well you dont need oxygen till you are above 15k, and that is still a very safe alt with out oxygen.
The highest I have been with out oxygen was 17k above sea level, but was only there for a few mins, then jumped out of plane. but at 17k with out oxygen, you are ok for about ten mins befor you start having problems, unless you smoke, then about 5 to 7 mins. and as I smoke, my finger tips and lips just started to turn blue when we jumped.
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wolf37
C.O.
THUNDER BIRDS