This report contains the collective views of an international group of experts and does not necessarily represent the decisions or the stated policy of the United Nations Environment Programme, the International Labour Organization, or the World Health Organization.

Concise International Chemical Assessment Document 61

First draft prepared by Prof. Fina Petrova Simeonova, Consultant, National Center of Hygiene, Medical Ecology and Nutrition, Sofia, Bulgaria; and Dr Lawrence Fishbein, Fairfax, Virginia, USA

Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organization, and the World Health Organization, and produced within the framework of the Inter-Organization Programme for the Sound Management of Chemicals.

World Health Organization

Geneva, 2004

The International Programme on Chemical Safety (IPCS), established in 1980, is a joint venture of the United Nations Environment Programme (UNEP), the International Labour Organization (ILO), and the World Health Organization (WHO). The overall objectives of the IPCS are to establish the scientific basis for assessment of the risk to human health and the environment from exposure to chemicals, through international peer review processes, as a prerequisite for the promotion of chemical safety, and to provide technical assistance in strengthening national capacities for the sound management of chemicals.

The Inter-Organization Programme for the Sound Management of Chemicals (IOMC) was established in 1995 by UNEP, ILO, the Food and Agriculture Organization of the United Nations, WHO, the United Nations Industrial Development Organization, the United Nations Institute for Training and Research, and the Organisation for Economic Co-operation and Development (Participating Organizations), following recommendations made by the 1992 UN Conference on Environment and Development to strengthen cooperation and increase coordination in the field of chemical safety. The purpose of the IOMC is to promote coordination of the policies and activities pursued by the Participating Organizations, jointly or separately, to achieve the sound management of chemicals in relation to human health and the environment.

WHO Library Cataloguing-in-Publication Data

Hydrogen cyanide and cyanides : human health aspects.

(Concise international chemical assessment document ; 61)

1.Cyanides - adverse effects 2.Hydrogen cyanide - adverse effects 3.Risk assessment

4.Environmental exposure I.International Programme on Chemical Safety II.Series

ISBN 92 4 153061 8        (LC/NLM Classification: QV 632)

ISSN 1020-6167

©World Health Organization 2004

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Technically and linguistically edited by Marla Sheffer, Ottawa, Canada, and printed by Wissenchaftliche Verlagsgesellschaft mbH, Stuttgart, Germany

FOREWORD

Concise International Chemical Assessment Documents (CICADs) are the latest in a family of publications from the International Programme on Chemical Safety (IPCS) — a cooperative programme of the World Health Organization (WHO), the International Labour Organization (ILO), and the United Nations Environment Programme (UNEP). CICADs join the Environmental Health Criteria documents (EHCs) as authoritative documents on the risk assessment of chemicals.

International Chemical Safety Cards on the relevant chemical(s) are attached at the end of the CICAD, to provide the reader with concise information on the protection of human health and on emergency action. They are produced in a separate peer-reviewed procedure at IPCS. They may be complemented by information from IPCS Poison Information Monographs (PIM), similarly produced separately from the CICAD process.

CICADs are concise documents that provide summaries of the relevant scientific information concerning the potential effects of chemicals upon human health and/or the environment. They are usually based on selected national or regional evaluation documents or on existing EHCs. Before acceptance for publication as CICADs by IPCS, these documents undergo extensive peer review by internationally selected experts to ensure their completeness, accuracy in the way in which the original data are represented, and the validity of the conclusions drawn.

The primary objective of CICADs is characterization of hazard and dose–response from exposure to a chemical. CICADs are not a summary of all available data on a particular chemical; rather, they include only that information considered critical for characterization of the risk posed by the chemical. The critical studies are, however, presented in sufficient detail to support the conclusions drawn. For additional information, the reader should consult the identified source documents upon which the CICAD has been based.

Risks to human health and the environment will vary considerably depending upon the type and extent of exposure. Responsible authorities are strongly encouraged to characterize risk on the basis of locally measured or predicted exposure scenarios. To assist the reader, examples of exposure estimation and risk characterization are provided in CICADs, whenever possible. These examples cannot be considered as representing all possible exposure situations, but are provided as guidance only. The reader is referred to EHC 170.¹

While every effort is made to ensure that CICADs represent the current status of knowledge, new information is being developed constantly. Unless otherwise stated, CICADs are based on a search of the scientific literature to the date shown in the executive summary. In the event that a reader becomes aware of new information that would change the conclusions drawn in a CICAD, the reader is requested to contact IPCS to inform it of the new information.

Procedures

The flow chart on page 2 shows the procedures followed to produce a CICAD. These procedures are designed to take advantage of the expertise that exists around the world — expertise that is required to produce the high-quality evaluations of toxicological, exposure, and other data that are necessary for assessing risks to human health and/or the environment. The IPCS Risk Assessment Steering Group advises the Coordinator, IPCS, on the selection of chemicals for an IPCS risk assessment based on the following criteria:

• there is the probability of exposure; and/or
• there is significant toxicity/ecotoxicity.

Thus, it is typical of a priority chemical that

• it is of transboundary concern;
• it is of concern to a range of countries (developed, developing, and those with economies in transition) for possible risk management;
• there is significant international trade;
• it has high production volume;
• it has dispersive use.

The Steering Group will also advise IPCS on the appropriate form of the document (i.e., a standard CICAD or a de novo CICAD) and which institution bears the responsibility of the document production, as well as on the type and extent of the international peer review.

The first draft is usually based on an existing national, regional, or international review. When no appropriate source document is available, a CICAD may be produced de novo. Authors of the first draft are usually, but not necessarily, from the institution that developed the original review. A standard outline has been developed to encourage consistency in form. The first draft undergoes primary review by IPCS to ensure that it meets the specified criteria for CICADs.

The second stage involves international peer review by scientists known for their particular expertise and by scientists selected from an international roster compiled by IPCS through recommendations from IPCS national Contact Points and from IPCS Participating Institutions. Adequate time is allowed for the selected experts to undertake a thorough review. Authors are required to take reviewers’ comments into account and revise their draft, if necessary. The resulting second draft is submitted to a Final Review Board together with the reviewers’ comments. At any stage in the international review process, a consultative group may be necessary to address specific areas of the science. When a CICAD is prepared de novo, a consultative group is normally convened.

The CICAD Final Review Board has several important functions:

• to ensure that each CICAD has been subjected to an appropriate and thorough peer review;
• to verify that the peer reviewers’ comments have been addressed appropriately;
• to provide guidance to those responsible for the preparation of CICADs on how to resolve any remaining issues if, in the opinion of the Board, the author has not adequately addressed all comments of the reviewers; and
• to approve CICADs as international assessments.

Board members serve in their personal capacity, not as representatives of any organization, government, or industry. They are selected because of their expertise in human and environmental toxicology or because of their experience in the regulation of chemicals. Boards are chosen according to the range of expertise required for a meeting and the need for balanced geographic representation.

Board members, authors, reviewers, consultants, and advisers who participate in the preparation of a CICAD are required to declare any real or potential conflict of interest in relation to the subjects under discussion at any stage of the process. Representatives of nongovernmental organizations may be invited to observe the proceedings of the Final Review Board. Observers may participate in Board discussions only at the invitation of the Chairperson, and they may not participate in the final decision-making process.

This CICAD on hydrogen cyanide and cyanides (human health aspects) was prepared by Prof. Fina Petrova Simeonova and Dr Lawrence Fishbein, based principally on the Agency for Toxic Substances and Disease Registry toxicological profile for cyanide (ATSDR, 1997) and the Joint FAO/WHO Expert Committee on Food Additives document on cyanogenic glycosides (JECFA, 1993). The source documents and a description of their review processes are presented in Appendix 1. A comprehensive literature search of several online databases was performed in October 2002 to identify any relevant references published subsequent to those cited in the source documents. This CICAD was first discussed at the 10th Final Review Board meeting, held in Monks Wood, United Kingdom, on 16–19 September 2002. Following revision, it was discussed again and approved as an international assessment at the 11th Final Review Board meeting, held in Varna, Bulgaria, on 8–11 September 2003. Participants at the 10th and 11th Final Review Board meetings are listed in Appendices 2 and 3. The drafts discussed at these meetings were peer reviewed before the meetings; information on the peer review process is presented in Appendix 4. The International Chemical Safety Cards on hydrogen cyanide, sodium cyanide, potassium cyanide, calcium cyanide, cyanogen, cyanogen chloride, acetone cyanohydrin, and potassium ferricyanide, produced by the International Programme on Chemical Safety (IPCS, 1999a,b, 2000b, 2001, 2002a,b,c,d), have also been reproduced in this document.

Cyanides comprise a wide range of compounds of varying degrees of chemical complexity, all of which contain a CN moiety, to which humans are exposed in gas, liquid, and solid form from a broad range of natural and anthropogenic sources. While many chemical forms of cyanide are used in industrial application or are present in the environment, the cyanide anion CN^– is the primary toxic agent, regardless of origin.

Hydrogen cyanide is a colourless or pale blue liquid or gas with a faint bitter almond-like odour. Hydrogen cyanide is used primarily in the production of substances such as adiponitrile, methyl methacrylate, chelating agents, cyanuric chloride, methionine and its hydroxylated analogues, and sodium and potassium cyanide. Hydrogen cyanide is also used as a fumigant in ships, railroad cars, large buildings, grain silos, and flour mills, as well as in the fumigation of peas and seeds in vacuum chambers.

Other cyanides, such as sodium and potassium cyanide, are solid or crystalline hygroscopic salts widely used in ore extracting processes for the recovery of gold and silver, electroplating, case-hardening of steel, base metal flotation, metal degreasing, dyeing, printing, and photography. They are also widely used in the synthesis of organic and inorganic chemicals (e.g., nitriles, carboxylic acids, amides, esters, and amines; heavy metal cyanides) and in the production of chelating agents.

Anthropogenic sources of cyanide release to the environment are diverse. Releases to air include chemical manufacturing and processing industries, such as metallurgical industries and metal plating, and extraction of gold and silver from low-grade ores. Other sources include volatilization from cyanide wastes disposed of in landfills and waste ponds, emissions from municipal solid waste incinerators, biomass burning, fossil fuel combustion, including vehicle emissions, fumigation operations, and the production of coke or other coal carbonization procedures.

Hydrogen cyanide is formed during the incomplete combustion of nitrogen-containing polymers, such as certain plastics, polyurethanes, and wool. Hydrogen cyanide is present in cigarette smoke.

Non-point sources of cyanide released to water can result from runoff from cyanide-containing anti-caking salts used on roads, migration from landfills, and agricultural and atmospheric fallout and washout. Point sources of releases to water include discharges from gold mining plants, wastewater treatment works, iron and steel production, and organic chemical industries.

Principal natural sources of cyanides are over 2000 plant species, including fruits and vegetables, that contain cyanogenic glycosides, which can release cyanide on hydrolysis when ingested. Among them, cassava (tapioca, manioc) and sorghum are staple foods for hundreds of millions of people in many tropical countries. Known cyanogenic glycosides in plants include amygdalin, linamarin, prunasin, dhurrin, lotaustralin, and taxiphyllin. Hydrogen cyanide is released into the atmosphere from natural biogenic processes from higher plants, bacteria, and fungi.

In air, cyanide is present as gaseous hydrogen cyanide, with a small amount present in fine dust particles. Cyanides have the potential to be transported over long distances from their respective emission sources.

The majority of the population is exposed to very low levels of cyanide in the general environment. There are, however, specific subgroups with higher potential for exposure. These include individuals involved in large-scale processing of cassava and those consuming significant quantities of improperly prepared foods containing cyanogenic glycosides, such as cassava, speciality foods such as apricot pits, and bitter almonds. Other subgroups with greatest potential for exposure include those in the vicinity of accidental or intended releases from point sources, active and passive smokers, and fire-related smoke inhalation victims.

