Here in The Netherlands for classrooms we have regulations that aim for a classroom to stay below 950ppm CO2. I think almost every high school kid can remember how drowsy a full, poorly ventilated classroom would get.

Like I said I’m usually a scientific man but this is such a basic thing that anyone with any daily life experience would immediately recognize this 4000ppm (or whatever) as wrong.

Humans are also quite sensitive to noticing when a room is “stuffy”, so, yeah.