Spacecraft Maximum Allowable Concentrations for Selected Airborne Contaminants: Volume 4 (2000)

Chiu-Wing Lam, Ph.D., and King Lit Wong, Ph.D.

Johnson Space Center Toxicology Group

Medical Operations Branch

Houston, Texas

Hydrogen cyanide (HCN) is a colorless liquid with a boiling point close to room temperature. Thus, at ambient temperature, HCN exists essentially as a gas. HCN has a faint odor of bitter almonds (Sax 1984; ACGIH 1991).

HCN is used mainly in the production of acrylonitrile, methyl methacrylate, sodium cyanide, and cyanuric chloride (Hartung 1982). HCN can be generated when acid is added to cyanide salts (Gosselin et al. 1984) or, to a lesser extent, when alkaline cyanide, especially calcium cyanide, is exposed to water or moisture (NIOSH 1976). HCN is volatile; its volatilization increases as pH

decreases or as temperature increases. Potassium cyanide (KCN) and sodium cyanide (NaCN) are basic; the pH of a 0.1N KCN solution is 11. When these alkaline salts are neutralized, the majority of cyanide ions (CN^-) are converted to HCN. At pH 8,93% of cyanide exits as HCN; at pH 7,99% is HCN (Towill et al. 1978, as cited in ATSDR 1997). Alkaline cyanide salts are used extensively in industrial processes, including the manufacture of dyes, pigments, or nylon; cleaning; and electroplating (NTP 1993). Neither HCN nor cyanide salts have ever been used in any payload experiment aboard the space shuttle.

Pyrolysis of polyurethane foam or polyacrylonitrile can produce HCN (Wooley et al. 1979). Polyurethane foam that was pyrolyzed at 700°C released HCN at 0.2% of the polymer weight; the amount of HCN increased 10-fold at 900°C. When polyacrylonitrile was pyrolyzed at 400°C, 700°C, or 900°C, HCN yields were 1%, 6%, or 36% of the polymer weight, respectively. Burning wool, silk, paper, or nylon also can generate HCN (Terrill et al. 1978). HCN was detected in thermodegradation of electrical cables containing polyimide and polyfluorocarbon (Bourdin 1991); these cables were similar to those used aboard U.S. spacecraft. However, HCN has not been generated in the few thermodegradation events that occurred aboard spacecraft; the pyrolysis did not involve urethane foam or other polymers containing nitrogen atoms.

Lowry et al. (1985) detected HCN in 12% of the fires examined in Dallas. HCN concentrations reached 15 ppm in 10% of those fires; the maximum HCN concentration detected was 40 ppm. Increased serum concentrations of thiocyanate (a metabolite of HCN) was found in 12% of the fire fighters, after accounting for the contributions from cigarette smoking.

HCN is a very weak acid with a dissociation constant of 4.93 × 10^-10 and pKa of 9.31 (CRC 1985). At pH 2, the ratio of CN^- to HCN is 4.89 × 10^-8; that value indicates that essentially all of any ingested KCN or NaCN is converted to HCN in the stomach. The nonionized form is rapidly absorbed, and thus cyanide salts are rapidly fatal by the oral route (ATSDR 1997). At pH 7.4, the ratio of CN^- to HCN is 0.012, which indicates that about 1% of the HCN absorbed from the stomach or lung is ionized in the blood or intracellular fluid, and about 99 mole-percent of KCN or NaCN given parenterally exists as HCN in the body fluid. Thus, data on KCN or NaCN given enterally or parenterally are useful for

toxicological assessment of HCN exposures. However, given the rapid conversion of cyanide ions to HCN and the rapid absorption of HCN in the stomach, a bolus dose will generate a sudden high blood concentration, whereas continuous uptake from inhalation exposures will produce a different blood profile. Moreover, cyanide given orally (such as in drinking water) is subject to the first-pass effect through the liver uptake and detoxification. The first-pass processes can greatly affect the distribution of HCN to target organs. For example, Gettler and Baine (1938) reported that the proportionate tissue concentrations of HCN were lung > brain> heart>liver in two dogs and one human who died after exposure to HCN vapor at unknown concentrations. HCN concentration in the brain was half that in the lung in both dogs and the human. Liver HCN concentrations in both dogs were half those in the brain; for the human, the liver HCN was two-thirds that in the brain. In contrast to those results from inhalation exposures, six rats given KCN or NaCN at 4 mg CN^-/kg orally and an unspecified number of rabbits gavaged with 11.9-20.3 mg CN^-/kg had five times the HCN in the liver as in the brain (Ahmed and Farooqui 1982; Ballantyne 1983). These data demonstrate that the first-pass effect must be considered in assessing the effects of cyanide in the brain, the primary target organ. However, the first-pass effect will have little influence on the toxicity outcome resulting from the metabolic products of cyanide (such as thiocyanate) on extrahepatic organs.

Wolfsie and Shaffer (1959) predicted that HCN can readily diffuse across cellular membranes and be absorbed in the lung. Landahl and Herrmann (1950) compared HCN concentrations in inhaled versus exhaled gases from two volunteers who inhaled (by mouth) 450 mL of HCN at 0.46-4.6 ppm in 1.5 s and held their breath for 2 s. Under those conditions, the lung retained 58.5% of the inhaled HCN. When the holding time was doubled, the absorption increased to 73%. Nasal inhalation and mouth exhalation yielded nasal absorption estimated at 10-20%. Thus, about 75% of HCN inhaled through the nose in normal breathing would be retained in the body. In monkeys exposed to HCN via face mask, HCN uptake was rapid, and the blood cyanide concentration reached steady state in only 10-20 min (Purser et al. 1984).

Trace amounts of cyanide are present normally in healthy subjects; it probably comes from the breakdown of cyanogenic food, bacterial actions in the gastrointestinal tract, and cigarette smoking (Ansell and Lewis 1970). Yamanaka et al. (1991) reported that mainstream cigarette smoke contains HCN at 40-70 ppm, and side-stream cigarette smoke contains less than 5 ppm. Urinary cyanide concentrations average about 0.07 µg/mL in nonsmokers and 0.17 µg/mL in smokers (Ansell and Lewis 1970).

