The pesticide-based poison gas used in Nazi gas chambers at Auschwitz-Birkenau was Zyklon B.
Zyklon B was a commercial fumigant made from hydrogen cyanide absorbed into porous pellets, with an irritant added to warn users of its presence. It had been developed and marketed for pest control, not originally as a weapon for mass murder.
At Auschwitz-Birkenau, SS personnel used it in gas chambers to murder Jews and other prisoners, especially from 1942 onward. The pellets released hydrogen cyanide when exposed to air, causing fatal poisoning by preventing cells from using oxygen. The method was part of the camp’s industrialized system of mass murder.
A common mix-up is confusing Zyklon B with carbon monoxide. Carbon monoxide was used in some other Nazi killing operations, including gas vans and the Operation Reinhard camps, while Auschwitz is particularly associated with Zyklon B.