The first gas chamber for execution purposes was built in Arizona in 1920. It consisted of an airtight chamber with gasketed doors and windows, a gas generator, an explosion proof electrical system, an air intake and exhaust system, provision for adding ammonia to the intake air and mechanical means for activating the gas generator and air exhaust. The air intake consisted of several mechanically operated valves. Only the hardware has changed to the present.

The gas generator consisted of a crockery pot filled with a dilute solution (18%) of sulfuric acid with a mechanical release lever. The chamber had to be scrubbed with ammonia after the execution, as did the executee. Some 25 - 13-gram sodium cyanide pellets were used and generated a concentration of 3200 ppm in a 600 cubic foot chamber.
In the years that followed, other states adopted the HCN gas chamber as a mode of execution and design techniques changed. Eaton Metal Products designed, built and improved most of the chambers. Most had two chairs and were fitted with a vacuum system to guarantee a negative pressure and only inward leakage. All systems employed the gas generator technique because it was the most effective and simplest procedure available up until the late 1960's. No system ever was designed to use, or ever used, Zyklon B.

The reason for this is quite simple. Zyklon B takes too long to evaporate (or boil off) the HCN from the inert carrier and requires heated air and a temperature controlled system. Not only is the gas not instant, but a danger of explosion always exists.

The overall gas mixture is generally below the lower explosion limit (LEL) of the gas air mixture of 0.32% (since the mixture should not normally exceed 3200 ppm), but the concentration of the gas at the generator (or as in the case of Zyklon B, at the inert carrier) is much greater and may well be 90% to 99% by volume. This is almost pure HCN and this condition may exist at points of time in pockets in the chamber. The ambient air temperature or the heated air temperature must be considerably higher and artificially controlled for Zyklon B (since evaporation is strictly a physical process), where, with the gas generator, the temperature can be lower and uncontrolled since the chemical reaction in the generator is self-catalytic after starting. Electrical contacts and switches must be kept at a minimum, explosion-proof and outside the chamber. Technology available only since the late 1960's has enabled the Missouri system, which will be the most advanced system ever built, to utilize a gas vaporizer and delivery system for liquid HCN, eliminating the dangerous of handling and disposal of the prussic acid residual after the execution.

Zyklon B, which would seem on the surface to have been a more efficient means of supplying gas and eliminating the prussic acid residue problem, was not the solution to the problem. In fact, the use of Zyklon B would have increased the execution time and therefore lengthened the time for handling the dangerous gas and, also, because of the heater requirements, caused a risk of explosion. An alternate solution would have been to heat the gas externally and circulate the gas/air mixture through plumbing outside the chamber and back into the chamber as the DEGESCH delousing equipment did, but this would only have caused a greater risk of leakage and hazard to the users. It is poor design and extremely dangerous to allow the gas outside the pressurized chamber. The DEGESCH equipment was intended to be utilized in the open, or in a well-ventilated area, and only in the presence of trained personnel and not with untrained people present.

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In the United States, Arizona, California, Colorado, Maryland, Mississippi, Missouri, Nevada, New Mexico, and North Carolina have utilized gas as a mode of execution. But because of the inherent dangers in handling the gas and the expensive maintenance costs for the equipment used, some states (Nevada, North Carolina and New Mexico) have legislated for lethal injection, either as the only, or as a choice procedure. Other states will probably follow. The author has been a consultant to the states of Missouri, California and North Carolina.

In any event, because of the cost of manufacture of HCN gas, and because of the excessive hardware and maintenance costs of the equipment, gas has been in the past, and still is, the most expensive mode of execution.