In adult humans, the half-lives of cyanide and thiocyanate in blood have been estimated at 20-60 min and 4-8 d, respectively (Ansell and Lewis 1970; Levine

and Radford 1978). The large difference between the two supports the notion that the metabolism of cyanide to thiocyanate is favored over the reverse reaction. By monitoring the blood cyanide concentration in rats for 2 to 60 min after gavage with KCN (1 mg/kg), Leuschner et al. (1991) calculated a half-life of 14 min. Cyanide concentrations in blood samples collected from dogs 5 to 245 min after a KCN intravenous (iv) injection (0.82 mg/kg) showed a biphasic elimination (T1/2α = 23 min; T1/2β = 5.5 h). Although these findings suggest that blood cyanide concentrations should decline significantly in the first 60 min after the exposure ends, Purser et al. (1984) found that exposing monkeys to HCN gas at 100-147 ppm for 30 min via face mask produced no appreciable decline in blood cyanide concentrations 1 h after the exposure. These authors concluded that cyanide has a fairly long half-life in monkeys. At exposure concentrations that high and for that length of time, the amounts of thiosulfate and 3-mercaptopyruvate, the two endogenous compounds that normally react with cyanide in the body (see below), might have been depleted to the extent that blood cyanide concentrations showed no appreciable decline 1 h after the exposure. In any event, all the exposed monkeys were incapacitated within 15 min.

Cyanide is metabolized through several pathways. Thiocyanate, the major metabolite, is formed from the reaction of cyanide either with thiosulfate, catalyzed by rhodanase (a mitochondrial enzyme found in many tissues, particularly liver), or with 3-mercaptopyruvate catalyzed by cyanide-sulfur transferase (Baumann et al. 1934; Himwich and Saunders 1948; Wood and Cooley 1956; Singh et al. 1989). A minor metabolic pathway is the reaction of HCN with cystine to form 2-aminothiazoline-4-carboxylic acid and 2-iminothizolidine-4-carboxylic acid (Wood and Cooley 1956). Other minor pathways include oxidation of HCN or thiocyanate to CO ₂, reaction with hydroxocobalamine to form cyanocobalamin, and conversion of HCN to formic acid, which enters one-carbon metabolism in the body (Boxer and Richards 1952; Ansell and Lewis 1970; Baumeister et al. 1975).

After a cyanide exposure, some of the body burden of HCN is exhaled unchanged, producing the characteristic odor of bitter almonds on the breath (Ansell and Lewis 1970). Boxer and Richards (1952) showed that rats exhaled cyanide gas following a subcutaneous injection of KCN at 0.65 mg/kg. However, most of the body burden is excreted in the urine as thiocyanate, and a small amount is metabolized to CO₂ (Ansell and Lewis 1970). Wood and Cooley (1956) reported that 80% of cyanide injected intraperitoneally in rats

was recovered as thiocyanate and 15% was recovered as 2-aminothiazoline-4-carboxylic acid.

Thiocyanate formation from cyanide is quite rapid. Pettersen and Cohen (1985) showed that the thiocyanate concentration in plasma of mice doubled in 5 min and peaked at 30 min after a subcutaneous injection of KCN at 4 mg/kg (a sublethal dose). Some thiocyanate can be converted back to cyanide by thiocyanate oxidase in red blood cells (Goldstein and Reiders 1953).

When rats were given potassium cyanide in drinking water at a daily dose of 40, 80, or 150 mg/kg for 13 w, both blood cyanide and urinary thiocyanate concentrations increased proportionally with the doses (Leuschner et al. 1991). The authors concluded that exposing rats to the maximum tolerated dose did not saturate cyanide detoxification pathways. Blood cyanide concentrations remained fairly constant in each dosing group during the test.

Okoh (1983) showed that rats fed 2 mg KCN daily for 6 months showed no statistically significant changes in the fractions of the dose excreted in urine, feces, and expired air. These results suggest that neither the metabolism nor the pattern of excretion of cyanide is affected by long-term cyanide intake.

HCN, the nonionized form of cyanide, can permeate tissues much more readily than cyanide ions and is distributed widely throughout the body (Wolfsie and Shaffer 1959). The toxicity, especially the acute one, of cyanide is due mainly to its inhibition of cytochrome oxidase. HCN, a small molecule, can diffuse very rapidly into the mitochrondria, where it binds to cytochrome C oxidase and forms a stable but reversible coordination complex with the heme (F⁺⁺⁺) sites. The inhibition of cytochrome oxidase by cyanide in the tissue prevents oxygen utilization in situ (Albaum et al. 1946). Organs that are very sensitive to tissue hypoxia, such as the brain and heart, are the primary targets of cyanide toxicity. Blockage of oxygen utilization in tissues resulted in an accumulation of oxyhemoglobin. Venous blood becomes bright red or cherryred, a characteristic sign of cyanide intoxication (Gosselin et al. 1984).

Ballantyne and Bright (1979) showed that cytochrome C oxidase activities were reduced by 76% in the myocardium and 54% in the cerebral cortex of rabbits immediately after they were killed by an intramuscular injection of KCN at 8 mg of CN^- per kilogram. In another study, rats injected subcutaneously with NaCN at 3, 6, 9 or 12 mg/kg (LD₅₀ 9 mg/kg) showed inhibition of brain cytochrome C oxidase by about 2%, 22%, 35%, or 42%, respectively, 20 min after the injection (Tadic 1992). It took only 3.5 to 8 min to inhibit 50% of the cytochrome oxidase in the brains of rats injected intraperitoneally with a lethal

dose of NaCN (5 mg/kg) (Albaum et al. 1946). In mice administered KCN intraperitoneally at 10 mg/kg, 83% of the brain cytochrome oxidase was inhibited 3 min after the injection (Isom and Way 1976). Rapid inhibition of cytochrome oxidase in other organs also has been reported; hepatic cytochrome oxidase was inhibited maximally (about 80%) in mice 5 min after an intraperitoneal injection of KCN at 5 mg/kg (Isom et al. 1982).

