Since oxygen plays a rather vital role in our functioning, I'm curious why we can't detect low oxygen levels. Or perhaps we do detect them but only when it's dangerously low. How has it been confirmed that we detect CO2 levels instead of low oxygen?
You are totally unaware of your brain shutting down as the oxygen level decreases. People lose the ability to do arithmetic or simple puzzles including blocks in holes, but by the time that happens youre too far gone to recognize what that means.
(The above is just going off my memory of my physiology class from years ago. Apologies if I misremembered something.)