This one is simple:
(enphasis added).30. How long does it take to ventilate fully an area fumigated by Zyklon-B?
The IHR says:
Normally about 20 hours. The whole procedure is extremely involved and technical. Gas masks have to be used and only well-trained technicians are employed.
No. The "20 hours" figure is irrelevant for a variety of reasons.
First of all, the figure is intended to apply to ordinary, unventilated, commercial- or home-use buildings. One should not reenter an ordinary building within that period of time, because there is little if any forced ventilation. Furthermore, ordinary items like carpets, drapes, furniture, and so on lengthen the time required to restore fresh air. The Nazi gas chambers, on the other hand, were empty concrete rooms, forcibly ventilated, so even five minutes was enough to recycle the air.
So the nazis exterminated the jews by gassing empty rooms? The usual story says that the rooms were packed with people. Ther would have been lots of small spaces inside the pile of corpses filled with HCN-gas, making the ventilation procedure very difficult. And how about corpses falling over the Zyklon-B pellets? There would still be lots of HCN in them even after hours of ventilation.
If the "20 hours ventilation period" above was true, this would mean that the corpses of people executed using cyanide gas in US gas chambers would remain tied to the chair 20 hours after they were killed.
No, the U.S. gas chambers have an extremely good ventilation system, and the chambers don't have any air pockets that makes ventilation troublesome. If the gas chambers are stacked with people, many hours of ventilation would be needed even in the modern U.S. chambers