In the study by Pettersen and Cohen (1985), 30 min after mice were given a subcutaneous injection of KCN at 4 mg/kg (i.e., 10 min following the peak inhibition), cytochrome oxidase activity in the heart recovered fully, and that in the brain recovered to 85% of the pre-injection level. From this trend of recovery, the brain cytochrome oxidase could be expected to have recovered fully 40 min after the injection (i.e., 20 min after the peak inhibition). The hepatic cytochrome oxidase activity in mice recovered fully by 10, 20 or 25 min after a KCN injection intraperitoneally at 1, 3, or 5 mg/kg, respectively. In rats exposed to KCN, the enzyme recovered fully 1 h after a 3 or 5 mg/kg injection; recovery time was doubled when the dose was increased to 8 mg/kg.

Cyanide is an extremely potent and fast-acting poison regardless of the route of exposure. Typical symptoms of acute poisoning from a lethal dose include headache, vertigo, lack of motor coordination, nausea, vomiting, tachypnea, weak pulse, cardiac arrhythmia, convulsion, coma, and death. Pathological findings might include petechiae of the brain, meninges, and pericardium; cerebral and pulmonary edema; and tracheal congestion with hemorrhage (NTP 1993). These effects result mostly from direct inhibition of cellular respiration by binding cytochrome oxidase in the brain and heart, the two primary targets of cyanide poisoning. The toxic effects of this direct-acting poison are similar in humans and in animals (NTP 1993).

Being the most sensitive to tissue hypoxia, the brain is a primary target of cyanide toxicity. Signs and symptoms of acute cyanide poisoning reflect cellular hypoxia and anoxia and often are nonspecific. Exposures to high

concentrations of HCN vapor can produce flushing, headache, tachypnea, and dizziness within 30 s. Toxic effects progress to stridor, irregular breathing, coma, seizures, and death within 10 min (Ellenhorn and Barceloux 1988). The onset of toxic sequelae is less rapid in oral exposures than in other routes of exposure because of slower entry of cyanide into the circulation and passage through the portal system, where the liver detoxifies some cyanide through the first-pass effect. However, the rapid uptake of HCN from the stomach and the ease with which HCN passes through the blood-brain barrier make this toxin rapidly dangerous even by the oral route. Because of the dangerous nature of cyanide and the small margin of safety, very few controlled human studies have been conducted on the cyanides.

Flury and Zernik (1931) and Henderson and Haggard (1943) stated that exposing humans to HCN at 20-40 ppm for several hours produced only slight symptoms. The authors also noted that a 1-h exposure to HCN at 50-60 ppm could be tolerated without serious consequences but that an exposure at 100-240 ppm for 30 min or more is dangerous. However, it is not clear whether this information, which referred to work originally done by Lehman (1919) and cited by McNamara (1976), had been obtained from human experiments or from extrapolations from rabbit data.

Peden et al. (1986) reported that 12 men who were exposed to unknown concentrations of HCN in separate industrial accidents experienced dizziness (n = 8), dyspnea (n = 8), a shaky feeling (n = 6), headaches (n = 4), nausea (n = 4), and unconsciousness (n = 5). All the unconscious victims rapidly regained consciousness ''probably less than 10 min" after having been removed from the accident sites. However, the headaches persisted for up to 8 h after hospital admission. Similar symptoms and toxic signs also were reported by Nagler et al. (1978) in three cases of HCN poisoning at unknown concentrations from the accidental addition of 0.5 kg of a cyanide salt to a sulfuric acid bath in the electroplating department of a factory in Belgium. Those victims experienced semiconsciousness, headaches, nausea, sinus bradycardia, and atrial fibrillation.

Grubbs (1917) generated HCN gas in a practically airtight room by dropping 0.5 NaCN per 1000 ft³ space into acid, which generated HCN at about 240 ppm. Several human volunteers (the exact number was not specified) breathed the HCN atmosphere for 2 min without showing any symptoms, but a similar HCN atmosphere had "at other times caused dizziness." Also, no toxic effects were noted for human volunteers breathing HCN gas generated by dropping 0.75 oz of NaCN into acid per 1000 ft³ of space (estimated HCN concentration of 360 ppm) for 1.5 min. Barcroft (1931) quoted a 1923 report by Katz and Longfellow: "Men employed in fumigation with HCN have been tested while at rest in 250 ppm of air for 2 min and in 350 ppm for 1.5 min, but felt no dizziness, although possibly on exertion they might have done so." According to

McNamara (1976), Katz and Longfellow were describing the data that Grubbs had gathered with human subjects.

Bonsail (1984) reported the case of a worker poisoned in a tank that contained residual hydrazodiisobutyronitrile, which decomposes to HCN upon exposure to water. This man was in the tank for about 6 min, during which time he was exposed to HCN at about 500 mg/m³ (460 ppm) and probably also to hydrazodiisobutyronitrile vapor. He collapsed in the tank and was rescued; he was comatose, with marked conjunctivitis, vomiting, and paralysis upon arrival at a hospital. After being placed on a ventilator and treated with sodium thiosulfate and phenytoin, the victim regained full consciousness within 72 h. He was discharged from the hospital 2 w later with only minor loss of peripheral vision.

Barcroft (1931), of the U.S. War Department, exposed a man and a dog to HCN at a nominal concentration of 625 ppm (the actual concentration was between 500 and 625 ppm) in an airtight chamber. The man was more tolerant than the dog. The dog became unsteady at 50 s and unconscious at 1 min 15 s. It made "crying sounds" and went into tetanic convulsions at 1 min 30 s, at which time the man left the chamber. He put on a respirator and went back into the chamber at 1 min 30 s to retrieve the dog and remained outside the chamber thereafter. At 5 min after the start of the experiment, the man developed a "momentary feeling of nausea." At 10 min, he had difficulty in concentrating in close conversation.

