Posted for "Eric M. Jones" <emjones@charter.net>:

Lack of oxygen is not sensed by the body. What happens when we feel "short
of breath" is that the body responds to excess of CO2 to tell the brain that
there is an oxygen shortage. This worked well on the African savannahs, but
where the oxygen is thin, the CO2 never gets excessive.

In the 1930's when all this was being worked out, it was claimed that
providing the aviator with 3% CO2 (!) would allow him to fly at 17k with the
mental performance of 12k. I leave it to flight surgeons to puzzle this
out---my guess is that it is true.

There were also animal experiments that detailed how long one could survive
at 75k and recover if you were then given oxygen (17 seconds or so as I
recall).

In skydiving one soon discovers that if you want to build a formation, no
advantage is gained above 13k without oxygen; that is, the longer freefall
time is offset by the poor coordination of the participants. I have jumped
from almost 16k (and above 13k for an hour) without oxygen and it is
extremely unpleasant.

Eric