Our atmosphere is 21% O2, and less than 0.05% CO2.
If that changed by 1% to 20% O2 and 1.05% CO2, we would all die. But not for asphyxiation or lack of O2, because the slight reduction in O2 would be unnoticeable. The drastic increase in CO2, on the other hand, would be catastrophic.
I think you missed the point.
Our atmosphere is 21% O2, and less than 0.05% CO2.
If that changed by 1% to 20% O2 and 1.05% CO2, we would all die. But not for asphyxiation or lack of O2, because the slight reduction in O2 would be unnoticeable. The drastic increase in CO2, on the other hand, would be catastrophic.