Flying High Unpressurized (2016)

Importantly, it can take real time for the mind to clear with restoration of oxygen, especially if oxygen is only partially restored, as with a descent to 17,000 or even 13,000 feet. On a standard day, the barometer at sea level is 29.92 inches of mercury, or 14.7 psi; 21% of air is oxygen, so the partial pressure of oxygen (pO ) at sea level is 21% of 14.7, or 3.1 psi. Unfortunately, as environmental pO drops, blood oxygen must follow sooner or later; and by the time you get to a pO in the 60 range, the curve steepens, so blood oxygen is dropping fast.

Source: www.planeandpilotmag.com