Workers may be exposed to cyanides during fumigation operations and the production and use of cyanides in many industrial processes — for example, electroplating, case-hardening of steel, and extraction of gold and silver from ores.

Cyanides are well absorbed via the gastrointestinal tract or skin and rapidly absorbed via the respiratory tract. Once absorbed, cyanide is rapidly and ubiquitously distributed throughout the body, although the highest levels are typically found in the liver, lungs, blood, and brain. There is no accumulation of cyanide in the blood or tissues following chronic or repeated exposure.

Approximately 80% of absorbed cyanide is metabolized to thiocyanate in the liver by the mitochondrial sulfur transferase enzyme rhodanese and other sulfur transferases. Thiocyanate is excreted in the urine. Minor pathways for cyanide detoxification involve reaction with cystine to produce aminothiazoline- and iminothiazolidinecarboxylic acids and combination with hydroxycobalamin (vitamin B_12a) to form cyanocobalamin (vitamin B₁₂); these end-products are also excreted in the urine.

The principal features of the toxicity profile for cyanide are its high acute toxicity by all routes of administration, with a very steep and rate-dependent dose–effect curve, and chronic toxicity, probably mediated through the main metabolite and detoxification product, thiocyanate. The toxic effects of cyanide ion in humans and animals are generally similar and are believed to result from inactivation of cytochrome oxidase and inhibition of cellular respiration and consequent histotoxic anoxia. The primary targets of cyanide toxicity in humans and animals are the cardiovascular, respiratory, and central nervous systems. The endocrine system is also a potential target for long-term toxicity, as a function of continued exposure to thiocyanate, which prevents the uptake of iodine in the thyroid and acts as a goitrogenic agent.

In humans, whereas slight effects occur at exposure levels of 20–40 mg/m³, 50–60 mg/m³ can be tolerated without immediate or late effects for 20 min to 1 h, 120–150 mg/m³ may lead to death after 0.5–1 h, 150 mg/m³ is likely to be fatal within 30 min, 200 mg/m³ is likely fatal after 10 min, and 300 mg/m³ is immediately fatal. The lowest reported oral lethal dose for humans is 0.54 mg/kg body weight, and the average absorbed dose at the time of death has been estimated at 1.4 mg/kg body weight (calculated as hydrogen cyanide). Sequelae after severe acute intoxications may include neuropsychiatric manifestations and Parkinson-type disease. Cyanide from tobacco smoke has been implicated as a contributing factor in tobacco–alcohol amblyopia. Long-term exposure to lower concentrations of cyanide in occupational settings can result in a variety of symptoms related to central nervous system effects.

Long-term consumption of cassava containing high levels of cyanogenic glycosides has been associated with tropical ataxic neuropathy, spastic paraparesis, and, in areas with low iodine intake, development of hypothyroidism, goitre, and cretinism. While exposure to cyanide has been crudely estimated to be 15–50 mg/day in endemic areas in some such cases, owing to the limitations of data on exposure and potential impact of confounders such as malnutrition, low protein content of the diet, vitamin deficiencies, and iodine status, the available data do not provide meaningful information on dose–response for cyanide.

Data on end-points other than acute toxicity are somewhat limited. This is attributable in large part to difficulties in conducting, for example, investigations of repeated-dose or chronic toxicity due to the high acute toxicity of the compound. Cyanides are weakly irritating to the skin and eye; data on sensitizing properties or carcinogenicity of hydrogen cyanide or its alkali salts have not been identified. Although somewhat limited, the weight of evidence of available data indicates that cyanide is not genotoxic and that it induces developmental effects only at doses or concentrations that are overtly toxic to the mothers.

Available data in human populations are considered inadequate as a basis for characterization of dose–response for chronic ingestion of cyanide. In a 13-week repeated-dose toxicity study in which cyanide was administered in drinking-water, there were no clinical signs associated with central nervous system effects or histopathological effects in the brain or thyroid of rats or mice exposed to doses up to 12.5 mg and 26 mg cyanide/kg body weight per day, respectively. At 12.5 mg cyanide/kg body weight per day, there were slight changes in the reproductive tract in male rats, which, although they apparently would not affect fertility in rats, are possibly significant to humans. The no-observed-adverse-effect level (NOAEL) for these effects was 4.5 mg/kg body weight per day. The examination of neurotoxicity in this study was limited to clinical observation and optical microscopy in autopsy. The few available studies specifically intended to investigate neurotoxicity, while reporting adverse effects at exposure levels of 1.2 mg cyanide/kg body weight per day in rats and 0.48 mg cyanide/kg body weight per day in goats, suffer from weaknesses that preclude their quantitative assessment.

In relation to characterization of concentration–response for repeated-dose toxicity for inhalation (relevant principally to the occupational environment), in three separate studies in rats, there were no adverse systemic effects in rats exposed to acetone cyanohydrin, which is rapidly hydrolysed to hydrogen cyanide at physiological pH, at concentrations up to 211 mg/m³ (corresponding to a concentration of 67 mg hydrogen cyanide/m³). The steepness of the dose–effect curve is illustrated by the observation of 30% mortality among rats exposed part of the day to 225 mg acetone cyanohydrin/m³ (71 mg hydrogen cyanide/m³).

Adverse effects of exposure to the low concentrations of cyanide that are generally present in the general environment (<1 µg/m³ in ambient air; <10 µg/litre in water) are unlikely. Acute cyanide intoxications may arise from eating apricot kernels, choke cherries, and other stone fruit kernels with high concentrations of cyanogenic glycosides. Inadequately prepared cassava, when constituting the major part of the diet, may be hazardous.

Hydrogen cyanide (HCN) is a colourless or pale blue liquid or gas with a faint bitter almond-like odour. Common synonyms are hydrocyanic acid and prussic acid. Hydrogen cyanide is a very weak acid, with a pK_a value of 9.22 at 25 °C. It is soluble in water and alcohol. Hydrogen cyanide is commercially available as a gas or as a technical-grade liquid in concentrations of 5, 10, and 96–99.5%. Phosphoric acid is added to liquid hydrogen cyanide as a stabilizer to prevent decomposition and explosion (ATSDR, 1997). Some important physical and chemical properties of hydrogen cyanide are summarized in Table 1.

The conversion factors² for hydrogen cyanide in air (at 20 °C and 101.3 kPa) are as follows:

1 ppm = 1.12 mg/m³

1 mg/m³ = 0.890 ppm

Table 1: Physical and chemical properties of hydrogen cyanide (CAS No. 74-90-8).^a

^a From ACGIH (2001); DECOS (2002).

^b Hine & Weimar (1965); Edwards et al. (1978); Gaffney et al. (1987).

Sodium cyanide (NaCN) is a white hygroscopic crystalline powder with a faint bitter almond-like odour. Common synonyms are cyanide of sodium and hydrocyanic acid, sodium. Commercially available sodium cyanide generally achieves a purity of 95–98%. The aqueous solution of sodium cyanide is strongly alkaline and rapidly decomposes. Sodium cyanide produces hydrogen cyanide on contact with acids or acid salts.

Potassium cyanide (KCN) is a white deliquescent solid with an odour of hydrogen cyanide. Common synonyms are hydrocyanic acid, potassium salt and cyanide of potassium. Potassium cyanide is commercially available at a 95% purity. An aqueous solution of potassium cyanide in water is strongly alkaline. Potassium cyanide also produces hydrogen cyanide on contact with acids or acid salts.

Calcium cyanide (Ca(CN)₂), also commonly called cyanide of calcium, calcid, or calsyan, is a white crystalline solid. Its aqueous solution gradually liberates hydrogen cyanide. Cyanides such as sodium cyanide, potassium cyanide, and calcium cyanide form strong complexes with many metals (Table 2).

Cyanogen is a colourless toxic gas with an almond-like odour. Common synonyms are carbon nitrile, dicyanogen, ethane dinitrile, and oxalic acid dinitrile. Cyanogen is slowly hydrolysed in aqueous solution, yielding oxalic acid and ammonia. The conversion factors for cyanogen in air at 20 °C and 101.3 kPa are as follows:

1 ppm = 2.16 mg/m³

1 mg/m³ = 0.462 ppm

Table 2: Physical and chemical properties of selected cyanide compounds.^a

^a From Windholz (1983); ACGIH (2001); ECETOC (2004).

Cyanogen chloride is a colourless gas. Its common synonym is chlorine cyanide, and its common trade name is Caswell No. 267. Cyanogen chloride releases hydrogen cyanide by hydrolysis. Its conversion factors in air are:

1 ppm = 2.56 mg/m³

1 mg/m³ = 0.391 ppm

Common synonyms of acetone cyanohydrin are ACH, 2-cyano-2-propanol, 2-methyllactonitrile, and 2-hydroxy-2-methyl propanenitrile. It dissociates on standing to liberate hydrogen cyanide. Its boiling point is 120 °C (with decomposition to hydrogen cyanide and acetone). Its conversion factors in air are:

1 ppm = 3.54 mg/m³

1 mg/m³ = 0.283 ppm

The half-time of ACH in water was reported to be 9 min (Ellington et al., 1986); further studies reported that this hydrolysis to acetone and hydrogen cyanide was pH dependent, and half-times of 58, 27, and 8 min were observed at pH 4.8, 6.3, and 6.8 (ICI, 1993). In a more recent study, similar findings were reported (half-times of 54.7, 31.2, 5.4, and 4.0 min at pH 6.00, 6.40, 6.86, and 7.00, respectively) (Frank et al., 2002).

Some chemical properties of other cyanides are given in Table 2. Copper cyanide is a white to cream-coloured solid. Its common name is cuprous cyanide, and its synonym is cupricin. Potassium silver cyanide occurs as white crystals; its common synonym is potassium dicyanoargentate. It is sensitive to light. Sodium ferrocyanide decomposes at 435 °C, forming sodium cyanide.

Cyanogenic glycosides are produced naturally by many plants; when hydrolysed, they produce hydrogen cyanide. Chemical structures of some commonly occurring cyanogenic glycosides are depicted in Figure 1.

Further chemical and physical properties of hydrogen cyanide and some cyanides are summarized in the International Chemical Safety Cards included in this document.

Fig. 1: Cyanogenic glycosides in major edible plants (JECFA, 1993)

Amygdalin occurs in (among others) almonds, dhurrin in sorghum, linamarin in cassava, lotaustralin in cassava and lima beans, prunasin in stone fruits, and taxiphyllin in bamboo shoots.

Cyanides in environmental media are usually collected in sodium or potassium hydroxide solution and measured by spectrophotometry (Agrawal et al., 1991), colorimetry, or ion-specific electrode or by headspace gas chromatography with a nitrogen-specific detector or electron capture detector (Maseda et al., 1989; Seto et al, 1993). Cyanide in aqueous matrices is usually measured by colorimetric, titrimetric (US EPA, 1983), or electrochemical methods after pretreatment to produce hydrogen cyanide and absorption in sodium hydroxide solution. Total cyanide (irrespective of origin) includes all of the available cyanide in a sample; in drinking-water, it is measured by semi-automated colorimetry (EPA Method 335.4) as well as by selective electrode, ultraviolet/distillation/spectrophotometry, and ion chromatography (EPA Method 300.0) (US EPA, 1993a). Free cyanide can also be determined by one method (SM-4500-CN-F) approved for drinking-water compliance monitoring analysis that does not require distillation, the specific ion electrode method (US EPA, 2003a). Weak acid dissociable cyanide analysis (used principally by the precious metals mining industry) includes those cyanide species liberated at moderate pH 4.5, such as aqueous hydrogen cyanide and cyanide anion, the majority of copper, cadmium, nickel, zinc, silver, and tin complexes, and others with similar low dissociation constants. Weak acid dissociable cyanide can be determined in wastewaters by a ligand exchange/flow injection/amperometric technique (EPA Method 1677) (Milosavlievic et al., 1995; US EPA, 1997).

