asphyxiant

Gas or vapor that can cause unconsciousness or death by suffocation due to lack of oxygen. Asphyxiation is one of the major potential hazards of working in confined or enclosed spaces where chemicals are used or stored. The chemical vapors can become so concentrated that they reduce the normal oxygen level (about 25 percent) in the air to dangerous levels (19 percent or lower). Carbon monoxide works as an asphyxiant by reducing the blood's ability to carry oxygen.