Cyanide is toxic by ingestion, inhalation and skin contact. It is present in some insecticides, rodenticides, metal polishes, electroplating solutions and fumigants, and is used in a variety of metallurgical processes. Apricot kernels contain cyanogenic glycosides which are cleaved by gastrointestinal flora to liberate hydrogen cyanide.
Cyanide is used widely in the chemical industry, in laboratories and in photographic processing. Burning plastic tends to liberate hydrogen cyanide.
Cyanide reversibly inhibits cellular oxidising enzymes which contain ferric iron such as cytochrome oxidase. This has the effect of uncoupling oxidative phosphorylation. As a result, a range of clinical features are produced which are primarily due to tissue hypoxia.
The ingestion of 50 mg of hydrogen cyanide, or 200-500 mg of its salts, is likely to be fatal. Poisoning is enhanced by an empty stomach and high gastric acidity. A full stomach may delay symptoms for up to 4 hours. Inhalation of hydrogen cyanide may produce symptoms within seconds, and death within minutes.
The measurement of blood levels has little clinical significance but may be of forensic importance. Generally, serum levels greater than 1 mg/litre are fatal. Smokers may have incidental serum cyanide levels of 0.1 mg/litre.