A chromatographic technique with fluorescence detection is used to detect trace amounts of cyanide in blood cells (Chinaka et al., 1998). Cyanide in biological tissue and fluids can be measured spectrophotometrically after reaction with methaemoglobin.

Since many cyanides are unstable and emit volatile hydrogen cyanide gas, sampling, storage, and analysis must be done with caution, preferably immediately upon collection.

The three commonly used techniques (colorimetric, titrimetric, and electrochemical) may all suffer from interference problems, unless proper precautions are taken (ATSDR, 1989).

Metals suppress the transformation of cyanide to formic acid, thus lowering the measured hydrogen cyanide concentration (Dolzine et al., 1982). Carbonyl compounds also decrease the hydrogen cyanide recovery (Honig et al., 1983), as in the case with soybean samples, in which carbonyl compounds occur naturally.

Sodium thiosulfate can interfere with potentiometric (Sylvester et al., 1982) or colorimetric analysis (Ganjeloo et al., 1980). Care should be taken, since it is often used as an antidote to treat chemical poisoning.

Continuous monitoring of cyanide is also available using equipment based on diffusion and amperometric detection of hydrogen cyanide (NIOSH, 1976).

Detection limits for the different methods for hydrogen cyanide range from 0.8 to 400 mg/m³ for air samples, from 0.04 to 200 µg/litre for aqueous samples, and from 0.8 to 300 µg/litre for biological samples. NIOSH Method 7904 for workplace air has a limit of detection of 2.5 µg cyanide (NIOSH, 1994).

Hydrogen cyanide is ubiquitous in nature. It is found in the stratosphere and non-urban troposphere (US EPA, 1990). It is released into the atmosphere from biomass burning, volcanoes, and natural biogenic processes from higher plants, bacteria, algae, and fungi (Fiksel et al., 1981; Cicerone & Zellner, 1983; Way, 1984; ATSDR, 1997; Li et al., 2000). An estimate of the amount of cyanide released to the environment from natural biogenic processes is not available (ATSDR, 1997).

Cyanide occurs naturally as cyanogenic glycosides in at least 2000 plants (Figure 1). Amygdalin (d-mandelonitrile-beta-d-glucoside-6-beta-d-glucoside) has been found in about 1000 species of plants, including cassava (tapioca, manioc), sweet potato, corn, cabbage, linseed, millet, and bamboo, in pits of stone fruits, such as cherries, peaches, and apricots, and in apple seeds (JECFA, 1993; Sharma, 1993; Padmaja, 1995). It is also present in bitter almonds and American white lima beans (Ermans et al., 1972). After ingestion, linamarin can be hydrolysed by either cassava linamarase or an endogenous beta-glucosidase to yield d-glucose and ACH (Frakes et al., 1986a).

4.2.1 Production

Hydrogen cyanide is principally produced by two synthetic catalytic processes involving the reaction of ammonia and natural gas (or methane) with or without air. It is also obtained as a by-product in the production of acrylonitrile by the ammoxidation of propylene, which accounts for approximately 30% of the worldwide production of hydrogen cyanide.

Sodium and potassium cyanides are principally prepared by the direct reaction of hydrogen cyanide with the respective alkali in closed systems (European Chemicals Bureau, 2000a,b). Sodium cyanide is also prepared to a lesser extent by melting sodium chloride with calcium cyanamide or by heating sodium amide salt with carbon.

Calcium cyanide is produced by the reaction of coke, coal, and limestone.

Cyanogen chloride is a reaction product of organic precursors with hypochlorous acid in the presence of ammonia and may be formed as a by-product of the chloramination of water (WHO, 1996; IPCS, 2000a).

ACH was first produced in the 1930s as an intermediate in the production of methyl methacrylate from hydrogen cyanide. It is currently produced from the liquid-phase reaction of hydrogen cyanide and acetone in the presence of an alkali catalyst at atmospheric pressure (ECETOC, 2004).

Hydrogen cyanide capacity is generally treated as the sum of purposeful direct synthesis and that derived as a by-product of acrylonitrile production. Annual US hydrogen cyanide capacity by 11 companies in 1991 was 666 000 tonnes. US production of hydrogen cyanide from 1983 to 1989 rose from 300 000 to 445 000 tonnes (Pesce, 1993). Output of hydrogen cyanide in the USA was 545 000 tonnes in 1992 (Cohrssen, 2001). Worldwide annual production and capacity of hydrogen cyanide in 1992 were estimated to be 950 000 and 1 320 000 tonnes, respectively (Pesce, 1993; Cohrssen, 2001). It has been estimated that the present total annual production of hydrogen cyanide worldwide is 1.4 million tonnes (Mudder & Botz, 2000).

4.2.2 Use

In 1983, the major end uses of hydrogen cyanide in the USA were in the production of adiponitrile (200 000 tonnes), ACH (128 000 tonnes), cyanuric chloride (28 500 tonnes), sodium cyanide (69 000 tonnes), chelating agents (15 800 tonnes), and nitrilotriacetic acid (10 100 tonnes) and for miscellaneous uses (20 000 tonnes) (US EPA, 1990). Hydrogen cyanide is also used in the production of methyl methacrylate, methionine and its hydroxylated analogues, and potassium cyanide (ATSDR, 1997; ECETOC, 2004).

Sodium cyanide is extensively employed in a large number of industrial processes, including electroplating and case-hardening of metals; the extraction (cyanidation) of gold and silver from ores; base metal flotation; coal gasification; and the fumigation of ships, railroad cars, buildings, grain silos, flour mills, seeds in vacuum chambers, and soil. Large quantities of sodium cyanide are used to introduce cyano groups into organic compounds, in particular through a reaction with organic halogen compounds to yield nitriles. The nitriles can then be converted to a variety of carboxylic acids, amides, esters, and amines. Potassium cyanide is used for electrolytic refining of platinum, for metal colouring, and as an electrolyte for the separation of gold, silver, and copper from platinum (Eisler et al., 1999; Patnaik, 1999; ACGIH, 2001; ECETOC, 2004). Cyanide salts are used as chelating agents, and the complex cyanides of copper, zinc, and cadmium are used in electroplating processes, principally the plating of iron, steel, and zinc (ECETOC, 2004).

Calcium cyanide is used chiefly as a fumigant, because it readily releases hydrogen cyanide when exposed to air; as a fertilizer, defoliant, herbicide, and rodenticide; as a stabilizer for cement; and in stainless steel manufacture (ACGIH, 2001).

Cyanogen is used as a fumigant, as a fuel gas for welding and cutting heat-resistant metals, and as a rocket and missile propellant (ATSDR, 1997).

Cyanogen chloride is used as a fumigant gas and as a reagent in chemical synthesis.

Cuprous cyanide is used in plating baths for silver, brass, and copper–tin alloy plating (ATSDR, 1997), as an antifouling agent in marine paint, and as an insecticide and fungicide (Windholz, 1983).

Potassium silver cyanide is used in silver plating and as a bactericide.

Potassium ferricyanide is used chiefly for blueprints, in photography, for staining wood, in calico printing, and in electroplating.

Sodium ferrocyanide is used in ore flotation, as an anti-caking agent in rock salt, and in photography for bleaching, toning, and fixing.

Sodium nitroprusside has been used as an antihypertensive agent and in congestive heart failure and is used for deliberate induction of hypotension during certain neurosurgical procedures.

ACH is used in preparative transcyanohydrination reactions.

4.2.3 Release to the environment

More than 30 large-scale accidental releases of cyanide to water systems have been reported since 1975; these include transportation accidents, pipe failures, and tailings dam-related releases (Korte et al., 2000; Mudder & Botz, 2000).

Non-point sources of cyanide released to water can result from runoff from cyanide-containing anti-caking salts (i.e., sodium ferrocyanide) used on roads, migration from landfills, and agricultural and atmospheric fallout and washout (ATSDR, 1997).

The extraction of gold from low-grade ores by cyanidation processes was estimated to result in a worldwide emission of 20 000 tonnes of hydrogen cyanide into the atmosphere (Korte & Coulston, 1998). Another estimate suggested that currently 45 300 tonnes of cyanide are used in the USA in the cyanidation process. The wastes from these processes result in large cyanide-containing ponds near the mining operations (Clark & Hothem, 1991; Henny et al., 1994; Ma & Pritsos, 1997; Eisler et al., 1999).

The major point sources of cyanide release to water are discharges from gold mining plants, publicly owned wastewater treatment plants, iron and steel production, and the organic chemical industries. An estimated 3 billion litres (i.e., 3 × 10⁹ litres) of wastes containing cyanides were generated in the USA in 1983, principally from spent cyanide plating bath solutions from electroplating operations (except for precious metals) and from spent stripping and cleaning bath solutions from electroplating operations (Grosse, 1986).

During cassava starch production, large amounts of cyanoglycosides are released and hydrolysed by plant-borne enzymes, leading to cyanide concentrations in wastewater as high as 200 mg/litre (Siller & Winter, 1998).

The major sources of cyanide released to air, in addition to exhaust from vehicle emissions, are diverse, including chemical manufacturing (hydrogen cyanide, methyl methacrylate, acrylonitrile); processing industries, such as metallurgical industries and metal plating (i.e., electroplating metals and finishing [metal polishes]); extraction of gold and silver from low-grade ores; volatilization from cyanide wastes disposed of in landfills and waste ponds; the production of coke or other coal carbonization procedures; emissions from municipal solid waste incinerators; and direct release of cyanides to the atmosphere resulting from fumigation operations, combustion of polyurethanes, acrylonitrile, and polyamide plastics, and combustion of wool, silk, and fibres (Carotti & Kaiser, 1972; Fiksel et al., 1981; ATSDR, 1997; Eisler et al., 1999).

An estimated total of 1 million tonnes of hydrogen cyanide, amounting to 73.1% of the total environmental releases in the USA, was discharged to the air from manufacturing and processing facilities (ATSDR, 1997).

The estimated amounts of hydrogen cyanide released to air in 1976 from the most common non-industrial sources were as follows: agricultural pest control, 62 tonnes; incineration, 8.2–82 tonnes; and tobacco smoke, 5.9–340 tonnes (Fiksel et al., 1981; ATSDR, 1997).

In 2001, from various locations in the USA, about 1300 tonnes of hydrogen cyanide were released on- and off-site; 540 tonnes were emitted to the atmosphere, 0.1 tonne was released to surface waters, 770 tonnes were injected into Class I wells,³ and 0.42 tonne was released to land (US EPA, 2003c). In 2001, from various locations in the USA, approximately 3400 tonnes of cyanides (not otherwise specified) were released on- and off-site; 220 tonnes were emitted to the atmosphere, 47 tonnes were released to surface waters, 1800 tonnes were injected into Class I wells, and 1300 tonnes were released to land (US EPA, 2003c).

Hydrogen cyanide has been found following the combustion of a number of synthetic polymers. The maximum yield of hydrogen cyanide per gram of polyurethane foam ranged from 0.37 to 0.93 mg under non-flaming conditions and from 0.5 to 1.02 mg under flaming combustion (Sklarew & Hayes, 1984). Hydrogen cyanide concentrations in the off-gas from the shale oil retorting process ranged from 7 to 44 mg/m³ (Sklarew & Hayes, 1984).

One cigarette without a filter liberates 500 µg hydrogen cyanide, while filter cigarettes liberate only 100 µg in mainstream smoke. Hydrogen cyanide concentrations in mainstream and sidestream smoke ranging from 280 to 550 µg/cigarette and from 53 to 111 µg/cigarette, respectively, have been reported; sidestream:mainstream ratios of hydrogen cyanide concentrations ranged from 0.06 to 0.50 (ATSDR, 1997). The level of hydrogen cyanide found in Canadian cigarette smoke under International Organization for Standardization standard smoking conditions were as follows: mainstream smoke, 32–156 µg/cigarette; and sidestream smoke, 77–136 µg/cigarette (Health Canada, 2002).

