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Description of key information

Sodium or potassium cyanide is an alkali salt of the anion, cyanide, CN-, which is the solitary functional group which defines its chemical and toxicologic activity.  The salt is soluble in water, resulting in the immediate formation of HCN, as the pKa value (dissociation constant) is  9.11 at 30°C.  At the physiological pH of about 7, cyanide salts are distributed in the body as HCN and are not present as either the salt or the free CN‾ ion.  Data are available for the estimation of dose descriptors and DNELs in humans, thus avoiding extrapolation from animal species.  Calculations on LD50 values from animal studies provide comparable values for DNELs.

Key value for chemical safety assessment

Acute toxicity: via oral route

Endpoint conclusion
Dose descriptor:
LD50
Value:
5.09 mg/kg bw

Acute toxicity: via inhalation route

Endpoint conclusion
Dose descriptor:
LC50
Value:
103 mg/m³ air

Acute toxicity: via dermal route

Endpoint conclusion
Dose descriptor:
LD50
Value:
11.28 mg/kg bw

Additional information

HCN and its alkali salts are very toxic compounds by all routes of exposure.  Observations from animal studies describes systemic toxicity involves panting-breathing, ataxia, convulsions, coma and death. The acute oral LD50 in rats is approximately 5 mg/kg bw. On intact dry skin in female rabbits, the LD50 of the salt is high, over 200 mg/kg bw; when the skin is abraded or moistened, allowing the salt to form HCN, the dermal LD50 radically decreases to 7-11 mg/kg bw. The salt itself does not volatilize to allow study of inhalation toxicity, but if placed in water below a pH of 11, HCN vapour can be generated. The inhalation toxicity of HCN has been well studied, although guideline studies of a duration of 4 hours are not available. The LC50 for rats is less than 120 mg/m3 (measured 1 h exposure) and greater than 68 mg/m3 (measured 6 h exposure). The ECETOC Task Force on Cyanides (2007) developed interspecies scaling and extrapolation data to estimate that the LC50 for humans for a 4 h exposure is 103 mg/m3, and the LC01 for humans for a 4 h exposure is 45 mg/m3. The 15-minute LC01 in humans is estimated to be 173 mg/m3.

Based on experience in human cases involving intentional suicide or inadvertant occupational exposure, data are available to estimate NOAEL values or dose descriptors for humans. Rieders (1971) found that 200 mg of inorganic cyanide salt was the minimal lethal dose for oral exposure. For dermal exposure, 100 mg of inorganic salt was the minimal lethal dose, although this exposure was much slower. Cyanide salts, when hydrated on the surface of the skin, form HCN which moves through skin 30 times faster than the salt (Dugard, 1987). The Scientific Committee on Occupational Exposure Levels (SCOEL, 2005) recommended a short-term exposure limit of 5 mg CN-/m3 from HCN, NaCN and KCN. Taking into account molar conversion, a value of the inhalation limit is 9.4 mg/m3.

Justification for classification or non-classification

The classification and labeling of cyanide salts is dictated by Annex VI of Regulation (EC) No. 1272/2008, Entry 006-007-00-5, indicating Acute Category 2 for oral toxicity, Acute Category 1 for dermal toxicity, and Acute Category 2 for inhalation toxicity.   However, as the values for acute oral toxicity and acute inhalation toxicity are close to the threshold for the more toxic category, all acute toxicity classes are set to Category 1.  Both the oral and inhalation routes of exposure carry a skin notation.