Exposing monkeys to HCN at 100, 125 or 150 ppm produced semiconsciousness or unconsciousness in 17, 14, or 8 min, respectively (Purser et al. 1984). When the exposure was terminated, the animals recovered within 10 min to a fairly active state. The effects of HCN in monkeys and other laboratory animals were compared in an earlier study conducted by Dudley et al. (1942). The symptoms in monkeys exposed at 125 ppm for 12 min were described as "distinctly toxic." Clinical signs in cats, according to the authors, were "markedly toxic" after exposure to the same concentration for 7 min, but no symptoms were observed in rabbits. Exposing dogs at 35-60 ppm (for an unspecified duration) led to vomiting, convulsions, or death, but the dogs could tolerate HCN at 30 ppm. Guinea pigs could tolerate HCN at 200 ppm for 1.5 h without toxic signs. Dudley et al. (1942) concluded that sensitivity to HCN toxicity increases progressively in guinea pigs, rabbits, monkeys, cats, and dogs.

One of two rats exposed to HCN at 50 ppm showed violent agitation, paralysis, unconsciousness, and gasping after 3 min of exposure (Moss et al. 1951).

The other rat survived the exposure for an unspecified duration without paralysis. Rats and mice injected intraperitoneally with KCN at 3-5 mg/kg became unconscious in 1-1.5 min and recovered at about the same time as the hepatic cytochrome oxidase activity had returned to normal (Schubert and Brill 1968).

HCN produces central-nervous-system (CNS) effects through blocking cytochrome C oxidase. Prolonged hypoxia or anoxia, which can occur when HCN concentrations are high enough or the exposure duration is long enough, can result in permanent brain damage.

Long-term cerebellar spasmodic symptoms were documented in one man after he recovered from a coma after acute HCN intoxication (Fiessinger et al., 1938). Another man was comatose for 7 h after ingesting about 1 g of KCN in an attempted suicide; he acquired parkinsonism after the incident (Uitti et al. 1985). This man had hallucinations and delusions over the first few days after regaining consciousness; in the weeks that followed, he had difficulty expressing himself, personality changes, and depression. Four months after the cyanide ingestion, he experienced marked and generalized rigidity, bradykinesia, and tremors of the tongue, eyelids, and arms. He died 2 years later from an overdose of imipramine and alcohol. Autopsy revealed a shrunken striatum of spongy consistency with widespread lacunar formation, focal atrophy of cerebellar folia, and resolved laminar necrosis in the occipital lobes.

When 11 rats and 11 monkeys were infused with NaCN solution (0.07-0.15 mg/min-kg for rats and 0.05-0.10 mg/min-kg for monkeys) for approximately 35-120 min, 4 rats and 4 monkeys developed brain damage (Brierley et al. 1976, 1977). The animals were killed within 4 d after the exposure. All eight animals showed white-matter brain damage, but only one rat and one monkey suffered gray-matter damage.

The time course of brain damage induced by acute HCN intoxication was investigated by Levine and colleagues (Levine and Stypulskowski 1959;). Rats exposed for 20-45 min to an unspecified HCN concentration (sufficient to

incapacitate the rats within 10 min) showed initial neuronal damage followed by incomplete myelination loss of the damaged neurons. Extensive brain-tissue injury was observed 1 to 2 d after the exposure. At 4 mo after the exposure, the lost neural cells in corpus callosum, striatum, and hippocampus had been replaced by glial cells and vessels (Levine and Stypulskowski 1959; Hirano et al. 1968). However, the remyelination process in the CNS was slow and incomplete (Hirano et al. 1968).

Blockage of oxygen utilization resulting from binding of HCN on cytochrome C has a devastating effect on the heart. The initial effect is tachycardia, followed by bradycardia. Dysrhythmias and hypotension often precede peripheral vascular collapse (Ellenhorn and Barceloux 1988).

Electrocardiographic (EKG) changes in four men executed by HCN inhalation at unspecified concentrations were monitored and documented by Wexler et al. (1947). During the first 7 min of the inhalation, the heartbeat occasionally slowed to varying degrees with periods of either absence of P waves or irregular P waves. All four men also experienced A-V dissociation. After the first 7 min, the heartbeat slowed even further, followed by either heart block or ventricular fibrillation.

Three workers were poisoned by HCN vapor when one accidentally added 0.5 kg of cyanide salt into a sulfuric acid bath, generating 280 g HCN (Nagler et al. 1978). One worker, who was about 1 m away from the bath, immediately became semiconscious and developed atrial fibrillation. Another worker rushed to rescue the man and soon complained of headache, nausea, and throat irritation; when admitted to a hospital, the rescuer had crushing chest pain, profuse diaphoresis, vomiting, and tachycardia. Another worker who had been standing 3 m from the bath experienced throat irritation, nausea, vomiting, profuse diaphoresis, sinus bradycardia (36 beats per min), and crushing chest pain.

Purser et al. (1984) observed that cardiac changes after HCN exposures were accompanied by incapacitation and were always preceded by a hyperventilatory

episode. Hyperventilation was inevitable upon incapacitation. During a 30-min exposure of monkeys to HCN at 147 ppm, the heart rate slowed and the amplitude of T waves either increased or decreased. EKG data from the monkeys showed that HCN could produce EKG changes only in severe intoxication.

HCN poisoning in humans produces biphasic changes in respiration (i.e., initial rapid and deep respiration), followed by slow and irregular respiration (Parmenter 1926; Wood and Cooley 1956).

