O2
Gentlemen:
It's been a long time since my basic, high-altitude physiological training, but I seem to recall the figure FL250...
Yes, it's true, at high altitudes, due to the partial pressure of gasses, you have to be "force fed" oxygen in order to get the lungs to absorb what they need. This is what the "Emergency" selector can be used for...among other things (e.g. clearing a mask in the case of smoke, etc.). If I remember correctly, at FL250, this phenomenon begins to manifest...but, really becomes critical around the low to mid thirties.
Fortunately, the new-style masks do this for you. Please refer to Airbus FCOM 1.35.20 P7. With regard to the "Emergency" pressure selector:
"Note: Overpressure supply is automatically started, when cabin pressure exceeds 30,000 feet."
For those aircraft equipped with the older-style masks, it is necessary to manually force-feed yourself until you can get below at least the low thirties...preferably maybe FL250 (in my humble opinion).
In any case, without pressure feed at high altitudes, you can suck like crazy...just before you pass out....
Pantload