Partial Pressures & % Oxygen Available at Sea Level

At sea level there is a pressure equivalent to 10 meters of water pressing down on all of
us all the time. This is because of the weight of the air above us in the atmosphere. When
you travel up a mountain, there is less air above you in the atmosphere, so the pressure on
your body is less. 

The important effect of this decrease in pressure is this: in a given volume of air, there are
fewer molecules present. This is really just another way of saying that the pressure is lower.
(This is called Boyle's law.) The percentage of those molecules that are oxygen is exactly
the same: 21%. The problem is that there are fewer molecules of everything present,
including oxygen.

 

Elevation M/Ft
Atmospheric Pressure
% Oxygen that is
available at sea level
0  
760mm
1013mb
100%
1000m/3,280ft  
679mm
89%
3000m/9,842ft  
527mm
70%
5000m/16,403ft  
420mm
54%
5364m/17,600ft Everest Base Camp
401mm
52%
6065m/19,900ft Camp I
367mm
47%
6492m/21,300ft Camp II (ABC)
347mm
45%
7470m/24,500ft Camp III
305mm
39%
7925m/26,000ft Camp IV
287mm
37%
8848m/29,029ft Everest Summit
253mm
32%