The average rate of emission of hydrogen cyanide by automobile exhaust was reported to be 7–9 mg/km for cars not equipped with catalytic converters and on the order of 0.6 mg/km for cars with catalytic converters operating under optimum conditions in the mid- to late 1970s (ATSDR, 1997).

Cyanogen chloride is formed as a reaction product of organic precursors with hypochlorous acid in the presence of ammonia and may be formed as a by-product of the chloramination of water (e.g., via the reaction of humic substances with chlorine and chloramine used for water disinfection) (Ohya & Kanno 1987; WHO, 1996; IPCS, 2000a). In the USA, 35% of the surface water plants and 23% of the groundwater plants using chloramine as a primary or secondary disinfectant report cyanogen chloride formation (US EPA, 2002).

Cyanogen is generated in the combustion of nitrogen–carbon compounds and appears in automobile exhaust gases and gases from blast furnaces (CHEMINFO, 1998).

Cyanide is present in the air mostly as a gas, and cyanides have the potential to be transported over long distances from their respective emission sources.

5.1.1 Air

Cyanide is found in ambient air as hydrogen cyanide and to a smaller extent in particulate matter. The concentration of hydrogen cyanide measured since 1981 in the northern hemisphere’s non-urban troposphere ranged from 180 to 190 ng/m³ (Cicerone & Zellner, 1983; Jaramillo et al., 1989).

Ambient air monitoring data for cyanides in Bulgaria in areas near petrochemical plants showed concentrations ranging from 0.2 to 0.8 µg/m³ (annual average value) (Kaloyanova et al., 1985).

Cyanide has been detected at levels of 20–46 mg/m³ in the air near large-scale cassava processing facilities in Nigeria (Okafor & Maduagwu, 2000).

5.1.2 Water

Cyanides, reported as cyanide, hydrogen cyanide, sodium cyanide, potassium cyanide, calcium cyanide, or copper(I) cyanide, have been detected in surface water samples at 70 of the 154 hazardous waste sites where they were studied in the USA; they have also been detected in groundwater samples at 191 of the 419 waste sites studied and in leachate samples of 16 of the 52 sites studied. The median concentrations in the positive samples were 160 µg/litre for groundwater, 70 µg/litre for surface water, and 479 µg/litre for the leachates (HazDat, 2003).

Data from the US National Urban Runoff Program in 1982 revealed that 16% of urban runoff samples collected from four cities across the USA contained cyanides at levels of 2–33 µg/litre (ATSDR, 1997).

According to the US Environmental Protection Agency’s (EPA) STORET database, the mean cyanide concentration in most surface waters in the USA is less than 3.5 µg/litre. Data from the late 1970s to early 1980s indicated that the levels are higher only in limited areas and may exceed 200 µg/litre (ATSDR, 1997).

In 1978, a US EPA survey of drinking-water supplies showed that about 7% of the supplies had cyanide concentrations greater than 10 µg/litre (US EPA, 1993a). Cyanogen chloride is one of the 18 compounds that occur most frequently (8 of 10 city surveys) in potable water within the framework of the US National Organic Reconnaissance Survey (Bedding et al., 1982). In a survey in 1987 of over 35 drinking-water supplies, the quarterly median cyanogen chloride concentrations in drinking-water ranged from 0.45 to 0.80 µg/litre (from 0.19 to 0.34 µg cyanide/litre) (Krasner et al., 1989; ATSDR, 1997). More current data regarding the cyanide and cyanogen chloride levels in drinking-water are lacking.

Levels of 1.58–7.89 mg cyanide/litre have been found in natural water sources near large-scale cassava processing facilities in Nigeria (Okafor et al., 2001).

5.1.3 Soil

Cyanide has been identified in the soil of hazardous waste sites in the USA; the median concentrations for the positive sites were 0.8 mg/kg in the subsurface soil (found at 77 sites of the 124 studied) and 0.4 mg/kg in the topsoil (51 positive sites out of 91 sites) (HazDat, 2003).

Cyanide-containing wastes are commonly found in soils at former manufactured gas plant sites in the USA. Most concentrations of cyanide compounds at the manufactured gas plant sites are below 2000 mg/kg. The most prevalent types of cyanide compounds are iron-complexed forms, e.g., ferric ferrocyanide (Prussian blue), rather than the highly toxic free cyanide forms. Iron-complexed cyanides, dominated by the ferrocyanide ion, comprise over 97% of total cyanides in either weathered or unweathered soils (Shifrin et al., 1996).

5.1.4 Food

Many edible plants contain cyanogenic glycosides, whose concentrations can vary widely as a result of genetic and environmental factors, location, season, and soil types (Ermans et al., 1980; JECFA, 1993). Some of the foodstuffs and their cyanide contents are shown in Table 3. Cassava tubers vary widely in their cyanogenic glycoside content, although most varieties contain 15–400 mg cyanide/kg fresh weight. Occasionally varieties of cassava tubers contain 1300–2000 mg cyanide/kg fresh weight, and cassava leaves contain 1000–2000 mg cyanogenic glucosides/kg on a dry matter basis (Padmaja, 1995). Fermentation of cassava pulp for 96 h during gari production reduced the hydrogen cyanide content by 50%; soaking of sliced cassava for 24 h, 40%; and sun-drying, some 15% (Kendirim et al., 1995). It should be noted that the ranges of cyanide concentrations shown in Table 3 are very broad in several cases (i.e., cereals and their products, soy protein products, and apricot pits), which may be due to their different sources and differences in analytical procedures; as well, the values may reflect the older literature.

Hydrogen cyanide can be produced by hydrolytic reaction catalysed by one or more enzymes from the plants containing cyanogenic glycosides. In kernels, for example, this reaction is catalysed by the enzyme emulsin (Lasch & El Shawa, 1981) when the seeds are crushed and moistened. Amygdalin (which is also present in cassava, bitter almonds, and peach stones) is converted to glucose, benzaldehyde, and hydrogen cyanide (Figure 2) (IPCS, 1992). Hydrogen cyanide release can occur during maceration, which activates intracellular beta-glucosidases. This reaction can also result from chewing, which causes the enzyme and the cyanogenic glycosides stored in different compartments to combine (Ermans et al., 1980; Nahrstedt, 1993). The reaction occurs rapidly in an alkaline environment, and the hydrolysis is complete in 10 min. Hydrolysis is possible in an acid solution and takes place slowly.

Fig. 2: Hydrolysis of amygdalin

Liberation of hydrogen cyanide from cyanogenic glycosides occurs usually after ingestion and hydrolysis by the glycosidases of the intestinal microflora and, to a lesser degree, by glucosidases of the liver and other tissues (Padmaja, 1995). However, hydrolysis may also occur during the preparation of the food, which may account for the short interval between ingestion and the appearance of signs of poisoning in some accidents (Lasch & El Shawa, 1981).

Table 3: Cyanide concentrations in food products.^a

^a From Nartey (1980); Honig et al. (1983); JECFA (1993); ATSDR (1997).

5.1.5 Other

Laetrile (another name for amygdalin derived from apricot kernels), which was formerly used as an anticancer agent, releases cyanide upon metabolism. Bitter almonds and apricot pits containing cyanogenic glycosides are still sold in health food stores and over the Internet (Suchard et al., 1998). Other drugs, such as sodium nitroprusside, which is used as an antihypertensive and in congestive heart failure (Guiha et al., 1974; Tinker, 1976; Aitken et al., 1977; Schultz, 1984; Rindone & Sloane 1992), also liberate hydrogen cyanide in the body. In sodium nitroprusside, the CN^– moiety represents 44% by weight of the molecule. Some aliphatic nitriles that are widely used in the chemical industry — i.e., acetonitrile (IPCS, 1993), acrylonitrile (IARC, 1999), succinonitrile, and adiponitrile — also release cyanide upon metabolism (Willhite & Smith, 1981).

5.2.1 General population

The general population may be exposed to cyanide from ambient air, drinking-water, and food.

Based on an atmospheric hydrogen cyanide concentration of 190 ng/m³ and an average daily inhalation of 20 m³ air, the inhalation exposure of the general US non-urban, non-smoking population to hydrogen cyanide is estimated to be 3.8 µg/day (ATSDR, 1997).

Based on a daily drinking-water consumption of 2 litres for an adult, the daily intake of cyanogen chloride is estimated to be 0.9–1.6 µg (equivalent to 0.4–0.7 µg of cyanide) (ATSDR, 1997) for cyanogen chloride concentrations in water of 0.45–0.80 µg/litre (0.19–0.34 µg cyanide/litre).

Among the general population, subgroups with the highest potential for exposure to cyanide include active and passive smokers, individuals involved in large-scale processing of foods high in cyanogenic glycosides, individuals consuming foods high in cyanogenic glycosides, and, to a lesser degree, fire-related smoke inhalation victims.

Human exposure to cyanide by dietary intake is estimated to be potentially of major significance for cassava-consuming populations; cassava has been estimated to be the staple food for 500 million people. However, data on the concentrations of cyanides in the total diet are lacking; hence, the daily cyanide intake from food cannot be calculated. For human consumption, cassava can be eaten raw, cooked, or grated and roasted into flour and eaten as "gari," which is the common form in Nigeria (Kendirim et al., 1995). In Mozambique, it was estimated that in families affected by the "mantakassa" disease (spastic paraparesis; see section 8), the daily intake of cyanogens was 14–30 mg (as cyanide) at the time of a mantakassa epidemic in 1981 (Ministry of Health, Mozambique, 1984b). In Nigeria, it was estimated that the intake of hydrogen cyanide in the tropical ataxia-endemic areas may be as high as 50 mg/day (Osuntokun, 1981).

Urinary excretion of thiocyanate has been applied in the biological monitoring of exposure to cyanogenic glycosides, especially among cassava-consuming populations. The average urinary thiocyanate concentration among children in the Bandundu region of the Democratic Republic of the Congo (formerly Zaire) was 757 µmol/litre in the south and 50 µmol/litre in the north (both populations consumed cassava as their staple diet, but the cassava was well processed in the north and inadequately processed in the south). These concentrations can be compared with an average of 31 µmol/litre in a non-smoking Swedish reference population (Banea-Mayambu et al., 2000). In the same Bandundu region, it was shown that there was a marked seasonal variation in urinary thiocyanate concentrations in the villages with a high "konzo" (spastic paraparesis) incidence (563–627 µmol/litre in the dry season and 344–381 in the wet season), while the average in non-konzo areas was 241 µmol/litre (Banea-Mayambu et al., 1997). In Mozambique, the average urinary thiocyanate levels among healthy children from areas with epidemic spastic paraparesis varied between 33 and 1175 µmol/litre, whereas levels in areas with no paraparesis were between 18 and 400 µmol/litre (Casadei et al., 1990). In Nampula province in Mozambique, where spastic paraparesis epidemics had been observed in 1981–1982 and during the civil war in 1992–1993, average urinary thiocyanate concentrations among schoolchildren in five areas were between 225 and 384 µmol/litre in October 1999 (Ernesto et al., 2002). In Malawi, in an area where cassava was typically soaked for 3–6 days for processing to flour, urinary thiocyanate concentrations were between 2 and 410 µmol/litre, with a median of 32 µmol/litre (Chiwona-Karltun et al., 2000).

5.2.2 Occupational exposure

The principal routes of occupational exposure to cyanides are via inhalation and, to a lesser degree, skin absorption. Skin absorption may be significant under some circumstances — for example, when airborne concentrations are very high, such as in fumigation operations. It may occur as well when personal protection is inadequate and operators are splashed.

