Can someone provide an explanation why CO2 concentrations above 1000 ppm have such a negative influence given the fact that CO2 concentration in lungs (at rest) never falls below 10000 ppm?
I'm not a doctor, but I would consider it in terms of flow and throughput, rather than,—metaphorically—the amount of water the pipe can hold.
It simply makes the baseline higher. If you want to go to extreme cases, check carbogen
https://en.wikipedia.org/wiki/Carbogen