Bob Engelhardt writes:

It's the rise in CO2 concentration
in our lungs that makes it so hard to hold your breath for very long.
There's still plenty of oxygen in your lungs but the rising CO2 says:
BREATH! BREATH!!

CO2 is not just an inert oxygen-displacing gas like other shielding gases
or fuels. Respiration takes O2 in and sends CO2 out. Too little pO2 or too
much pCO2 in the ambient will kill you, either one. Yes, CO2 is a sensible
respiratory stimulant in low concentrations, and not toxic in the sense
that it is something your body produces and a strong whiff will not hurt
you. But even with enough pO2 it doesn't take much pCO2 in the ambient air
to prevent you from clearing CO2 from the blood and getting very
distressed, even if the rest is pure O2.