Hydrogen Cyanide

Hydrogen cyanide is usually included among the CW agents causing general poisoning. There is no confirmed information on this substance being used in chemical warfare. However, it has been reported that hydrogen cyanide was used by Iraq in the war against Iran and against the Kurds in northern Iraq during the 1980's. Hydrogen cyanide has high toxicity and in sufficient concentrations it rapidly leads to death. During the Second World War, a form of hydrogen cyanide (Zyklon B) was used in the Nazi gas chambers.

At room temperature, hydrogen cyanide is a colourless liquid which boils at 26 oC. The most important route of poisoning is through inhalation. Both gaseous and liquid hydrogen cyanide, as well as cyanide salts in solution, can also be taken up through the skin. Its high volatility probably makes hydrogen cyanide difficult to use in warfare since there are problems in achieving sufficiently high concentrations outdoors. On the other hand, the concentration of hydrogen cyanide may rapidly reach lethal levels if it is released in confined spaces.

The most important toxic effect of hydrogen cyanide is by inhibiting the metal-containing enzymes. One such enzyme is cytochromoxidase, containing iron. This enzyme system is responsible for the energy-providing processes in the cell where oxygen is utilized, i.e., cell respiration. When cell respiration ceases, it is no longer possible to maintain normal cell functions, which may lead to cell mortality.

Symptoms of cyanide poisoning vary and depend on, for example, route of poisoning, total dose and the exposure time. If hydrogen cyanide has been inhaled, the initial symptoms are restlessness and increased respiratory rate. Other early symptoms are giddiness, headache, palpitations and respiratory difficulty. These are later followed by vomiting, convulsions, respiratory failure and unconsciousness. If the poisoning occurs rapidly, e.g., as a result of extremely high concentrations in the air, there is no time for symptoms to develop and exposed persons may then suddenly collapse and die.

The treatment against cyanide poisoning given to civilians is based on encouraging and speeding-up the body's own ability to excrete cyanide and to bind cyanide in the blood. The enzyme rhodanese is present in the body, mainly in the liver, and together with sulphur transforms cyanide into thiocyanate, which is passed out in the urine. By supplying sulphur in the form of sodium thiosulphate (Na2S2O3) the detoxification can be speeded up. The cyanide ion has high affinity to trivalent iron (Fe3+). The divalent iron in blood haemoglobin can be oxidized to trivalent, which leads to the formation of methaemoglobin which binds cyanide ions. The formation of methaemoglobin can be achieved by supplying sodium nitrite (NaNO2) or dimethylaminophenol (DMAP).

Cyanide can also be bound by metallic ions supplied to the blood in suitable form. Among others, cobalt can be supplied in the form of a cobalt complex or as hydroxycobalamin (vitamin B12).

In cases of poisoning with hydrogen cyanide it is of the utmost importance that countermeasures are immediately introduced. For this reason, a medical antidote (PAPP, para-aminopropiophenone) for use as a pretreatment is being developed in the United Kingdom.

Relationship between concentration and effects when inhaling hydrogen cyanide

Concentration (mg/m3)  Effect 
300  Immediately lethal 
200  Lethal after 10 minutes 
150  Lethal after 30 minutes 
120-150  Highly dangerous (fatal) after 30-60 min. 
50-60  Endurable for 20 min. - 1 h without effect 
20-40  Light symptoms after several hours 

Source: A FOA Briefing Book on Chemical Weapons