In monkeys and mice, HCN's respiratory effects are monophasic. Respiration rate was not affected in monkeys exposed to HCN for 30 min at 80 ppm or less. At 90 ppm, hyperventilation was noted 20 min into the exposure. At 180 ppm, however, hyperventilation was almost immediate. In monkeys exposed to HCN at 147 ppm for 30 min, hyperventilation (about 130% increase in minute volume most of the time) developed within 0.5 min and lasted until 13 min into the exposure (Purser et al. 1984). Purser and co-workers attributed the hyperventilatory response to HCN's stimulatory effect on respiration. In contrast, Matijak-Schaper and Alarie (1982) found that HCN in mice slowed respiration, which they hypothesized as being due to depression of the respiratory centers. In mice exposed for 30 min to HCN at 23 or 120 ppm, the respiratory rate was reduced by 20% or 80%, respectively.

HCN is rapidly lethal if inhaled at sufficient concentrations. NcNamara (1976) estimated the average fatal concentration for humans to be 546 ppm for a 10-min exposure. According to Henderson and Haggard (1943), an exposure at 200 to 480 ppm for 30 min is fatal to humans. However, according to Dudley et al. (1942), HCN at 270 ppm can cause death immediately.

When Grubbs (1917) exposed four rats to an estimated HCN concentration of 240 ppm for 1 h, two rats died in 50 min and one died after the 1-h exposure. Moss et al. (1951) reported that one rat exposed to HCN at 25 ppm and one of two rats exposed at 50 ppm for unspecified periods survived the exposure; the other rat died after being exposed at 50 ppm for 8 min. Moss and co-workers further noted that exposure at 100 ppm for 5.5 min or at 200 ppm for 10 min was also lethal to rats.

The lethality of HCN also has been investigated in dogs exposed at 530 to 2200 mg/m³ (480 to 2000 ppm) for up to 10 min (Jandorf and Bodansky 1946) (Table 15-1). Etteldorf (1939) reported that dogs exposed to HCN at 40 mg/m³ (36 ppm), a concentration that induced convulsions, for 15 min or longer usually died. A 10-min exposure to HCN at 40 mg/m³ killed one of three dogs; the two survivors required 24 to 72 h to recover. None of four dogs exposed to HCN at 40 mg/m ³ for 8 min died; however, one developed CNS sequelae manifested by weakness and "dementia" 48 h after the exposure. In dogs, convulsions are always preceded by prodromal signs, such as lacrimation, salivation, defecation, and urination. If the HCN exposure was stopped during the prodromal stage, dogs always recovered.

Dudley et al. (1942) reported that an exposure to HCN at 90 ppm for several hours also was fatal to dogs. Exposing rats to HCN at 110 ppm for 1.5 h produced death; 315 ppm was fatal to guinea pigs, rabbits, and cats. The relative sensitivities of guinea pigs, rats, and rabbits to the effects of HCN also was determined by exposing these animals to HCN in a static gas chamber (Thautman 1933). For guinea pigs exposed to HCN at a nominal concentration of 900 ppm for an average duration of approximately 6 min, 10 of 25 (40%) died. Exposing rats and rabbits to HCN at a nominal concentration of 450 ppm for about 3 min killed 10 of 32 (31%) rats and 2 of 17 (12%) rabbits. Thautman concluded that rats were more sensitive than guinea pigs and rabbits to the lethal effects of HCN.

Species differences in the lethality of HCN also have been reported by others. According to Vernot et al. (1977), mice were more sensitive than rats; the 1-h LC₅₀ was 323 ppm for mice (95% confidence limits 276-377 ppm) and 484 ppm for rats (95% confidence limits 442-535 ppm). In another study, Barcroft (1931) noted that "the monkeys were only beginning to show signs of unsteadiness when the dogs died." According to Barcroft, sensitivity to HCN, from least to most, follows the order of monkey, goat, rabbit, rat, dog, pigeon, and canary. Barcroft's data on monkeys, rats, and dogs are shown in Table 15-2.

The toxicity of HCN in 36 chronically exposed workers in three electroplating factories in Egypt was reported by E1 Ghawabi et al. (1975). These subjects, all nonsmokers, were exposed to HCN and probably also to cyanide salt mist in the course of their 5 to 15 y of work in the factories. During the 2-mo study, breathing-zone air samples and urine samples were collected three times a week from each worker. Average HCN concentrations in the three facilities were 6.4 ± 6.9, 8.1 ± 8.2, and 10.4 ± 10.9 ppm (mean ± standard deviation).

Atmospheric cyanide concentrations correlated well with urinary thiocyanate concentrations. Past and present medical histories showed that these workers had significantly higher incidences of headaches, weakness, changes in taste and smell, giddiness, vomiting, and dyspnea than 20 male control subjects who were of the same age range and socioeconomic status and who had "never been exposed to any chemical hazard." Because the articles to be electroplated were washed in petrol (gasoline) baths before being treated in the cyanide baths, workers could have been exposed to a variety of volatile organic compounds as well as to cyanide. The authors did not say whether workers in factory A, where HCN concentrations were the highest, had a higher incidence of complaints, nor did they provide any indication of whether any of the symptoms noted had been experienced during the 2-mo survey period. Lacking this information and other details, assessment of any correlation between symptoms and exposure concentrations is difficult. Nevertheless, the symptoms noted above are similar to those resulting from acute cyanide exposures; therefore, it cannot be ruled out that the clinical toxicity experienced by these workers might have resulted from periods of exposures to higher concentrations than those measured during the survey.

CNS symptoms similar to those documented by E1 Ghawabi et al. (1975) were reported by Radojicic (1973, as cited in NIOSH 1976). Radojicic's subjects were 43 cyanide-exposed workers and 20 nonexposed workers in a Yugoslavian electroplating plant. According to the NIOSH report, Radojicic's subjects were exposed to cyanide at concentrations of 6 to 11 mg/m³ (5.5 to 10.0 ppm), and most of the exposed workers complained of headaches, fatigue, weakness, pain, tremors of the hands and feet, and nausea.

