You seem to be assuming that the only physiologically-relevant factor is the ratio of O2 to CO2 in the air. I am pretty sure that is wrong: for example, dissolv

You seem to be assuming that the only physiologically-relevant factor is the ratio of O2 to CO2 in the air. I am pretty sure that is wrong: for example, dissolved CO2 makes the blood significantly more acidic whereas dissolved O2 does not make it more basic. Also, the US Navy's submarine service, the Space Shuttle and the Apollo missions solved the problem by removing CO2, not by adding other gasses.