AC, HCN, Blausaure

Jan Medema, Ph.D.
BC Defense Consultant
Benthuizen, The Netherlands

HCN has been mentioned on several occasions as a potential terrorist weapon. For instance for the possible attack on the London Subway and as possible agent that killed the dogs in the Al-Qaida tapes shown by CNN some time ago. In addition some experts (Dr Sands) mentioned it as the CWA of choice for Al Qaida. Finally, when an expert was asked in a television interview whether HCN could be sprayed from an aircraft the answer was no. In order to assess the viability of the use of HCN in Terrorist incidents I advise all the experts to read the literature, unfortunately from a long time ago, because two or three generations back it was already shown that most of the today expert views are wrong. The experiments done in the past cannot be repeated but the answers are still valid. Cyanide has been suggested for the first time as a weapon by a pharmacist in Berlin to defend Berlin against attacks from Napoleon. The Prussian soldiers should dip their bayonets in KCN just before the battle.

In 1915 the allied forces look desperately for new agents to counter the attacks from Germany. The French decide to develop HCN as a chemical weapon. They had to stabilize the agent (it often exploded or went up in flames). In order to raise a sufficient concentrated cloud the 75-mm shells had to be fired by a special gun with a high rate of fire. The end product called Vincenite was never assessed as very effective. It has been said that the Germans even did not border to mask if they smelled the bitter almonds.

The Germans went directly for phosgene, but advised by the French the British also considered the use, the agent was very effective when tested on dogs. However, they were saved by a brave professor, who declared that dogs were many times more sensitive than humans were. In order to proof his point he went into the Gas chamber together with a dog and let himself be exposed to HCN at a certain concentration for a certain time. The dog died in a few minutes, the professor who was reading a book easily survived an exposure of half an hour.

The use of HCN pops up again in the Second World War. The Germans find out that the Soviets have sprayers to disperse HCN from aircraft. In first instance this is assessed as very unusual because HCN vapor is lighter than air and never would reach the ground. Despite that a field trial is run in Munster which failed. In 1942 the Germans capture the Soviet sprayers and repeat the field trial with great success most dogs in the trial area died. The description by Hirsch is very detailed, indicating the dosage at several positions in the trial grid. Analyzing these data assuming a LCt50 of 5000 mg.min/m3 it appears that only in a very small fraction of the grid humans were at danger. Hirsch does not describe why the German sprayers did not work and the Soviets did.

At the end of the sixties an East German handbook for Radiological, Chemical and Biological Defense is issued in the DDR, after a short introduction the medically oriented handbook describes the threat from NATO countries. Two amazing threat agents are mentioned one is Blausaure Schnee, HCN Snow, and the other about fluorine compounds like PFIB. (The latter part will be the subject of another publication). The term snow indicated that the HCN was solid and thus very cold. Cooling through decompression does not work very well in case of HCN and flying at altitude will cool the liquid but not to a degree that it would freeze. The most likely explanation for the snow effect is that the liquid drops are quite large and evaporate during the free fall. The drops are cooled and condense/freeze water from the atmosphere. (I would love to verify this explanation but unfortunately this kind of sprayers were not available in NATO countries.) At least the Soviets showed that it is possible to spray HCN from aircraft but that also in these concentrated attacks the concentration was seldom high enough to kill humans.

The toxicity of HCN was studied on several occasions and it appears that it is best described by the toxic load concept, not Habers rule: C x t = constant but rather Cn x t =constant with n as large as 3. This means that if the LCt 50 for one-minute exposure is 5000 mg.min/m3, than the exposure time required at a concentration of 1000 mg/m3 is more than 2 hours. The reason is that HCN decays rapidly in the human system. The description with the toxic load concept is empirical. Actually a correct expression could be derived if the kinetics from uptake and decay were known exactly. The result of this toxic load effect is that protection inside houses (lower concentration) works very well. Even a mask that does not adsorb the HCN very well would slow down the ingress of HCN during an attack and reducing the concentration sufficiently to render the HCN harmless.

The Al-Qaida tapes clearly indicate that terrorist are thinking of resorting to chemical weapons. The explanation that it was a nerve agent does not hold very long. The dogs I have seen become unconscious from a nerve agent exposure (they were revived by therapy and lived happily ever after) did that very differently from which was shown on the tapes. The pilot plant scale chemistry that was demonstrated indicated indeed that something like HCN was generated, but there is no proof at this point. If it was HCN the tapes showed a completely useless experiment and demonstrated once more these people have no respect for life being it human or dog. Reassuring for the western world is that the degree of sophistication is low and maybe there is time to stop them before they really plan more dangerous things.

Should HCN be disregarded as CWA is a legitimate question. The answer is no, because on several places in the world tens of tons of HCN are stored in tanks. If those quantities are released in a short period the concentration down wind will be large enough to kill. As a first line of defense measures should be developed to protect the civilians. Protection in houses in side a sealed room works here. Even better would be to open up after the cloud has passed. A far better approach would be to change the production process in particular the logistics aspects involving HCN in such a way that the storage tanks can be reduced by one or two orders of magnitude. This should not lead to frequent transport of HCN over the road or by train. HCN should be generated locally. The same principles should be applied to other Toxic Industrial Chemicals like Phosgene.

A recent warning for a possible HCN attack was for the London Tube. Again instead of frightening people the media would have done better in telling them how to behave. The Sarin attacks in Tokyo with at least a 100 times more potent agent caused 13 killed and 1000 exposed. The other 4000 had mostly psychological problems. With simple counter measures casualties from a HCN attack can be greatly reduced. For one there always is a serious draft in the London Subway. Maybe at some points even anti draft measures might have been taken. (For instance in the Metro in Singapore sliding doors are used to reduce the draft). If that is the case remove them. The higher the wind speeds the lower the dosage and the lower the concentration. When people in trains smell something unusual open up the windows and leave the train at the next stop. Don’t get excited breath as little as possible. I would not go that far to give the World War I soldiers emergency remedy: urinate on a piece of cloth and breath through it. But what really helps is to wear the small do it yourself mask as long as they have some active carbon in them, the dust part of the filters by the way would work very well against biological attacks. Not that you can count on it to be safe against all forms of chemical or biological terrorist attacks. However, if the whole population takes these precautions the casualty potential is reduced to such a degree that it is no longer worth the effort for the terrorist.

Author’s Note: The information in this article is derived from: Callinicus, In defense of Chemical Warfare by Haldane (1923); Der Chemische Krieg by Hanslian (1937); The Story of the Chemical Brigade by Foulkes (1924); The US version of the Hirsch report by Walter Hirsch (probably UK 1948)