Fortunately our bodies are very well equipped to detect high CO2 concentrations (in our bloodstream - but that tracks outside concentration pretty reliably) - y

Fortunately our bodies are very well equipped to detect high CO2 concentrations (in our bloodstream - but that tracks outside concentration pretty reliably) - you will feel like you're suffocating and have an irresistible urge to move to fresh air. Unlike other gases (CO (without the 2), helium, argon, even nitrogen) that we don't "feel" at all, so they can displace oxygen and cause us to lose consciousness or even die, without much warning from our bodies.