It is still possible to pass out from Hypoxia without building up CO2
levels back to the amount needed to trigger the chemoreceptors to tell
your brain "I GOTTA BREATHE". It has been the cause of more than one
drowning/diving accident.

Some years back I saw a GREAT film on this. They take several guys,
just sitting on a couch, and hook them up to full face masks, bypassing
a CO2 scrubber, and an auxilliary air cannister. So they can just sit
there and breathe normally.

Then they shut off the fresh air, so they are breathing the same air
over and over. After a couple of minutes they start breathing hard,
and finally start grabbing for their masks and gasping for air. The "I
can't stand it any longer and I have to breathe" response.

Then, after they have recovered and are back on the original system,
they shut off the fresh air and ADD the CO2 scrubber. The guys just
sit there on the couch for a few minutes, and slowly, calmly, one by
one, pass out!

jmk