There is normally still plenty of O2 left in the air, so you are conscious and panic when too much CO2 is around (actually, in your bloodstream rather than arou

There is normally still plenty of O2 left in the air, so you are conscious and panic when too much CO2 is around (actually, in your bloodstream rather than around). If a huge amount of CO2 displaces everything else quickly, there is suddenly no O2 at all and you just faint, then die.