I am well aware that air isn’t just Oxygen, Nitrogen and Carbon Dioxide. I didn’t mention these other components because they are not relevant to the point I was making. CO2 concentrations can easily exceed 1,000 ppm indoors and in heavily populated cities. CO2 really only drops to around 400 ppm far away from populated areas. Your body relies on CO2 concentration in your blood (technically bicarbonate) to stimulate breathing. It does not use Oxygen concentration to do so. This is why some industrial environments can be incredibly dangerous if Nitrogen, Argon etc. displace Oxygen. Youll breathe in and out just fine until the lack of Oxygen knocks you unconscious.