If you actually ask the questions, you'll arrive at a different conclusion than Irving, who denies even the well-documented fact that Jews were gassed at Auschwitz.
"Documents including blueprints of the killing installations as well as orders for construction materials and Zyklon B (the deadly hydrogen cyanide preparation used for gassings at Auschwitz and Majdanek...carbon monoxide exhaust was used at other camps) survived the war as did some of the actual gassing facilities themselves. Photos clandestinely taken by prisoners of Auschwitz-Birkenau even show the disposal of corpses removed from the gas chamber. The manufacture, distribution and use of the deadly gas was clearly demonstrated at the "Zyklon B Trial" in March 1946, Hamburg, Germany. Two of the defendants, Bruno Tesch and Karl Weinbacher, the owner and a major executive of a company that manufactured the gas were sentenced to death after notes of their trips to Auschwitz disproved their contention that they were unaware that the poison was used to kill inmates.
Jean-Claude Pressac, a one-time skeptic of the gas chambers, had undertaken a careful study of Auschwitz in which he analyzed a wide variety of camp documents, photos, reports and blueprints. Pressac, who had at one time been intrigued by the Holocaust-denying theories of Robert Faurisson, concluded that his original skepticism could no longer be supported in the face of the evidence. In 1989, the Klarsfeld Foundation published his study, Auschwitz: Technique and Operation of the Gas Chambers, in which Pressac demonstrates the use of the gas chambers of Auschwitz-Birkenau in the murders of hundreds of thousands of people."
[motlc dot wiesenthal dot com/site/pp.asp?c=gvKVLcMVIuG&b=394667]
Jean-Claude Pressac was a French chemist who decided to research the Holocaust in an attempt to disprove it, but when he saw the evidence first-hand, he realized he was a big fat idiot.
"Pressac visited Auschwitz Birkenau ten times between 1979 and 1984, according to registered letter from the Auschwitz Museum director Kazimierz Smoleń, sent on 11 September 1985. Pressac used only authentic documents concerning the construction of crematoria and the gas chambers, which originated from the Nazi German office of Zentral Bauleitung der Waffen SS. Museum staff provided him with assistance, convinced of his honourable intentions.[5] Pressac was given access to blueprints which had survived due to being located in the construction office rather than the administrative offices. The analysis of material proof convinced him that his former views shaped by the Faurisson case were in error."
en.wikipedia.org/wiki/Jean-Claude_Pressac