Workers involved in electroplating, metallurgy, pesticide application, firefighting, gas works operations, tanning, blacksmithing, metal cleaning, photoengraving, photography, and the manufacture of steel, cyanides, adiponitrile and other nitriles, methyl methacrylate, cyanuric acid, dyes, pharmaceuticals, or chelating agents have the potential to be occupationally exposed to higher concentrations of cyanide (Prohorenkov & Kolpakov, 1978; Philips, 1989; IPCS, 1992; Banerjee et al., 1997).

A number of illustrative levels of cyanide in the breathing zone of workers in working environments monitored at different production facilities in the USA during the period 1976–1982 have been reported. The concentration of cyanide in air at a plating facility of a national airline was 0.001–0.004 mg/m³ (NIOSH, 1982). Concentrations of hydrogen cyanide in air at a plating facility of an electrical and electronic company in Virginia, USA, ranged from 0.07 mg/m³ in a salt pot room to 4.3 mg/m³ in a stripping tank (NIOSH, 1976). The concentration of cyanide in air at a plating facility in Ohio, USA, was 1.7 mg/m³ (NIOSH, 1978).

Hydrogen cyanide is readily absorbed following inhalation, oral, and dermal exposure. Following exposure to cyanide in the atmosphere, toxic amounts of cyanide are absorbed with great rapidity through the bronchial mucosa and alveoli (ATSDR, 1997). Humans retained 58% of the hydrogen cyanide in the lungs after inhaling the gas through normal breathing (Landahl & Herrmann, 1950; ATSDR, 1997). Alkali metal cyanides are rapidly absorbed from the gastrointestinal tract. Absorption is affected by the presence of food in the gut, the pH of the gut, and the lipid solubility of the cyanide compound.

Gastrointestinal absorption of inorganic cyanide salts is slower than pulmonary absorption, and the onset of symptoms is delayed and the severity of symptoms diminished compared with inhalation. When simple cyanide salts such as potassium and sodium cyanide are ingested, free cyanide ion can rapidly bind hydrogen ion to form hydrogen cyanide in the highly acidic medium of the stomach. Essentially all cyanide ingested as cyanide salts will form hydrogen cyanide and will be quickly absorbed. However, after oral intake, only part of the dose reaches the blood due to first-pass metabolism by the liver (ECETOC, 2004).

Liquid cyanide compounds are easily absorbed through intact skin upon direct contact due to their lipid solubility and rapid epidermal penetration. Skin absorption of vapours of hydrogen cyanide is also possible when the air concentrations are high. The amount and rate of absorption of cyanides from aqueous solutions or atmospheric hydrogen cyanide depend upon the presence of moisture in the skin, concentration and pH of the solution, the surface area of contact, and the duration of contact (Dugard, 1987). In vitro studies with human skin have shown that penetration of sodium cyanide in aqueous solution through skin decreases with increasing pH (increasing dissociation), reflecting the more rapid absorption of the undissociated hydrogen cyanide. The permeability constant measured for the cyanide ion in aqueous solution was 3.5 × 10^–4 cm/h, and that calculated for hydrogen cyanide was 1 × 10^–4 cm/h (Dugard, 1987).

Hydrogen cyanide has a pK_a of 9.22; thus, at physiological pH (about pH 7), hydrocyanic acid is distributed in the body as hydrogen cyanide and is not present as the free cyanide ion. Hence, the form of cyanide to which exposure occurs, the salt or the free acid, does not influence distribution, metabolism, or excretion from the body (ECETOC, 2004). Inhaled or percutaneously absorbed hydrogen cyanide passes immediately into the systemic circulation. The distribution of cyanide to the various tissues is rapid and fairly uniform. Somewhat higher levels are generally found in the liver, lungs, blood, and brain. The tissue levels of hydrogen cyanide were 0.75, 0.42, 0.41, 0.33, and 0.32 mg/100 g of tissue in lung, heart, blood, kidney, and brain, respectively, in a man who died following inhalation exposure to hydrogen cyanide gas (Gettler & Baine, 1938; Ballantyne, 1983a; ATSDR, 1997; ECETOC, 2004). In contrast, high proportions of ingested sodium and potassium cyanide will pass through the liver and are detoxified by the first-pass effect.

The major portion of cyanide in blood is sequestered in the erythrocytes, and a relatively small proportion is transported via the plasma to target organs. Cyanide is concentrated in red blood cells at a red blood cell to plasma ratio of 199:1; levels in plasma reflect tissue levels better than levels in whole blood or erythrocytes. Small but significant levels of cyanide are found in normal blood plasma (<140 µg/litre) and other tissues (<0.5 mg cyanide/kg) of humans without known occupational cyanide exposure (Feldstein & Klendshoj, 1954). These levels are related mostly to exposure to cyanogenic food, vitamin B₁₂, and tobacco smoke. A detailed survey of normal plasma cyanide levels in 10 cases showed a maximum level of 106 µg/litre, with a mean of 48 µg/litre (Feldstein & Klendshoj, 1954). After cessation of exposure, plasma cyanide levels tend to return to normal within 4–8 h (Feldstein & Klendshoj, 1954; Ansell & Lewis, 1970).

In rats dosed by gavage, highest concentrations of cyanide were found in the liver, followed by the lungs and blood (Yamamoto et al., 1982). After inhalation exposure, the highest concentrations of cyanide in rats were found in the lungs, followed by the blood and liver.

Cyanide has not been shown to accumulate in the blood and tissues following oral exposue to inorganic cyanide (ATSDR, 1997), and no cumulative effect on the organism during repeated exposure has been demonstrated. There is a cumulative effect of exposure to thiocyanate (from the breakdown of cyanogenic glycosides in food plants), resulting in thyroid toxicity, including goitre and cretinism (Nahrstedt, 1993).

A number of illustrative levels of cyanide in organs and blood after oral intake in humans (Ansell & Lewis, 1970; ATSDR, 1997) and rabbits (Ballantyne, 1983a) have been reported. For a given exposure route, whole blood and serum cyanide levels are quite similar for different species (Ballantyne, 1983a).

Although cyanide can interact with substances such as methaemoglobin in the bloodstream, the majority of cyanide metabolism occurs within the tissues. Cyanide is metabolized in mammalian systems by one major route and several minor routes. The major route of metabolism for hydrogen cyanide and cyanides is detoxification in the liver by the mitochondrial enzyme rhodanese, which catalyses the transfer of the sulfane sulfur of thiosulfate to the cyanide ion to form thiocyanate (Figure 3) (Williams, 1959; Ansell & Lewis, 1970). About 80% of cyanide is detoxified by this route. The rate-limiting step is the amount of thiosulfate. While rhodanese is present in the mitochondria of all tissues, the species and tissue distributions of rhodanese are highly variable. In general, the highest concentrations of rhodanese are found in the liver, kidney, brain, and muscle, but the supply of thiosulfate is limited (Aminlari et al., 1994). Rhodanese is present in rat nasal mucosal tissues, particularly in the olfactory region, at a 7-fold higher concentration (on a per milligram of mitochondrial protein basis) than in the liver (Dahl, 1989). Dogs have a lower overall activity of rhodanese than monkeys, rats, and rabbits (ATSDR, 1997).

Fig. 3: Basic processes involved in the metabolism of cyanide (ATSDR, 1997)

A number of other sulfur transferases can also metabolize cyanide, and albumin, which carries elemental sulfur in the body in the sulfane form, can assist in the catalysis of cyanide to thiocyanate as well (Sylvester et al., 1982; Westley et al., 1983). Cyanide and thiocyanate can also be metabolized by several minor routes, including the combination of cyanide with hydroxycobalamin (vitamin B₁₂a) to yield cyanocobalamin (vitamin B₁₂) (Boxer & Rickards, 1952) and the non-enzymatic combination of cyanide with cystine, forming 2-iminothiazoline-4-carboxylic acid, which appears to be excreted without further change (Rieders, 1971) (Figure 3).

In studies with rats orally administered potassium cyanide and maintained for up to 4 weeks on either a balanced diet or a diet lacking the sulfur amino acids L-cystine and L-methionine, a strongly positive linear relationship was found between blood cyanide and plasma cyanate (OCN^–) concentration (Tor-Agbidye et al., 1999). It was suggested that in Africa, where there are protein-deficient populations whose levels of sulfur-containing amino acids are low, cyanide (from prolonged use of cassava) may conceivably be converted to cyanate, which is known to cause neurodegenerative disease in humans and animals.

While absorbed cyanide is principally excreted as thiocyanate in the urine, traces of free hydrogen cyanide may also be excreted unchanged in the lungs, saliva, sweat, or urine (Hartung, 1982), as carbon dioxide in expired air, or as beta-thiocyanoalanine in saliva and sweat (Friedberg & Schwartzkopf, 1969; Hartung, 1982; JECFA, 1993).

Thiocyanate was found in the urine of non-exposed people at average concentrations of 2.16 mg/litre urine for non-smokers and 3.2 mg/litre urine for smokers (Chandra et al., 1980). Urinary excretion of thiocyanate was monitored in a man after ingestion of about 3–5 g potassium cyanide (15–25 mg cyanide/kg body weight) (Liebowitz & Schwartz, 1948; ATSDR, 1997). The results indicated that the patient excreted 237 mg of thiocyanate over a 72-h period. This quantity was substantially more than the normal average amount of thiocyanate in urine, which varies from 0.85 to 14 mg/24 h (ATSDR, 1997).

The limiting factor in cyanide metabolism is the low concentration of the sulfur-containing substrates in the body — primarily thiosulfate, but also cystine and cysteine. The rate of spontaneous detoxification of cyanide in humans is about 1 µg/kg body weight per minute (Schultz et al., 1982), which is considerably slower than in small rodents (Schubert & Brill, 1968) or dogs (Lawrence, 1947).

After administration of an intravenous dose of 3–4 mg potassium cyanide to beagle dogs, blood levels decreased in a manner consistent with first-order elimination kinetics for the first 80 min (Bright & Marrs, 1988). The half-time for this phase was about 24 min, corresponding to an elimination rate constant of 0.03/min. After 80 min, the blood cyanide concentrations fell at a slower rate, with a half-time of 5.5 h. In rats, after a single oral dose, the blood elimination half-time of cyanide was 14.1 min, corresponding to a rate constant of 0.05/min (Leuschner et al., 1991).

Rats treated orally with 2 mg cyanide/kg body weight excreted 47% of the dose in the urine within 24 h (Farooqui & Ahmed, 1982). A [¹⁴C]cyanide intake study with rats (exposed to a regular intake of cyanide in the diet for 3 weeks) indicated the existence of a gastrointestinal circulation of thiocyanate, in which a substantial amount of thiocyanate, which was excreted into the stomach contents of the rat, was reabsorbed by the intestine into the body fluid, to be partly excreted in the urine and partly resecreted into the gastric contents (Okoh & Pitt, 1982). The relative proportion of cyanide to thiocyanate in body fluids is about 1:1000 (Pettigrew & Fell, 1973). The half-time for hydrogen cyanide elimination is about 1 h (Ansell & Lewis, 1970; IPCS, 1992).

Half-time values of the principal metabolite thiocyanate in humans have been reported as 4 h (Blaschle & Melmon, 1980), 2 days (Bödigheimer et al., 1979), and 2.7 days (Schultz et al., 1979). In patients with renal insufficiency, a mean half-time of 9 days was reported (Bödigheimer et al., 1979).

Levels of the cyanide metabolite thiocyanate in blood serum and plasma and urine have been employed as indicators of high cyanide exposure in humans (Lauwerys & Hoet, 2001). However, at low levels of occupational exposure, the relationship between exposure and urinary thiocyanate concentrations shows a wide inter- and intraindividual variation due to a variety of factors (e.g., diet); therefore, measuring cyanide and/or thiocyanate levels in blood and urine is not a reliable biomarker for exposure to low concentrations of cyanide.

