Hydrogen cynide and cyanide salts: sodium, potassium, calcium, as CN-.
Documentation of proposed values of occupational exposure limits (OELs)
Authors: Skowroń Jolanta , Konieczko Katarzyna 
Published: July 2020

Hydrogen cyanide (HCN) and its salts: potassium cyanide (KCN), sodium cyanide (NaCN) and calcium cyanide [Ca(CN2)] are very toxic. Hydrogen cyanide at ambient conditions is a colourless liquid or a colourless gas with the characteristic odour of bitter almonds. Sodium, potassium and calcium cyanides are white hygroscopic, crystalline solids with a slight HCN odour. Hydrogen cyanide is used mainly in a fumigation of ships, buildings, orchards and various foods, in electroplating, in the production of chelating agents such as EDTA, and in metal treatment processes. It is also used as a chemical intermediate. Cyanides are used in the extraction and recovery of gold and silver from ores, the heat treatment of metals, and electroplating. They are also precursors in chemical syntheses. Workers from metal, electrochemical, plastics, pharmaceutical, textile, chemical and food industries are exposed to these compounds. In 2008–2013, there were no workers exposed to the concentration of hydrogen cyanide and sodium, potassium and calcium cyanides exceeding the maximum admissible ceiling concentration MAC(C) 5 mg/m3 (the national database maintained by the Regional Sanitary Station in Bydgoszcz). Hydrogen cyanide and cyanides are irritating to mucous membranes and skin. They are absorbed by inhalation, dermal and oral exposure. The acute hydrogen cyanide and cyanides poisoning indicate a great danger and hazard, because these compounds are quickly absorbed into the body and their effects are present within a few minutes after the start of exposure. Exposure to sodium cyanide at a concentration of 286 mg/m3 or to hydrogen cyanide at a concentration greater than 300 mg/m3 for 1 min may be fatal. Sodium, potassium or calcium cyanides at concentrations of 25 mg/m3 are direct hazards to life and health of workers if exposure lasts about 30 min and without respiratory protection. For hydrogen cyanide this value was established as 56 mg/m3. The development of symptoms of acute poisoning by hydrogen cyanide or cyanides in humans occurs in three phases: breathlessness and excitement, convulsions and paralysis. The results of studies of subchronic and chronic exposures of workers to cyanides by inhalation indicate that symptoms of exposure were associated with changes in the central nervous system (headache, weakness, changes in the sensation of taste and smell) and damage to the thyroid (enlargement, changes in uptake of iodine, elevated concentration of thyroid stimulating hormone TSH and a reduction of thyroid hormones T3 and T4). Other studies suggest that chronic exposure to hydrogen cyanide in the hardening plant of metals caused decrements in lung functions among workers. Hydrogen cyanide and cyanides, both in aqueous solution, applied to the conjunctival sac or on the skin is quickly absorbed into the body of animals in amounts sufficient to cause toxic effects and death. In rats and mice treated with sodium cyanide in drinking water at a dose of 4.5 mg/kg bw/day for 13 weeks, no significant changes in biochemical and haematological parameters of peripheral blood and histopathological findings in the internal organs were observed. There were no pathological changes in the respiratory, cardiovascular, nervous system and kidneys in rats which were feed with hydrogen cyanide over two years. Calculated NOAEL was approximately 10.4 mg/kg body weight. There is no available data on the carcinogenicity of hydrogen cyanide and cyanides in human and animals. Positive effects were obtained in one study only, in which hydrogen cyanide was tested with Salmonella typhimurium strain TA 100 in the absence of metabolic activation, while the other strains employed in this study yielded negative results. Cyanides did not show mutagenic activity in the tests in vitro and in vivo. On the basis of the studies on hamsters, teratogenic effects of sodium cyanide were observed. This compound was toxic for pregnant mothers and caused an increase in fatal resorption and malformations in an offspring. The results of the study of workers exposed to hydrogen cyanide and cyanides and with changes in thyroid were the basis for calculating MAC (NDS) value. The LOAEL value was establishes as a concentration of 4.7 mg/m3. The MAC of 1 mg/m3 (calculated CN–) was established for hydrogen cyanide and the inhalable fraction of sodium, potassium, calcium cyanides was accepted. Due to totally different mechanism of action of hydrogen cyanide and cyanides (sodium, potassium, calcium) in chronic exposure (effects on the thyroid gland) and in the acute exposure, which is primarily associated with inhibition enzymatic system of cytochrome c oxidase, which prevents cells from using oxygen (histotoxic hypoxia), for these compounds the ceiling value MAC(C) of 5 mg/m3 was not changed. Such an approach is a deviation from the basic methodology adopted by the Group of Expert and the Interdepartmental Commission for MAC and MAI. MAC and ceiling MAC(C) values for these substances should be establish due to the different effects of critical action and mechanisms of action in the acute and chronic condition. This approach is consistent with the DECOS Committee (Dutch Expert Committee on Occupational Standards) from 2002. According to the committee, the acute human data show the most sensitive effect, i.e., death. The steepness of the dose-response relationship and the severity of the acute effects in humans imply at the same time that utmost care should be taken to prevent this exposure level from being exceeded, not even for a short time. Therefore, the committee proposed to establish a ceiling value for the acute health effects of 10 mg/m3 for hydrogen cyanide. The Scientific Committee on Occupational Exposure Limit Values (SCOEL) proposed an OEL value of 1 mg/m3. However, since the acute effects in humans are severe (i.e., death) and show a rather steep dose-response relationship, peak exposures should be avoided. Based on the steepness of the dose-response relationship and the severity of the acute effects in humans a STEL of 5 mg/m3 is recommended as CN– from any combination of the three compounds. Based on the very high skin permeability measured for hydrogen cyanide and cyanide anions in aqueous solutions, a skin notation is recommended for hydrogen cyanide and sodium, potassium, calcium cyanides.