A gas chamber is an airtight room used for killing people or animals by means of gas. The gas used is either poisonous, or it causes asphyxiation (chokes the victim). 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. The victims die from choking (asphyxiation) from being made to breathe the poisonous gas.
The gas chamber was first used in Nevada, United States in 1924, as the government of Nevada did not want to use an electric chair, and considered hanging to be cruel and inhumane. Gee Jon was the first person to be executed in a gas chamber. The gas chambers became notoriously infamous in the Nazi Germany as they were first used in the "T-4" programme (killing disabled people) and then in the Holocaust for mass murdering of Jews, Gypsies and other people that the Nazis did not like.
By the 1990s, the gas chamber had become less popular in the United States because people began to feel that it was too painful of a way to die. When Donald Eugene Harding was executed in Arizona in 1992, witnesses described it "violent death" and an "ugly event". California stopped the use of gas chamber in 1995 as the officials found that dying from hydrogen cyanide poisoning, which can take up to around fifteen minutes, is an extremely painful way to die. The last person to be executed in the gas chamber in the United States was Walter LaGrand in Arizona in 1999. It is unlikely that anybody else will be executed in the gas chamber because it is likely that it would be ruled "cruel and unusual punishment" and thus against the Constitution.