Holocaust Gas Chambers
The Nazis started their use of poison gas in 1939, as a tool for killing mentally and physically disabled patients in hospitals. They called this euthanasia, because they felt that these people were deemed to be unworthy of living, but it was essentially part of their systemic murder. There were six different killing centers throughout Nazi-occupied Europe that were established for this specific purpose. In Bernburg, Brandenburg, Grafeneck, Hartheim, Hadamar and Sonnenstein, people were killed using chemically-created carbon monoxide or lethal injections. The gas seemed to be quicker and easier, though, so it was often the preferred option.
In 1941, Germany invaded the Soviet Union and developed the mobile killing unit known as the Einsatzgruppe, which was focused on mass shootings of civilians. At this time, they started experimenting with the possibility of using killing vans for mass gassing, which allowed them to kill large numbers of women and children without the mental anguish and with a lower expense. Later that year, they decided that shipping Jews to concentration camps or extermination camps would be more effective, both time and cost wise, and started building gas chambers in various camps where they would go on to kill millions of Jews and other 'inferior' citizens.
Chelmno, a camp in Poland, was the first to kill people using gas, but it used the mobile van units initially. In 1942, finally, they used stationary gas chambers at Treblinka, Sobibor and Belzec, camps that were all located in Poland. Victims would be unloaded from cattle cars and instructed to undress as they had to shower to be 'disinfected'. Some were beaten or yelled at along the way if they didn't comply. People were told to enter the room (what they thought was a shower) with their arms raised so that more people could fit in. This increased efficiency, but it also killed them faster because more people meant they suffocated faster because there was less oxygen to spare.
At Auschwitz, the largest extermination camp in the Nazi regime during the Holocaust, they wanted a more efficient, effective way to use gas. They experimented with a known fumigation chemical, Zyklon B, which in pellet form became a lethal gas when it hit the open air. It was quick and effective, and became the chosen method for use in gas chambers during the Holocaust in Auschwitz, where at the height of the transits, as many as 6,000 Jews were gassed every single day.