There are two types of asphyxiants, simple & chemical.

Simple asphyxiants displace oxygen from your hemoglobin (examples are methane, hydrogen, and nitrogen)

Chemical asphyxiants prevent the joining of oxygen & hemoglobin at the cellular level.Such as Carbon monoxide or hydrogen sulfide. (carbon monoxide will bind with your hemoglobin at over 200x the rate of oxygen forming carboxyhemoglobin)

That's why a chemical asphyxiant will usually require treatment in a hyperbaric chamber to force the oxygen back into the cells versus simple asphyxiants which can be treated with oxygen via a non-rebreather mask.