Beaver
The time of usefull consiousness reduces with altitude.
18,000ft 10-15min
25,000ft 2-3 min.
35,000ft 45sec
45,000ft 12sec
Hypoxia: Reduced pressure in the alveloi rapidly reduces the transfer of oxygen into the bloodstream, first cells affected are those with the greatest oxygen requirement. (brain & eye)
Hence our companys procedure that if operating at or above FL 410 then 1 pilot must have a demand O2 mask on.