That makes sense, thanks, since our threshold for co2 is less than 0.5%.
I may have worded my question poorly; I’m more asking why low oxygen is a problem vs low nitrogen. In retrospect, my climate focus may be distracting. It was what made me wonder about this in the first place, but the medical and scuba points are much more relevant. That has little to do with co2 (I think?) and more to do with the relative compounds in our air.
I’m still confused why we hear about oxygen but never nitrogen. Another example: when we look for habitable planets, the focus is ‘oxygen rich atmosphere’, but not ‘nitrogen rich’.