Comment by UniverseHacker

Even if you are right and there is no objectively measurable impaired abilities, the things I can detect directly are themselves extremely undesirable.

I feel irritable, and fatigued/sleepy when CO2 is high. Increased breathing rate by itself activates an undesirable sympathetic nervous system response, that anyone can notice immediately with deliberate breathwork.

Also, it seems likely to me that the same poor air exchange that leads to high co2 causes respiratory disease to spread more rapidly, and with a higher initial viral titer.