supplemental (additional) pressure to force the O2 through the cell walls at altitude.


Dalton’s Law (Partial Pressure)


159.5 mm Hg 0.3 mm Hg


600.2 mm Hg 760mm Hg


Dalton’s Law: The pressure exerted by a mixture of gases is equal to the sum of the partial pressures of each gas in the mixture.


The pressure exerted by a mixture of gases


is equal to the sum of the partial pressures of each gas in the mixture. Pt = P1 + P2 + ...+ Pn


760mm Hg = 159.5 + .3 + 600.2


7 High Altitude Awareness Training


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