Re: Great murder choice.
I once tried walking into a small room where there had been a rapid burn of petrol vapors. High concentration of CO2. Took one step in & virtually bounced off it as the breathing reflex kicked in with a vengence. Felt like dry heaving, but with my lungs instead of stomach.
By contrast, we had an environmental test chamber at work which was chilled by liquid nitrogen. We had to have oxygen meters installed to trigger an alarm if the O2 partial pressure in the room dropped too far, because of the exact reason you stated.