In The Hoax of the Twentieth Century I remarked that typhus was a great killer for the Germans in World War I, giving a reference published shortly after that war . The typhus of World War I, like that of World War II, was carried by lice. In response to this specific problem, the German company DEGESCH (Deutsche Gesellschaft für Schädlingsbekämpfung - German Pest Control Co.) developed the Zyklon B pesticide  and made it available for commercial use in 1923.
Zyklon B, referred to here merely as Zyklon, is a very effective pesticide consisting of liquid hydrogen cyanide (HCN, an acid) absorbed into some inert material such as wood pulp, with an irritant added to warn bystanders of its presence. HCN is also called "prussic acid" and in German "Blausaüre" (blue acid), because it is commonly used in blue dyes and tends to leave blue stains where it is used.
In its gaseous or liquid form HCN is very deadly and is used in American execution gas chambers, where it is traditionally generated by mixing an acid (normally sulfuric acid) with potassium cyanide or sodium cyanide, resulting in rapid release of HCN in its gaseous form.
HCN is useless as a battlefield gas because it is a true gas, slightly lighter than air, and disperses too rapidly for that application. World War I battlefield gases were actually dusts that hovered about the target area.
The pesticide Zyklon works somewhat differently. It is supplied in a very tightly sealed container. When a space (e.g. a barracks or building) is to be treated with it, that space is tightly sealed and trained personnel empty cans of Zyklon on the floor, preferably spreading the Zyklon out as much as possible. They then leave the space, close it, and wait the time required for the liquid HCN to pass into the gaseous form by evaporation, fill the space, and kill the target pests. Then the space is opened and ventilated.
This is a very time consuming process because the evaporation is slow and for safety reasons the time required for ventilation is lengthy. The boiling point of HCN is 25.6 C (78 F). That does not mean the space must have that ambient temperature in order for the gas to be released; water does not have to be brought to its boiling point in order to evaporate. However the process is slow at any lower temperature and especially slow in winter temperatures.
An information booklet for Zyklon  gives typical times of 2-72 hours for the gassing process and at least 10 hours for ventilation; the former depends very much on the temperature and the target pests and the latter on the physical properties of the space and its contents. For example, clothing and bedding should be beaten even after ventilation.
Another step recommended at the end of a Zyklon gassing and ventilation process is the test for residual HCN gas. The typical test uses a mixture of copper acetate and benzidene acetate and test paper which turns dark blue if the HCN level is dangerously high. A temperature of at least 15 C (60 F) is required to perform this test, so there are many circumstances under which it cannot be performed .
There have also been gas chambers designed specifically to use Zyklon to disinfest articles such as clothing with HCN, especially in Germany . With few exceptions the interiors of these gas chambers are heated, in order to accelerate the gassing process, and for other reasons. Gaseous HCN is water soluble, so high temperature is desired to reduce moisture in the gas chamber. Also, lice and some other pests are easier to kill at higher temperatures, because their metabolism rates are higher. Desired temperatures are in the range 25 - 35 C (77 - 95 F).
In such gas chambers the air - gaseous HCN mixture is expelled from the top when the gassing process is completed, for safety reasons and perhaps because fresh air that enters during the ventilation is cool and the warmer air - HCN gas mixture tends to the top of the chamber (as already mentioned gaseous HCN, by itself, is only slightly lighter than air).
A related point is that the standard German Zyklon gas chamber of the time used a circulatory system developed by DEGESCH. In this system the air - HCN mixture is continually recirculated, i.e. it continually exits and re-enters the gas chamber. Circulation greatly reduces the length of time required to generate the gas from the Zyklon and work on the target pests. At the conclusion of the gassing, expulsion of the gas and the introduction of fresh air are accomplished by opening and closing the relevant ports in this circulatory system. Overall, a gas chamber with circulation is about three times more effective than one without, i.e. can do about three times more work . The practical minimum time required to kill lice (among the most difficult and resistant creatures) with Zyklon is about 3/4 of an hour. In a heated gas chamber with circulation a total time of about an hour, for gas generation and killing of the lice, followed by a ventilation period of about 20 minutes, is attainable under practical operational conditions .
The best material for a gas chamber using HCN is steel. If bricks or concrete are used, then the interiors must be coated with a sealant to prevent retention of the gas in the walls of the gas chamber .
created: 6 January 1997.
last modified: 6 January 1997.
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