The people or animals are put into the gas chamber , the door is sealed airtight from the outside , and a gas , such as carbon monoxide or hydrogen cyanide , is put into the chamber . A gas chamber is an apparatus for killing humans or animals with gas , consisting of a sealed chamber into which a poisonous or asphyxiant gas is introduced . The most commonly used poisonous agent is hydrogen cyanide ; carbon dioxide and carbon monoxide have also been used .