Symptoms of cyanide toxicity can occur within seconds of inhalation of hydrogen cyanide or within minutes of ingestion of cyanide salts. Onset may be delayed up to 12 h after ingestion of cyanogenic glycosides, nitriles, or thiocyanates.

Inhalation LC₅₀ values of hydrogen cyanide in rats ranged from 158 mg/m³ for 60 min to 3778 mg/m³ for 10 s (Ballantyne, 1983a). Exposure of mice to cyanide resulted in similar LC₅₀ values (Higgins et al., 1972; Matijak-Schaper & Alarie, 1982), and LC₅₀ values of hydrogen cyanide in rabbits ranged from 2432 mg/m³ for 45 s to 208 mg/m³ for 35 min (Ballantyne, 1983a). The concentration of hydrogen cyanide in inhaled air markedly affects the acute toxicity: the total amount of hydrogen cyanide inhaled leading to death is disproportionately larger at low exposure levels than at high exposure levels (and thus the time leading to death is disproportionately longer) (Table 4). A similar dose rate dependence of acute toxicity was observed in hamsters: (pregnant) hamsters did not show signs of toxicity until they had received a total dose of 30–40 times the single subcutaneous LD₅₀ when given sodium cyanide subcutaneously by an osmotic minipump (Doherty et al., 1982).

Table 4: Acute inhalation toxicity of hydrogen cyanide vapour in rats.^a

^a From Ballantyne (1983a).

^b Details of calculation not provided.

A similar dose rate dependence has also been noted for oral exposure. While the single-dose gavage LD₅₀ of potassium cyanide was 10 mg/kg body weight in Sherman rats, no mortality was observed when a dose of 250 mg/kg body weight was given in the diet for 90 days. The author ascribed this remarkable difference to the difference in dose rate (bolus vs. dietary exposure): at the low dose rate, the liver is capable of detoxifying cyanide before it reaches the general circulation (Hayes, 1967).

The information on the relative sensitivity of various animals to hydrogen cyanide vapours is mainly based on early studies by Barcroft (1931). In an extensive series of inhalation experiments, in which different species of animals were exposed to usually 5–8 different concentrations of hydrogen cyanide and followed until death, he showed that the lethal time (time for 50% of animals to die at 1000 mg/m³) was 0.8, 1.0, 1.0, 1.0, 2.0, 2.0, 3.0, and 3.5 min for dogs, mice, cats, rabbits, rats, guinea-pigs, goats, and monkeys. Extrapolation to zero mortality gave the following maximal non-lethal concentrations to dogs, rats, mice, rabbits, monkeys, cats, goats, and guinea-pigs: 100, 100, 140, 180, 180, 180, 240, and 400 mg/m³. There was thus a rough inverse relationship between sensitivity to hydrogen cyanide and body size, dogs being a notable exception.

Severe dyspnoea has been observed in dogs exposed to 170–740 mg hydrogen cyanide/m³ for 2–12 min. Pulmonary oedema was found in some dogs at necropsy (Haymaker et al., 1952). Exposure of cynomolgus monkeys to 110–180 mg hydrogen cyanide/m³ led quickly to incapacitation; the time to incapacitation was inversely related to the level of exposure, being 8 min for 180 mg/m³ and 19 min for 110 mg/m³ (Purser et al., 1984). At 70 mg/m³ for 30 min, slight depression of the central nervous system was reported (Purser, 1984).

Following oral administration to rats, LD₅₀s of hydrogen cyanide, sodium cyanide, and potassium cyanide are very similar: 0.156, 0.117, and 0.115 mmol/kg body weight, respectively, i.e., 3–4 mg cyanide/kg body weight (Ballantyne, 1983a⁴). In mice, an LD₅₀ of 15.8 mg potassium cyanide (corresponding to 6 mg cyanide)/kg body weight has been reported (Ferguson, 1962). In rabbits, hydrogen cyanide, potassium cyanide, and sodium cyanide appear equitoxic on a molar basis (LD₅₀s of 0.092, 0.104, and 0.090 mmol/kg for hydrogen cyanide, sodium cyanide, and potassium cyanide, respectively); rabbits appeared to be somewhat more susceptible to cyanides than mice or rats (Ballantyne, 1983a).

Following application of cyanides in aqueous solution to the intact skin of New Zealand rabbits, the dermal LD₅₀s of hydrogen cyanide, sodium cyanide, and potassium cyanide were 0.260, 0.298, and 0.343 mmol/kg body weight, respectively (corresponding to 6.8, 7.7, and 8.9 mg cyanide/kg body weight) (Ballantyne, 1983a). The dermal toxicity of cyanide, especially of hydrogen cyanide, is markedly greater following application on abraded skin, which enhances the penetration of cyanide (LD₅₀s of 0.087, 0.220, and 0.30 mmol/kg for hydrogen cyanide, sodium cyanide, and potassium cyanide, respectively) (Ballantyne, 1987).⁵ Local contact may produce mild burns (IPCS, 1992).

In a range-finding toxicity study on ACH, the oral LD₅₀ in rats and dermal LD₅₀ in rabbits were both 17 mg ACH (5.2 mg cyanide)/kg body weight; 4-h inhalation mortality in rats was 2/6 at 220 mg/m³ and 6/6 at 440 mg/m³ (Smyth et al., 1962). In a 4-week inhalation study, in which the average measured concentrations of ACH were 33, 106, and 211 mg/m³, 3/10 male rats died after the first 6-h exposure at the highest concentration (Monsanto Co., 1985c). On this first day of exposure, the four measured concentrations of ACH in the chamber were 196, 214, 225, and 225 mg/m³, 225 mg/m³ being the highest individual concentration measured during the 1-month exposure (when no further mortality was observed). No similar acute mortality was observed in a 14-week study at exposures up to 204 mg/m³, in the male fertility study (202 mg/m³), or in the female fertility study (207 mg/m³) (see sections 7.3 and 7.6) (Monsanto Co., 1984a, 1985a,b).

There are no qualitative differences in acute poisoning between cyanide compounds, since the cyanide ion is the common agent that primarily inhibits tissue cytochrome oxidase activity in rats, mice, and rabbits, with resulting anoxia (Way, 1984; US EPA, 1988). Although acute oral doses of cyanide cause cardiovascular, respiratory, and neuroelectric alterations, many studies have shown that the brain is the organ most sensitive to cyanide toxicity. Death from cyanide poisoning is believed to result from central nervous system depression, subsequent to inhibition of brain cytochrome oxidase activity (Way, 1984). Typical signs of toxicity after inhalation of hydrogen cyanide in test species include rapid breathing, weak and ataxic movements, convulsions, loss of voluntary movement, coma, and decrease and irregularities in respiratory rate and depth preceding death (Ballantyne, 1983b; European Chemicals Bureau, 2000a,b).

7.2.1 Oral

Forty-six male adult inbred Wistar rats were used in four experimental groups and one control group and treated with 0, 0.3, 0.9, 3.0, or 9.0 mg potassium cyanide/kg body weight per day in the drinking-water for 15 days. This was equivalent to 0, 0.12, 0.36, 1.2, and 3.6 mg cyanide/kg body weight per day. The high-dose group exhibited a 70% lower body weight gain than the control animals. In qualitative histological analysis, without statistical treatment or morphometric analysis, changes were observed in the kidney, liver, and thyroid. Cytoplasmic vacuolation, considered to reflect hydropic degeneration of proximal tubular epithelial cells, was noted in animals treated at doses of 3.0–9.0 mg potassium cyanide/kg body weight per day and in hepatocytes of those animals treated at a dose of 9.0 mg potassium cyanide/kg body weight per day. A dose-dependent increase in the number of reabsorption vacuoles on follicular colloid in the thyroid gland was noted in all animals of the experimental groups. No changes were observed in serum triiiodothyronine (T₃), thyroxine (T₄), creatinine, or urea levels; a decrease was observed in serum alanine aminotransferase (ALAT) activity at the two lowest exposure levels. Serum aspartate aminotransferase (ASAT) was elevated by 30% at the two lowest dose levels and by 21% at the 3.0 mg potassium cyanide/kg body weight per day dose; it was decreased by 29% at the highest dose level (Sousa et al., 2002).

7.2.2 Inhalation

7.2.2.1 Hydrogen cyanide

No statistically significant increase in the incidence of histopathological changes in the lungs or cardiovascular tissues (e.g., myocardial ultrastructure) compared with the controls was noted in rabbits continuously exposed to 0.6 mg hydrogen cyanide/m³ for 1 or 4 weeks (Hugod, 1981).

Dogs exposed to 50 mg hydrogen cyanide/m³ for 30 min every third day for 28 days exhibited extensive vascular and cellular central nervous system lesions, including vasodilation and haemorrhages (Valade, 1952).

7.2.2.2 Acetone cyanohydrin

Sprague-Dawley rats (10 per sex and dose level) were exposed to ACH in the atmosphere at average concentrations of 33, 106, or 211 mg/m³ for 6 h/day, 5 days/week, for 4 weeks. This was equivalent to hydrogen cyanide concentrations of 10, 34, and 67 mg/m³ (Monsanto Co., 1985c). On the first exposure day, the highest concentration of ACH in the chamber was 225 mg/m³, corresponding to 71 mg hydrogen cyanide/m³, and 3 out of the 10 males at the highest dose level died after the first exposure (see section 7.1). Irritation of the nose and eyes was observed at the two highest exposure levels. High-dose females had decreased blood haemoglobin and elevated blood urea nitrogen concentrations, and total serum protein levels were decreased in high- and mid-dose males; these laboratory changes were within the reference range. No gross or microscopic changes were observed in a wide range of organs at autopsy. No changes were observed in thyroid function, with the exception of an elevated T₃ level in the mid-dose males. The NOAEL (the observed effect being irritation) reported from the study was 33 mg ACH/m³ (10 mg hydrogen cyanide/m³); this can be estimated to correspond to a daily dose of 2.7 mg cyanide/kg body weight per day.⁶

7.3.1 Oral

In 13-week studies, groups of F344/N rats and B6C3F₁ mice (10 of each sex) were administered 0, 3, 10, 30, 100, or 300 mg sodium cyanide/litre in drinking-water (NTP, 1993). The equivalent cyanide ion doses were 0, 0.2, 0.5, 1.4, 4.5, and 12.5 (in males) and 0, 0.2, 0.5, 1.7, 4.9, and 12.5 (in females) mg cyanide/kg body weight per day for rats and 0, 0.3, 1, 3, 9, and 26 mg cyanide/kg body weight per day for mice. No deaths, clinically significant effects on body or organ weights, or histopathological or clinical pathology changes were noted in either rats or mice. In particular, no lesions were found in the brain or thyroid gland. Effects on the reproductive organs were analysed in animals in the three highest dose groups. A slight (7–13%) but statistically significant reduction was observed in sperm motility and in the weight of cauda epididymidis in all studied groups of male rats. In the males of the 300 mg/litre group (12.5 mg cyanide/kg body weight per day), a statistically significant decrease was observed in the weight of the left epididymis, left cauda epididymidis, left testis, and the number of spermatid heads per testis. Sodium cyanide at concentrations of 100 and 300 mg/litre (4.9 and 12.5 mg cyanide/kg body weight per day, respectively) caused a statistically significant increase in the time spent by female rats in proestrus and diestrus relative to estrus and metestrus. In male mice, a statistically significant decrease in the left cauda epididymidis weights was noted at 26 mg cyanide/kg body weight per day, but no changes in sperm motility or spermatid head density were observed. No changes in the estrous cycle length in female mice were noted. The authors noted that the changes in male rats are consistent with a small but measurable adverse effect on reproduction. While these changes are insufficient to decrease fertility in rats, the relative sensitivity of humans to such changes is considered to be greater than that of rats; therefore, a potential for adverse reproductive effects exists in humans (NTP, 1993). One of the source documents (ATSDR, 1997) identified 12.5 mg cyanide/kg body weight per day as the lowest-observed-adverse-effect level (LOAEL), based on all the effects on reproductive organs observed in the male rats, and 4.5 mg cyanide/kg body weight per day as the NOAEL; the findings in female rats were not considered adverse (ATSDR, 1997).⁷

In a 13-week study, male Sprague-Dawley rats were administered potassium cyanide in drinking-water at a dose level of 40, 80, or 160/140 mg/kg body weight per day. These doses correspond to 16, 32, and 64/56 mg cyanide/kg body weight per day (Leuschner et al., 1989). Histopathological investigation of the brain, heart, liver, testes, thyroid, and kidneys did not reveal adverse effects. Urinary protein excretion was increased in dosed animals, and dose-dependent increases were observed in organ weights; these were interpreted to have arisen from decreased food and water consumption caused by decreased palatability.⁸

All rats survived, but there was a dose-dependent loss of body weight, an increase in thyroid weight, and a decrease of blood haemoglobin and serum T₄ levels in rats after 14 weeks on a diet containing 5 or 10 g potassium cyanide/100 g diet (corresponding to approximately 800 and 1600 mg cyanide/kg body weight per day) (Olusi et al., 1979).

