The gas chambers were not used for killing.

"In 1942, the crematorium area was constructed next to the main camp. It included the old crematorium and the new crematorium (Barrack X) with a gas chamber. There is no credible evidence that the gas chamber in Barrack X was used to murder human beings. Instead, prisoners underwent "selection"; those who were judged too sick or weak to continue working were sent to the Hartheim "euthanasia" killing center near Linz, Austria. More than 2,500 Dachau prisoners were murdered in the gas chambers at Hartheim. In addition, mass executions by shooting took place, first in the bunker courtyard and later in a specially designed SS shooting range. Thousands of Dachau prisoners were murdered there, including at least 4,000 Soviet prisoners of war following the invasion of the Soviet Union in 1941."

(http://www.autoadmit.com/thread.php?thread_id=5753736&forum_id=2).#49124786)