I think the other posters have it right. The specific concept behind
it, if I remember my science fiction books correctly, is 'partial
pressure'. With the open cycle (the one that hooks to your nose),
there's an open path direct to the low pressure area through your
mouth. The oxygen enters your lungs through osmosis, and if the
pressure on the inside of the hemoglobin in your pulmonary capillary is
greater then the pressure of the O2 against it, it just won't enter.

Another part of the problem may be that at high enough altitudes, CO2
may no longer be effective at triggering the breathing impulse.

I think the partial pressure issue is probably more relevant.

If someone here is a doctor or actually KNOWS the answer, feel free to
tell me where I pooched it up. My education comes from the likes of
Del Rey Publishing and Baen Books, not John Hopkins.

Ben Hallert
PP-ASEL