A fatality in a factory where NaCN solution was used to extract silver from photographic films led the U.S. Occupational Safety and Health Administration (OSHA) to suspend the factory's operations (Blanc et al. 1985). (OSHA cited the factory for failing to provide protective gloves and chemical vapor masks for its workers.) One day after the suspension, 24-h environmental monitoring showed an airborne HCN concentration of 15 ppm (24-h time-weighted average (TWA)), a result that suggests that workers had been exposed to cyanide at more than 15 ppm. The factory never reopened. A later survey was conducted with 36 former employees who had worked an average of 11 ± 10.4 mo (mean ± standard deviation) at that factory. Responses to questionnaires revealed that between 42% and 72% of the workers experienced, during their active employment, headache, nausea or vomiting, almond or bitter taste, eye irritation, loss of appetite, weight loss, epitaxis, easy fatigue, and rash (symptoms listed in decreasing order of occurrence). During the month before the interview, 11-36% of these former workers still experienced those symptoms, except that none were still losing weight. Positive correlations were noted between symp-

toms and doses when the extent of cyanide exposure was classified on the basis of job category, the number of times powdered cyanide was handled monthly, the number of direct skin contacts with cyanide solution per month, and the ingestion of food or beverages in production areas. Unfortunately, no airborne HCN concentrations were measured in the factory during production, so exposure concentration-response information was not available.

NIOSH (1976) reviewed two studies and concluded that HCN produced no toxic effects of HCN in the workers being exposed at 0-17 ppm (5 ppm, mean) in the Grabois (1954) study or at 4-6 ppm in the Hardy et al. (1950) study. In fact, Grabois measured only airborne HCN concentrations in an apricot-kernel processing plant and did not assess health effects. Hardy et al. cited the results from field studies conducted by Massachusetts State chemists, who measured air samples and did not study health effects. The exact concentrations to which Hardy's subjects were exposed were not determined.

In a study of 23 cyanide-exposed male workers and 20 control workers in an electroplating and case-hardening factory in India, Chandra et al. (1980) noted that cyanide-exposed workers experienced "typical symptoms of poisoning." Neither the types nor the incidence of symptoms were specified. The average airborne cyanide concentration in the factory was reported to be 0.45 mg/m³ (range 0.2 to 0.8 mg/m³). That concentration, equivalent to 0.4 ppm, is an order of magnitude lower than those reported by E1 Ghawabi et al. (1975), Radojicic (1973), and Hardy et al. (1950). That measurement is probably not correct. As Hardy et al. noted, an electroplating factory equipped with adequate engineering devices could maintain HCN at not more than 4-6 ppm in workers' breathing zones. It is doubtful that an electroplating factory operating in India nearly 20 y ago would have had engineering devices that could maintain HCN at less than one-tenth that concentration. Moreover, the average thiocyanate concentration in 24-h urinary samples collected from the cyanide-exposed workers (who were nonsmokers) in the Chandra et al. (1980) study was 0.57 mg/100 mL. If the average daily output of urine was about 1360 mL (690-2690 mL/d, n = 33 subjects; Liappis 1973), then Chandra et al. subjects excreted about 7.8 mg of thiocyanate daily. That value corresponds to about 13 ppm (12 mg/m³) of airborne cyanide in the workplace, as calculated from an equation derived by E1 Ghawabi et al. (1975). The estimated HCN concentration (13 ppm) is comparable with those reported by others (E1 Ghawabi et al. 1975; Radojicic 1973; Hardy et al. 1950). Thus, the symptoms observed in the Chandra et al. (1980) study were probably induced by exposures to cyanide at concentrations much more than 0.45 mg/m ³.

Chandra et al. (1988) also reported a case-controlled study of 111 cyanide-exposed workers in the electroplating and heat-treatment plants of two Indian factories (40 in factory A and 71 in factory B); 30 control workers were from

factory B. From clinical, behavioral, and biochemical data, Chandra and co-workers proposed an exposure limit for CN of 0.35 mg/m³ for workers in India. However, the rationale for this recommendation is questionable, because cyanide and thiocyanate concentrations in blood and urine (biochemical data) were treated as "indicators of cyanide toxicity." In addition, no clinical data were collected from the control group.

When cyanide at up to 300 ppm in drinking water was given to rats and mice for 13 w, clinical observations showed no evidence that cyanide produced CNS toxicity (NTP 1993; see below).

Cyanide in cigarette smoke is thought to cause tobacco amblyopia in certain smokers (Baumeister et al. 1975). This condition is characterized by central scotomata with ganglion-cell degeneration and vascular changes in the retina, as well as degeneration of the optic nerve. However, in addition to long-term cyanide exposure, deficiency of hydroxocobalamin, which is involved in one of cyanide's metabolic pathways, also is thought to contribute to this condition.

The effects of cyanide and thiocyanate on the CNS was assessed in rats that were fed a diet containing potassium cyanide at 500 ppm or potassium thiocyanate at 2240 ppm for 11.5 mo. Both compounds produced demyelination in the white matter of the spinal cord (Philbrick et al. 1979).

Crampton et al. (1979) reported that baboons exposed to KCN at 1 mg/kg subcutaneously for 5 d/w for 42 mo showed no neurological damage as assessed by neurological and histological examination. Lessell (1971) reported that subcutaneous exposure of rats to cyanide at increasing doses three times a week for 3 mo produced segmental demyelination of the optic nerve and necrosis in the corpus callosum. However, the doses required to produce these neurological lesions were so high (cumulative dose 200 to 300 mg/kg) that the exposure was lethal to 70-80% of the rats. However, in another study, giving cyanide

salts to rats and mice at up to 300 ppm in drinking water over a 13-w period produced no evidence of brain lesions on histopathological examination (NTP 1993).

The long-term effects of cyanide were evaluated by the National Toxicology Program (NTP 1993) in groups of 20 rats and 20 mice (10 of each sex) given NaCN in drinking water at concentrations of 0, 3, 10, 30, 100, or 300 ppm for 13 w. The average daily intake of NaCN is shown in Table 15-3. As noted above in the Toxicokinetics section, essentially all ingested cyanide is absorbed as HCN in the stomach. Thus, the intake of HCN can be calculated from the NaCN values (Table 15-3).

