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 reduction in O2 would be unnoticeable. The 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 reduction in O2 would be unnoticeable. The increase in CO2, on the other hand, would be catastrophic.