Dalton's Law = The pressure of a mixture of gases is the sum of the partial
pressures of the constituent gases.
In terms of the
composition of air, (78% N, 21% O2, 1% other, ie
"a mixture of gases"), this means 21% of atmospheric pressure is due
to O2.
Although these percentages
are constant with increasing height, the amount of 'inhalable' O2 reduces due
to decreasing pressure - this is essentially what 'Partial Pressure' (PP)
means, eg:
PP of O2 at MSL = 21% x
1013 = 213mb
PP of O2 at 30,000ft =
21% x 300 = 63mb
Human body very
susceptible to reductions in PP of O2 - anything below 55mm/Hg will induce
Hypoxia, ie reduced air pressure resulting in less O2
being pushed through the veins. Above 38,000ft, O2 needs to be administered
under pressure.