Two German firms, Tesch/Stabenow and Degesch, produced Cyclone B gas after they acquired the patent from Farben. Tesch supplied two tons a month, and Degesch three quarters of a ton. The firms that produced the gas already had extensive experience in fumigation.
After the war the directors of the firms insisted that they had sold their products for fumigation purposes and did not know they were being used on humans. But the prosecutors found letters from Tesch not only offering to supply the gas crystals but also advising how to use the ventilating and heating equipment. Hoess testified that the Tesch directors could not help but know of the use for their product because they sold him enough to annihilate two million people. Two Tesch partners were sentenced to death in 1946 and hanged. The director of Degesch recieved five years in prison." (Feig, Konnilyn. Hitler's Death Camps : The Sanity of Madness. NY: Holmes & Meier, Publishers, 1981.) (See also Breitman, Richard. The Architect of Genocide : Himmler and the Final Solution. MA: Brandeis University Press, 1992, pp. 203-204, for a discussion of the early involvement of Heerdt-Lingler)
From the statement of Hans Stark, registrar of new arrivals, Auschwitz (Klee, Ernst, Willi. Dressen, Volker Riess. The Good Old Days: The Holocaust As Seen by Its Perpetrators and Bystanders. NY: The Free Press, 1992, p. 255):
Zyklon-B is a powerful insecticide which serves as a carrier for the gas Hydrocyanic acid, or HCN. It usually comes in the shape of small pellets or disks. (See Breitman, 203, for more detail about the early use of the gas at Auschwitz) HCN is the cause of death following the application of Zyklon-B. While interacting with iron and concrete, it creates Hydrocyanic compounds, which Leuchter admitted were found in the ruins of the gas chamber in Krematoria II. His finding was confirmed by findings of the Polish government.
HCN is extremely poisonous to humans. It is used in execution gas chambers in the US; the first was built in Arizona in 1920.
As noted above, these "difficulties" were easily solved in 1920. Moreover, the Germans had a lot of experience with HCN, as it was extensively used for delousing.
There were two types of gas chambers in Auschwitz: those used for delousing clothes ("delousing gas chambers") and those used for killing people on a massive scale ("extermination gas chambers"). The delousing gas chambers were a standard feature, and were left intact by the SS (the extermination gas chambers were dynamited in an effort to conceal criminal traces).
HCN is much more effective on warm-blooded animals, including humans, than it is on insects. The exposure period (to HCN) is much greater in delousing operations than in homicidal gassings. This means that a much lower concentration is necessary to kill people than to get rid of lice, etc. In delousing, concentrations of up to 16,000 ppm (parts per million) are sometimes used, and exposure time can be up to 72 hours; while 300 ppm will kill people in fifteen minutes or so.
Therefore, the HCN in the extermination chambers hardly had time to form compounds on the walls. While some claim that the gas would need a lot of time to kill, because it would have to spread all over the chamber, it simply is not true; the gas chambers were not that large (those in Krematoria II and III were about 210 square meters), and the Zyklon-B was dropped from four openings (still visible in the ruins of the gas chambers). Since the concentration used was higher than the lethal one, death was very swift.
Furthermore, the delousing chambers are intact while the extermination chambers were blown up (a .GIF picture of the one of Krema II is available). Therefore, their walls have been exposed to the elements for the last 50 years. The ruins of the gas chamber of Krema II are covered with about 3 feet of water during certain periods of the year; HCN compounds easily dissolve in these surroundings. Nonetheless, so much gassing took place that some of the compound remained.
Summarizing, the walls of the extermination gas chambers were in contact with HCN for a much shorter time then those of the delousing chambers, and for the last 45 years were exposed to surroundings which dissolve the compounds, while the delousing rooms were not. Therefore it is obvious that less traces of compounds would remain in them.
This fact - that all, or most, of the compounds would vanish during 45 years of exposure - is clearly stated in the report written by the experts of the Cracow Institute of Forensic Research. (Also see The Leuchter FAQ).
But - as they admit themselves - the gas chamber of Krema I was used only for a short time, and than it was changed to an air-raid shelter. After the liberation of the camp, it was reconstructed to its original shape. This, and the fact that "only" about 10,000 people were murdered inside it (compared to 350,000 and 400,000 in Kremas II and III) explain why relatively small amounts of cyanide compounds remain. As for Kremas IV and V, they were completely destroyed by the SS before the Soviets liberated the camp.
Finally, cyanide compounds were found on the ventilation grills of the extermination chambers, proving beyond doubt that gassing did take place inside them.
The concentration of HCN necessary to cause death is nearly 200 times lower than that which causes explosion. Although the SS used a concentration higher than the lethal one, it was far below that causing explosion.
As a reference, one can look at "The Merck Index" and the "CRC handbook of Chemistry and Physics", or consult any manual dealing with toxicity and flammability of chemicals. For HCN, a concentration of 300 ppm (parts per million) kills humans within a few minutes, while the minimal concentration that can result in an explosion is 56,000 ppm.
If one disinfects a building in ordinary commercial use, it should not be reentered within 20 hours. That figure, however, has no meaning in relation to the extermination chambers, because they were forcibly ventilated. Fifteen minutes were enough to replace the air. When ventilation was not used, the Sonderkommando (prisoners used as forced labor) who took the bodies out had gas masks on.
The Germans had plenty of experience with gas, especially HCN, which was widely used for delousing. They knew how to work with it without getting hurt. It is absurd to use the 20 hour figure in this context, which does not assume forced ventilation and takes a huge safety factor into account. The SS didn't care much for the safety of the Sonderkommando who had to enter the gas chambers to take the corpses out.
Furthermore, what makes ventilation difficult and lengthy is the presence of rugs, furniture, curtains, etc. Needless to say, these were not present in the gas chambers - there was just bare concrete, making ventilation fast and efficient.
If the "20 hours ventilation period" above was true, this would mean that the corpses of people executed using cyanide gas in U.S. gas chambers would remain tied to the chair 20 hours after they were killed...
This claim stems from the fact that Hydrocyanic compounds were found on the ventilation grills of the gas chambers in Krematoria II and III (the chemical analysis was carried out by Dr. Jan Robel of the Cracow Forensic Institute in December 1945, and was part of the evidence in the trial of Auschwitz commander Höss). This proves that gassing did take place in that chamber. Zyklon-B cannot kill anaerobic bacteria - it kills only aerobic organisms. That means it would be useless for disinfecting corpses.
In closing, consider the testimony of SS private Hoeblinger: (Langbein)
Note Pvt. Hoeblinger's mention of gas masks - some Holocaust deniers insist that the SS-men dropping the gas would be killed by it, which leads one to speculate about their reading ability. Finally, the undeniable evidence that the SS ordered Degesch to remove the indicator odor, mandated under German law, which was added to the Zyklon B in order to provide a warning to human beings that the lethal stuff was nearby. I believe this demonstrated clear criminal intent - the SS would hardly have removed the indicator odor if they had intended, as the denial set insists, to use the gas only on insects and corpses... (Borkin, Joseph. The Crime and Punishment of I. G. Farben NY: The Free Press, 1979, pg. 123).
Source: The Nizkor Project