It would be like carbon monoxide poisoning, but worse
In late August 1941, Fritzsch tried out the effect of Zyklon B on Soviet POWs, whom he locked up in cells in the basement for this experiment. This method quickly proved to have been effective and soon became irreplaceable in mass gassing of hundreds of thousands of victims.
The Rudolf Report: Zyklon B for the Killing of Human ..
According to testimony of his superior , it was also Fritzsch who first came up with the idea of using poison gas for the purpose of mass murder. Fritzsch ordered the killing of Soviet POWs locked in cells in the basement of the Bunker while Höss went away on an official trip in late August 1941. Fritzsch tried out the effect of Zyklon B inside cells which were not air-tight, subjecting the victims to even more torturous death. Fritzsch repeated the test-killing of additional victims using Zyklon B soon thereafter in the presence of Höss. According to Höss the preferred method for the mass murders in using Zyklon B was devised on site.
Each extermination camp operated differently, yet each had designs for quick and efficient industrialized killing. While Höss was away on an official journey in late August 1941 his deputy, , tested out an idea. At Auschwitz clothes infected with lice were treated with crystallised prussic acid. The crystals were made to order by the chemicals company for which the brand name was Zyklon-B. Once released from their container, Zyklon-B crystals in the air released a lethal cyanide gas. Fritzch tried out the effect of Zyklon B on Soviet POWs, who were locked up in cells in the basement of the bunker for this experiment. Höss on his return was briefed and impressed with the results and this became the camp strategy for extermination as it was also to be at Majdanek. Besides gassing, the camp guards continued killing prisoners via mass shooting, starvation, torture, etc.