If the intake of NaCN from drinking water is assumed to be roughly continuous, then an equivalent continuous inhalation exposure concentration of HCN that would produce the same body burden can be estimated from the amount of water consumed. To accomplish that, we first used the estimates of daily HCN oral doses (in milligrams per kilogram) (Table 15-3) to calculate the daily HCN consumption for a 300-g rat or a 30-g mouse (Table 15-4). Then, the airborne HCN concentration for continuous exposure that yields the same daily HCN dose in the rat or mouse can be estimated. No data are available on the uptake efficiency of inhaled HCN in rodents; however, humans retain about 75% of

inhaled HCN in the body (Landahl and Herrmann 1950). If the same absorption efficiency is assumed for rodents and if the corresponding ventilation rates are 210 mL/min (0.30 m³/d) for a 300-g rat and 20 mL/min (0.029 m³/d) for a 30-g mouse (Lai 1991), then airborne concentrations for the rat or mouse can be estimated. The estimates are shown in Table 15-4.

No deaths were attributed to cyanide treatment in this 13-w study. No clinical signs or microscopic changes were noted in any organs, including brain or thyroid. Minimal changes were present in hematological, clinical chemistry, and urinalysis values; however, these changes were considered neither biologically significant nor related to the effects of cyanide. Weights of the left caudal epididymis in all groups of exposed males were significantly lower than those of the controls (see section on developmental toxicity).

In another study, no morphological or hematological changes were noted over a 2-y period in which rats were given food containing HCN (Howard and Hanzal 1955). The HCN was added to the feed by fumigation, after which the feed was stored in special jars. HCN concentration in the feed was either 190 ppm (range 80 to 300 ppm) or 76 ppm (52 to 100 ppm). Because a 250-g rat eats about 15 g of feed per day and inhales approximately 240 mL of air per minute (Sweet 1987; Mauderly 1986), the consumption of HCN at 190 or 76 ppm in the feed is equivalent to inhaling airborne HCN at 11 or 4.4 ppm, again assuming 100% absorption for the oral route and 75% for inhalation. Body weight was not affected during the first 91 w, but was lower at w 104.

In the study of Egyptian electroplating workers (E1 Ghawabi et al. 1975), 20 of the 36 HCN-exposed workers (56%) had goiter of a mild or moderate degree. The enlarged thyroids were reportedly firm and slightly nodular in 4 workers but soft and smooth in the remaining 16 workers. These 20 individuals had ''no clinical manifestations of hypo-or hyperthyroidism." No correlation was found between thyroid size or goiter incidence and duration of employment: 14 subjects were employed for less than 5 y, 14 for 5-10 y, and 8 for more than 10 y. The authors of this report attributed the goiter to thiocyanate formation in the body, quoting the finding of goiter in hypertension patients who had been treated with thiocyanate for 4 mo or longer (Potter 1944; Rawson et al. 1943). According to E1 Ghawabi and co-workers, the HCN-exposed workers accumulated iodine-131 in thyroid more rapidly than controls when the exposed workers returned to work after 2 d of nonexposure over the weekend.

Goiter also was reported by Hardy et al. (1950) in two workers exposed to HCN while they dipped red-hot metallic tools into a cyanide solution. The exposure concentrations of HCN were unknown. No palpable thyroid abnormalities were found among those who had worked for about a year in a silver recovery plant that used cyanide salt.

In the 13-w drinking-water study discussed above, the equivalent continuous inhalation exposure concentrations of HCN were estimated to be 0.24, 0.6, 1.9, 5.9, or 15.6 ppm for the rats and 0.3, 1.4, 3.8, 12.1, or 34.7 ppm for the mice. Histopathological examination of thyroid glands in both species revealed no evidence of toxicity. In rats, urinary thiocyanate concentrations increased as exposure concentrations increased (NTP 1993).

Longer exposures to higher concentrations of cyanide produced effects on the rat thyroid gland. Rats fed a diet containing potassium at 1500 ppm for 11.5 mo showed decreased rates of thyroxine secretion but no changes in plasma thyroxine concentration (Philbrick et al. 1979). However, exposure to potassium thiocyanate at 2240 ppm reduced thyroxine measurements and produced thyroid enlargement. The cyanide exposure also suppressed body-weight gain. Calculations of equivalent airborne HCN exposure concentrations reveal that daily ingestion of food containing KCN at 1500 ppm would give the same body burden of HCN as inhaling air containing HCN at 86.8 ppm.

In the aforementioned NTP (1993) study, sperm morphology and vaginal cytology were assessed in rats and mice that consumed cyanide at 30, 100, or 300 ppm in drinking water. As shown in Table 15-4, the equivalent inhalation exposure concentrations of HCN for rats were 1.9, 5.9, or 15.6 ppm; those for mice were 3.8, 12.1, or 34.7 ppm. Mild but statistically significant decreases in the weight of the left caudal epididymis were seen in the exposed male rats and in some of the male mice; marginal reduction of sperm motility was seen in the exposed rats but not in the mice (see Table 15-5). NTP (1993) concluded that these mild reproductive changes in rats are probably not biologically significant and might not decrease fertility. However, humans are considered more sensitive to such effects (NTP 1993).

Female rats in the 100- and 300-ppm groups had prolonged proestrus and diestrus. However, the lack of a dose-response relationship in these variables led the NTP to conclude that "these differences are spurious and the results of this study would need to be replicated before such changes could be unequivocally attributed to sodium cyanide exposure."

In vivo developmental toxicity evaluations of HCN have not been reported in the literature. In one study of chick embryo explants (Spratt 1950), sodium cyanide at more than 5 µM inhibited CNS development. Hamsters infused with sodium cyanide at 0.13 mmole/kg-h on d 6-9 of gestation showed fetal malformations, including limb and tail defects, hydropericardium, and encephaloceles, on d 11 of gestation (Doherty et al. 1982).

