The cognitive ability of humans is varies with the concentration of CO₂ in breathing air. But how does it vary?
A very high concentration causes sleepiness, and sleepiness is a state of reduced cognitive ability.
That means it does vary, but I think there is no sudden onset of the reduction.
I would be interested in, ideally, a function of some experimental correlate of cognitive ability by level of CO₂, but any theoretical or experimental information about the shape of the function is interesting.
I'm aware that the signal to noise ratio in an experimental measurement is high, but qualitative results are the most important.