The first execution by lethal gas in American history is carried out in Carson City, Nevada. The executed man was Tong Lee, a member of a Chinese gang who was convicted of murdering a rival gang member. Lethal gas was adopted by Nevada in 1921 as a more humane method of carrying out its death sentences, as opposed to the traditional techniques of execution by hanging, firing squad, or electrocution.
During a lethal gas execution, the prisoner is sealed in an airtight chamber and either potassium cyanide or sodium cyanide is dropped into a pan of hydrochloric acid. This produces hydrocyanic gas, which destroys a human body's ability to process blood hemoglobin. The prisoner falls unconscious within seconds and chokes to death, unless he or she holds his or her breath, in which case the prisoner often suffers violent convulsions for up to a minute before dying.
Lethal gas as a method of carrying out capital punishment was largely replaced by lethal injection in the late 20th century.