In a 3-month study, weanling male Wistar rats were given potassium cyanide at 0, 0.15, 0.3, or 0.6 mg/kg body weight per day daily (0, 0.06, 0.12, and 0.24 mg cyanide/kg body weight per day) by gavage (Soto-Blanco et al., 2002b). In plasma samples collected on the last day of the administration, no changes were observed in the concentrations of T₃, T₄, or glucose, while a decrease was observed in the concentration of cholesterol, significant at the highest dose (45%, P < 0.05). The authors made a qualitative statement that dose-dependent neuropathological findings were observed, including spheroids on the ventral horn of the spinal cord, neuron loss in the hippocampus, damaged Purkinje cells, and loss of cerebellar matter. No further details or statistical analysis was presented.

No effects were noted in Sprague-Dawley rats fed potassium cyanide at concentrations up to 187.5 mg/100 g diet (750 mg cyanide/kg diet) for 56 days. On protein-deficient diets, the lowest body weight gain was obtained at the highest dietary cyanide concentration (Tewe & Maner, 1985).

In a 40-week study in rabbits, the animals were fed potassium cyanide at a level of 1.76 g/kg diet (corresponding to 24–17 mg cyanide/kg body weight per day) (Okolie & Osagie, 1999). The weight gain of the treated animals was decreased by 33%; at the end of the experimental period, serum urea and creatinine levels were elevated, as were the activities of serum lactate dehydrogenase, sorbitol dehydrogenase, ALAT, and alkaline phosphatase.

In a neuropathological study (Soto-Blanco et al., 2002a), goats, 30–45 days old at the beginning of the study, were given potassium cyanide in milk (until weaning) and in drinking-water thereafter at a dose level of 0.3, 0.6, 1.2, or 3.0 mg (0.12, 0.24, 0.48, or 1.2 mg cyanide)/kg body weight per day for 5 months. In a qualitative morphological and immunohistochemical study, presence of gliosis and spongiosis in the medulla oblongata and spinal cord and gliosis in the pons and damage to Purkinje cells in the cerebellum were observed at the highest dose, but no increase in apoptotic cells was reported. Congestion and haemorrhage in the cerebellum were observed at the 0.48 mg cyanide/kg body weight per day group. No quantification or statistical analysis of the findings was presented.

Hyperactivity tremors, convulsions, and laboured breathing were noted in Sprague-Dawley rats exposed to 7.8 mg cyanide/kg body weight per day as copper cyanide for 90 days by gavage. No similar effects were reported at 1.45 mg cyanide/kg body weight per day. Laboured respiration occurred in rats exposed at a lower dose of 0.8 mg cyanide/kg body weight per day when administered potassium silver cyanide for 90 days (Gerhart, 1987).

The only sign of toxicity observed in male rats exposed by gavage to copper cyanide (14.5 mg cyanide/kg body weight per day) (Gerhart, 1986) or potassium silver cyanide (2.6 mg cyanide/kg body weight per day) for 90 days was an increase in testis weight (Gerhart, 1987). No effects were observed in female rats in either study. The NOAELs were 4.35 mg cyanide/kg body weight per day (Gerhart, 1986) and 0.8 mg cyanide/kg body weight per day (Gerhart, 1987).

Kamalu and co-workers (Kamalu & Agharanya, 1991; Kamalu, 1993) compared the effects of cassava containing linamarin with those of a diet containing an equivalent amount of sodium cyanide in three groups of growing dogs, each comprising six animals, for 14 weeks. One group was fed cassava (gari) as the carbohydrate source, which was expected to release 10.8 mg hydrogen cyanide/kg cooked food; another group was fed on the control diet with rice as the carbohydrate source, to which enough sodium cyanide was added at feeding time to release 10.8 mg hydrogen cyanide/kg cooked food (for both, the daily dose was 1.08 mg hydrogen cyanide/kg body weight). The control group was fed this rice diet without added sodium cyanide. Nephrosis and changed plasma free amino acid profile were observed in the sodium cyanide-treated group, while no effect was observed in the plasma glutamyltransferase, ALAT, or isocitrate dehydrogenase activities or in the histology of the liver, kidney, or myocardium. Adrenal hyperplasia and hypertrophy and pancreatic necrosis and fibrosis were observed. (In contrast, the gari diet caused generalized congestion and haemorrhage, periportal vacuolation of the liver, swelling, vacuolation, and rupture of the epithelial cells of the proximal convoluted tubule of the kidney, myocardial degeneration, adrenal gland degeneration, and pancreatic haemorrhage, necrosis, and fibrosis.) A 36% decrease in the serum T₃ concentration was noted, together with histological changes of the thyroid consistent with parenchymatous goitre. A significantly reduced frequency of testicular tubules in stage 8 of the spermatogenic cycle as well as marked testicular germ cell sloughing and degeneration were also observed.

7.3.2 Inhalation

7.3.2.1 Cyanogen

Groups of 30 male albino rats (Charles River) were each exposed for 6 months (6 h/day, 5 days/week) by inhalation to 0, 24, or 54 mg cyanogen/m³ (corresponding to 0, 25, and 56 mg hydrogen cyanide/m³). There were no effects on haematological or clinical chemistry parameters, gross pathology, or histopathology (liver, kidney, cardiovascular system) attributable to the cyanogen exposure. Body weights were significantly lower in rats exposed to 54 mg cyanogen/m³ than in the controls (Lewis et al., 1984).

In an inhalation study, groups of five rhesus monkeys (Macacca mulatta) were exposed to 24 or 54 mg cyanogen/m³ for 6 h/day, 5 days/week, for 6 months. This corresponded to 25 and 56 mg hydrogen cyanide/m³. There were no effects on haematological or clinical chemistry parameters attributable to the inhalation exposure. Total lung moisture content was significantly lower in both treatment groups than in control animals (Lewis et al., 1984).

7.3.2.2 Acetone cyanohydrin

Sprague-Dawley rats (15 per sex and dose level) were exposed to ACH at concentrations of 0, 36, 101, or 204 mg ACH/m³, 6 h/day, 5 days/week, for 14 weeks. The exposures were equivalent to 0, 11, 32, and 65 mg hydrogen cyanide/m³ (Monsanto Co., 1984a). There were no treatment-related deaths or significant changes in body weight gain or haematology. Irritation of the nose and eyes was observed, but no more in exposed than in non-exposed animals. A decrease in blood glucose was recorded in high- and mid-exposure females, and a decrease in total serum protein and globulin concentrations was noted in the mid- and low-dose females. A comprehensive microscopic evaluation of tissues revealed no abnormalities, and no changes in serum T₃ or T₄ levels were observed. The NOAEL reported from the study was 204 mg ACH/m³, corresponding to 65 mg hydrogen cyanide/m³. This can be estimated to correspond to a daily dose of 15 mg cyanide/kg body weight per day.⁹

In the male fertility study described in section 7.6.1, no mortality, clinical signs of toxicity, changes in body weight, or changes in gross necropsy were observed after 48 exposures to up to 202 mg ACH (64 mg hydrogen cyanide)/m³, 6 h/day, 5 days/week (Monsanto Co., 1985a).

In the female fertility study described in section 7.6.1, there was a dose-dependent increase in red nasal discharge of encrustation, but no other clinical signs of toxicity, mortality, changes in body weight, or changes in gross necropsy during or after 34–36 exposures to up to 207 mg ACH (66 mg hydrogen cyanide)/m³, 6 h/day, 7 days/week (Monsanto Co., 1985b).

Few data exist on the effects of chronic cyanide exposure. In a 2-year dietary study, weanling albino rats (10 per sex per group) were administered food fumigated with hydrogen cyanide (special jars were used in order to limit volatilization of hydrogen cyanide from the feed) (Howard & Hanzal, 1955). The average concentrations of cyanide in the feed were 0, 73, and 183 mg/kg diet, as estimated (US EPA, 1993b) based on the authors’ data for concentrations at the beginning and end of each food preparation and by assuming a first-order rate of loss for the intervening period and on the corresponding daily doses of 4.3 and 10.8 mg cyanide/kg body weight per day. No treatment-related effects on survival or growth rate, signs of toxicity, or haematological or histopathological changes in the organs examined (heart, lung, liver, spleen, gastrointestinal tract, kidneys, adrenals, thyroid, testes, uterus, ovaries, cerebrum, cerebellum, and brain) were observed in the treated male or female animals. A NOAEL of 10.8 mg cyanide/kg body weight per day was established.

The effects of cyanide on thyroid function were investigated in groups of 10 male weanling rats fed a semipurified casein-based diet for 11.5 months either supplemented by added methionine, vitamin B₁₂, and potassium iodide or without added vitamin B₁₂ and potassium iodide and with the methionine addition restricted to a third. Both dietary groups were divided into three: one served as the control, the second received 1500 mg potassium cyanide/kg, and the third received 2240 mg potassium thiocyanate/kg. The rats given the potassium cyanide would have received doses of 30 mg cyanide/kg body weight per day. Cyanide, but not thiocyanate, caused a consistent reduction in weight gain in the complete and restricted diet fed animals. Both cyanide and thiocyanate caused decreased thyroid gland activity in young rats, particularly in those fed the restricted diet. Depression of both plasma T₄ and the T₄ secretion rate, suggestive of depressed thyroid function, was found at 4 months, but to a lesser degree after 1 year. At autopsy, the animals were found to have enlarged thyroids, which suggested a mechanism of adaptation (Philbrick et al., 1979).

In order to study the possible contribution of cyanide exposure to malnutrition-related diabetes mellitus (see section 8), Okolie & Osagie (1999, 2000) fed New Zealand White rabbits potassium cyanide for 10 months (702 mg cyanide/kg diet, corresponding to approximately 20 mg/kg body weight per day). No effects were observed on the serum amylase activity, blood glucose concentration, or the morphology of the pancreas, while degenerative changes were reported in the liver and kidney. Similarly, 1-year feeding of cassava to rats induced no changes in blood glucose homeostasis or pancreatic histology (Mathangi et al., 2000).

Because of the small group size and limited exposure time in most of them, these studies are not informative with regard to the possible carcinogenicity of cyanides.

