It’s the co2 in your blood that you make in your body( not that you breathe in) that makes you want to breathe. You can breathe pure oxygen and you will still feel the need to breathe, because your body uses the oxygen to make co2 which then needs to be expelled.

What if that's not the case though. Spaces that are high in CO2 are dangerous because the CO2 has displaced the oxygen below a safe percentage. Maybe by scrubbing the CO2 we can increase the oxygen concentration.