@phoikoi @mike_bowler Hm, I'm no expert by any means but I'll take a shot at arguing about this anyways lol

So every CO₂ takes up one O₂ and, according to Wikipedia, dry air contains 20.95% O₂ and 0.04% CO₂. So at 1000ppm carbon dioxide concentration we've converted 0.06% O₂, resulting in an atmosphere of 20.89% O₂ and 0.1% CO₂.

I believe that for breathing processes partial pressures are relevant and, if I remember physics correctly, the amount of dissolved gas is directly proportional to the partial pressure of that gas in the air. Meaning we've increased the amount of dissolved CO₂ in our blood by 150% while we decreased the amount of O₂ by 0.29% (*)

So I guess it really is about the carbon dioxide as long as I'm not missing anything?

(*) 1 - 20.89 / 20.95 = 0.29%