Just want to point out that these are two different things:

1. detrimental effects of too much CO2

2. detrimental effects of too little O2

The short and long-term effects of both may be very different. The human body can adapt to the low O2 at high altitudes given enough time[0], but I don't know if the same applies to high levels of CO2.

[0] https://en.wikipedia.org/wiki/Altitude_sickness#Prevention