-=> Quoting Jay Hanig to Don Burke <=-
JH> The math necessary is fairly simple now.  We breath air at sealevel
with a partial pressure of oxygen of 3 psi.  Take the atmospheric
pressure of any altitude and multiply it by .2 to get the partial
pressure of O2 at altitude. That is based on air, which is 20% oxygen.
JH> In the case of the 50/50 mix of nitrox, multiply atmospheric pressure
(for whatever altitude interests you) by .5 to get the partial
pressure.   Any figure that you derive that has a value of less than 2
isn't doing so well. 2 psi is the equivalent of breathing air at 10,000
feet. 
When I was riding submarines, I went a couple of days at about 2.7 psi.
It was difficult to do anything complex under those conditions.
For a shorter period, it would be okay I suppose.
The limit on the other end is oxygen toxicity which occurs somewhere 
between 15 psi and 30 psi, depending on the individual.  
People have actually died from too much oxygen.
If there is no compelling medical reason to go above about 3.5 psi 
for an individual, the extra oxygen just boosts the fire hazard.
... Turning final runway "X"......
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