Sodium cyanide was not mutagenic in Salmonella typhimurium strains TA100, TA1535, TA97, or TA98 with or without exogenous metabolic activation (NTP, 1993). Potassium cyanide was not mutagenic in five strains (TA1535, TA1537, TA1538, TA98, and TA100) of S. typhimurium with or without metabolic activation (De Flora, 1981). Hydrogen cyanide induced mutations in S. typhimurium TA100 in the absence of S9 activation, but was not mutagenic to strain TA98 with or without S9 activation (Kushi et al., 1983). ACH did not induce mutations in S. typhimurium strains TA1535, TA1537, TA1538, TA100, or TA98 in a plate incorporation assay or at concentratons of 100–950 µg/ml in a CHO/HGPRT assay (Monsanto Co., 1983b,c).

DNA repair tests in Escherichia coli WP67, CM871, and WP2 with potassium cyanide were negative (De Flora et al., 1984). Potassium cyanide induced both time- and dose-dependent DNA fragmentation accompanied by cytotoxicity in rat thymocytes in vitro. Cyanide also induced DNA damage in baby hamster kidney cells (BHK-21) in vitro, where, unlike thymocytes, internucleosome DNA fragmentation was not observed (Bhattacharya & Rao, 1997). The cytotoxic mode of double strand breaks in the pathogenesis of DNA fragmentation was studied by Vock et al. (1998), employing an A549 human epithelial-like lung carcinoma cell line treated with potassium cyanide. Induction of double strand breaks by potassium cyanide was observed only after cell viability was reduced to less than 60%, indicating that double strand breaks were the consequence of extragenomic damage, as a secondary effect of high cytotoxicity in combination with cell lethality.

Sodium cyanide was a highly effective inducer of germline aneuploidy in Drosophila (Osgood & Sterling, 1991).

No statistically significant increases in the frequency of chromosomal aberrations or changes in mitotic index compared with control values were found in bone marrow cells from four groups of 24 male and 24 female Sprague-Dawley rats administered a single dose of ACH by oral gavage at levels of 0, 1.5, 5, or 15 mg/kg body weight with preparation intervals of 6, 12, and 24 h post-administration (Monsanto Co., 1984b).

No testicular DNA synthesis inhibition was detected in mice after a single oral potassium cyanide dose of 1 mg/kg body weight (Friedman & Staub, 1976).

Relatively few data are available on the reproductive and developmental toxicity of cyanides. No reproductive and developmental toxicity studies are available for hydrogen cyanide.

7.6.1 Effects on fertility

After 2 weeks on a diet containing 5 or 10 g potassium cyanide/100 g diet, female rats (10 per group) were mated with untreated males. No pregnancies resulted. The dose corresponds roughly to 1000 and 2000 mg cyanide/kg body weight per day (Olusi et al., 1979). There was a dose-dependent decrease in body weight gain, blood haemoglobin (18% and 23%), and serum T₄ concentration (54% and 75%).

In a male fertility study (Monsanto Co., 1985a), Sprague-Dawley rats (n = 15) were exposed by inhalation to ACH (0, 35, 101, or 202 mg/m³; 0, 11, 32, or 64 mg hydrogen cyanide/m³), 6 h/day, 5 days/week, during a period of 69 days (i.e., 48 exposure days). After the treatment period, the males were mated with three non-exposed females each. There were no effects on the mean body weight, clinical signs of toxicity, or anatomical changes in gross necropsy. The mating efficiency, number of live implants, and pre- and post-implantation losses were not different between treated and control groups.

Female Sprague-Dawley rats were exposed by inhalation (6 h/day, 7 days/week) for 21 days to ACH at at 38, 108, or 207 mg/m³ and then mated with untreated males. Exposure of the females, which was equivalent to 12, 34, and 66 mg hydrogen cyanide/m³, was continued until the day of mating, and the females were sacrificed at mid-gestation (gestation days 13–15). No treatment-related effects on female fertility were observed in any of the exposure groups. The only frequently observed clinical sign post-exposure was red nasal discharge or encrustation. The highest exposure in the study could be considered as the no-observed-effect level (NOEL) for female reproductive effects (Monsanto Co., 1985b).

7.6.2 Developmental toxicity

Preliminary experiments with pregnant Golden Syrian hamsters showed that at a dose rate of 0.125 mmol/kg body weight per hour, no effects on the fetus were observed, while at a dose rate of 0.133 mmol/kg body weight per hour or more, there was a 100% resorption rate and maternal deaths. Toxicity to dams increased with increasing dose levels and included shortness of breath, incoordination, reduced body temperature, and loss of body weight. Co-administration of thiosulfate eliminated the teratogenic effect, protecting the dams and fetuses from the toxic effects of cyanide (Doherty et al., 1982). In a follow-up experiment, pregnant Golden Syrian hamsters (5–7 animals per group) were continuously exposed to sodium cyanide from day 6 to day 9 of gestation at 0, 0.126, 0.1275, or 0.1295 mmol/kg body weight per hour by using osmotic minipumps implanted subcutaneously. These doses are equivalent to 0, 3.28, 3.32, and 3.37 mg cyanide/kg body weight per hour or 0, 78.7, 79.6, and 80.9 mg/kg body weight per day. The treatment induced a remarkable increase in resorptions as well as fetal malformations. These included non-closure of the neural tube, exencephaly, encephalocoele, and malformations of the heart and limbs or tail (Doherty et al., 1982).

Pregnant Wistar rats (10 animals per group) were fed a cassava diet liberating 21 mg hydrogen cyanide/kg diet, fortified with 500 mg potassium cyanide/kg diet, throughout gestation and lactation. This is equivalent to an estimated daily dose of 16 mg cyanide/kg body weight. No effects were observed on the number, mortality at birth, or body weight of offspring or weight gain of pups during lactation (Tewe & Maner, 1981a).

In equivalent studies with pregnant Yorkshire pigs, three groups of six animals were given potassium cyanide in the diet (30.3, 277, or 521 mg cyanide/kg diet)¹⁰ throughout gestation. No effects were seen on the number or weight of offspring or subsequent lactational performance. Pregnant sows treated at the highest dose level had proliferative changes in the kidney glomeruli and increased thyroid weights. Fetal spleen to body weight and head to body weight ratios in the high-cyanide group were significantly reduced (P > 0.05) compared with the low-cyanide exposed group (Tewe & Maner, 1981b).

In a preliminary study with limited experimental details, pregnant albino rats were fed milled cassava powder as 50% and 80% of their diet during the 15 days of gestation. Growth retardation and an increased frequency of resorptions were observed at both dose levels; in addition, limb defects were observed at the high dose. The weight gain of the dams was lowered; no further information on maternal toxicity was given. No indication of the cyanide content of the cassava diet was given. The effects observed may have resulted from nutritional deficiencies, such as the low protein content of cassava (Singh, 1981).

Groups of pregnant hamsters were fed diets consisting of two types of cassava meal, either a "low-cyanide" (sweet cassava meal) or a "high-cyanide" (bitter cassava meal) variety. These were mixed (80:20) with laboratory chow and administered on days 3–14 of gestation. The cyanide concentration of the sweet cassava meals was 0.6–0.7 mmol/kg; that of the bitter cassava meal was 5–11 (mean 7.9) mmol/kg.¹¹ Cassava-fed dams gained significantly less weight than did control animals (fed diet similar in nutritional value as cassava, but without cyanogenic glycosides), and their offspring showed evidence of fetotoxicity (reduced fetal body weight and reduced ossification of sacrocaudal vertebrae, metatarsals, and sternebrae). The bitter cassava also produced a significant increase in the number of runts compared with litters from dams fed either low-protein¹² or laboratory-stock diets. The only teratogenic effects noted were hydrocephalus in three animals in the low-cyanide (sweet cassava) test group and one encephalocoele found in one animal in the high-cyanide (bitter cassava) test group (Frakes et al., 1986b).

In a teratogenicity study, hamsters were given a single oral dose of linamarin (0, 70, 100, 120, or 140 mg/kg body weight, corresponding to 0, 7.4, 11, 13, or 15 mg cyanide/kg body weight per day) on day 8 of the pregnancy. The animals were killed on day 15 of the pregnancy, and the numbers of resorption sites, dead fetuses, and living fetuses were recorded. Living fetuses were examined for gross external malformations and for internal malformations using histopathological methods. Linamarin had no effect on fetal body weight, ossification, embryonic mortality, or litter size. At the two highest doses, which caused overt maternal toxicity (dyspnoea, hyperpnoea, ataxia, tremor, and hypothermia), vertebral and rib anomalies and encephalocoeles were observed (Frakes et al., 1985).

A single oral dose of d,l-amygdalin at gestational day 8 resulted in exencephaly, encephalocoele, and skeletal malformations at doses of >250 mg/kg body weight (>14 mg cyanide/kg body weight) in hamsters (these doses were also clearly toxic to the mothers). At the lowest dose tested, 200 mg/kg body weight (11 mg cyanide/kg body weight), fused ribs were observed in two offspring of one mother (maternal toxicity not reported). Encephalocoele and rib anomalies were also observed after a dose of d-prunasin (177 mg/kg body weight [16 mg cyanide/kg body weight]) in the absence of maternal toxicity in hamsters. No teratogenic effects were noted when hamsters received d,l-amygdalin (275 mg/kg body weight [16 mg cyanide/kg body weight]) intravenously (Willhite, 1982). The teratogenic effects found were considered to be due to cyanide released by bacterial beta-glucosidase in the gastrointestinal tract (Willhite, 1982).

Groups of 25 pregnant Sprague-Dawley rats were dosed by gavage on days 6–15 of gestation with 0, 1, 3, or 10 mg ACH/kg body weight (equivalent to 0, 0.3, 0.9, or 3 mg cyanide/kg body weight). Maternal toxicity was evidenced by slight reductions in body weight gain in the mid- and high-exposure groups, and statistically significant differences between the high-dose group and controls were found for the number of corpora lutea implantations per dam. There were no comparable differences in the number of viable fetuses per dam, post-implantation losses per dam, mean fetal body weight, or fetal sex distribution for all dose groups and the controls. The incidences of fetal malformations and developmental variations for all fetuses of treated animals and controls were also comparable. It was concluded that 10 mg ACH/kg body weight (3 mg cyanide/kg body weight) was not teratogenic in the rat in the presence of maternal toxicity (Monsanto Co., 1982, 1983a).

The effects of cyanide on behaviour were studied in fasted 25-week-old miniature pigs (12 litter mates: 5 females and 7 castrated males) randomized in four groups. The animals were dosed daily for 24 weeks with a single bolus of cyanide as aqueous potassium cyanide just prior to the daily feeding. The doses were 0, 0.4, 0.7, or 1.2 mg cyanide/kg body weight, chosen to be equivalent to those consumed by West Africans in their diet (Jackson et al., 1985). Every 6 weeks, thyroid function (T₃ and T₄) and fasting blood glucose were measured, but not thyroid-stimulating hormone (TSH). Daily observations were made of clinical signs and various behavioural measurements, including social, antagonistic, exploratory, learning, feeding, and excretory behaviour. In all treatment groups, dose-related decreases were evident from week 6 in blood levels of T₃ and T₄, and an increase in fasting blood glucose was noted, particularly in top-dose animals. Statistical analysis was not provided for each dose group versus control, but changes in top-dose animals appeared significant by week 18; by week 24, decreases of 35% for T₃ and 15% for T₄ and an increase of 60% in fasting blood glucose were observed. Behavioural observations revealed a picture of decreased high energy-demanding behaviour, such as exploration and aggression, slower eating, more frequent drinking, and shivering consistent with the decreased thyroid activity. A LOAEL of 1.2 mg/kg body weight per day could be suggested from this study (Jackson, 1988).

Dogs administered sodium cyanide in capsules at levels of 0, 0.5, 2, or 1–4 mg/kg body weight (one dog at each dose level) daily for 13–15 months showed severe signs of acute cyanide poisoning right after the daily dosing (the dog at the lowest dose died). In the autopsy, the only significant findings were degenerative changes in ganglia cells of the central nervous system, interpreted to be caus