Rats fed food containing HCN at 190 ppm (range 80 to 300 ppm) or 76 ppm (80 to 300 ppm) for 2 y showed no increase tumor incidence (Howard and Hanzal 1955). The equivalent inhalation concentrations of HCN were 11 or 4.3 ppm, respectively.

Kushi et al. (1983) reported that HCN is weakly mutagenic in Salmonella typhimurium.

HCN might potentiate the lethal effect of carbon monoxide (Moss et al. 1951). A 30-min exposure to CO at 2000 ppm produced no ill effects in rats; however, CO exposure at 2000 ppm with HCN at 10 or 20 ppm was lethal to some rats. Moss attributed the synergistic effect to respiratory stimulation by HCN. Synergism also was demonstrated for CO and potassium cyanide: Moore et al. (1991) reported a higher mortality in rats exposed to both CO (2000 to 3750 ppm) and KCN (3 to 7 mg/kg, intraperitoneal injection) than could be explained by an additive effect. The combination also resulted in higher concentrations of carboxyhemoglobin than CO poisoning alone, and more severe lactic acidosis than KCN intoxication alone.

Table 15-6 presents a summary of the toxicity data on HCN.

Table 15-7 presents exposure limits for HCN set by other organizations and Table 15-8 presents the SMACs established by NASA.

SMACs are derived in accordance with guidelines developed by the SMAC subcommittee of the Committee on Toxicology (NRC 1992). The SMACs are set by choosing the lowest values among the acceptable concentrations (ACs) (see Table 15-9). The major difficulty in setting exposure limits for HCN is the lack of good dose-response inhalation data from human and animal studies. Even with the data from a few epidemiological studies on HCN-exposed workers, the correlation between exposure concentrations and cyanide toxicity cannot be established with certainty. Most of the human studies were conducted to investigate the consequences of brief exposures to high concentrations. Most of the animal inhalation data also were obtained from brief exposures to high concentrations; these results on lethality or serious toxicity are of little value in setting exposure limits.

The brain is known to be the organ most sensitive to HCN toxicity. However, a recent NTP rodent study showed that the reproductive system also is sensitive to HCN. Chronic cyanide exposures have resulted in goiters in humans. Data on these toxicity end points will be considered in setting the SMAC values.

The most relevant data for setting exposure limits come from the reports of 36 workers in three electroplating factories where cyanide salts were used (El Ghawabi et al. 1975). Unfortunately, the subjects in those factories were exposed to other chemicals besides HCN. Symptoms reported from past and "present" medical histories and from interviews of these workers, who had worked for periods of a few years to more than 15 y in those facilities. HCN concentrations in the three factories during the 2-mo survey were measured at 6.4 ± 6.9, 8.1 ± 8.2, and 10.4 ± 10.9 ppm; the highest concentration was found in factory A, which had no ventilation. The most prevalent and least-severe symptoms were headache and weakness, which were reported by 80% of the workers (20-30% in controls). Vomiting and more-serious CNS effects were reported by 44% of the subjects during their tenures. The authors did not mention whether any symptoms were present during the survey period. The symptoms were reported by the workers who had worked for many years in these factories. The concentrations reported were obtained by analyzing air samples taken several times from each factory during the 3-mo survey. It is likely that the more serious symptoms, such as vomiting, were the result of brief exposures to high HCN concentrations at work. Therefore, it is reasonable to conclude that 8 ppm would likely produce no more than mild CNS effects (e.g., mild headache), which would be acceptable for 1-h exposures in a spacecraft. Therefore, 8 ppm is set as the 1-h AC for HCN. The concentration is reduced by half to 4 ppm to ensure that exposure would produce no more than slight CNS effects. This AC is further reduced to 1 ppm as the AC for 7-d, 30-d and 180-d exposures. HCN at 1 ppm is not expected to produce any CNS effects.

When rats and mice were fed drinking water containing NaCN at 300 ppm, which is equivalent to HCN at 15.6 ppm in the air (see Table 15-4), they had

mild but statistically significant decreases in the weights of the left epididymis and left cauda epididymis. Statistically significant changes in testicular weight, sperm counts and sperm motility were also observed in rats but not in mice (NTP 1993). NTP concluded that those changes are probably not biologically significant and are insufficient to decrease fertility in rats; however, NTP cautioned that "humans are considered to be relatively more sensitive than rats to such changes." Thus, 300 ppm in drinking water or the equivalent of 16 ppm continuous (24-h) inhalation exposure was treated as the no-observed-adverse-effect level (NOAEL) for rats. A species factor of 10 is used to account for the possibility that humans are more sensitive to the reproductive toxicity of HCN. Therefore, the AC for 7 d and 30 d is set at 1.6 ppm (NOAEL ÷ 10). The 180-d AC is calculated below:

El Ghawabi et al. (1976) reported that mild goiter was detected in 44% of the 36 Egyptian workers working for up to 15 y in poorly ventilated electroplating factories where the HCN concentration varied greatly. The great variations in exposure length and concentrations make these data unsuitable for setting SMACs. A recent 13-w study on HCN toxicity to all major organs and tissues, including thyroid, has more direct relevance to the assessment of HCN exposures (≤ 180 d) in spacecraft. In the latter study, rats and mice were fed water containing as much cyanide as 300 ppm; histopathological examination revealed no thyroid lesions. As discussed above, the exposure concentration is roughly equivalent to a 24-h continuous airborne HCN concentration of 15.6 ppm for the rats or 34.7 ppm for the mice. Rats are known to be more sensitive than mice to the effects of chemicals on the thyroid; therefore, the equivalent exposure of 15.6 ppm to rats are used to establish ACs. A species factor of 10 is applied to obtain an AC of 1.6 ppm for a 30-d exposure. A factor of 2 is applied to obtain an AC of 0.8 ppm (which was rounded up to 1 ppm) for 180-d exposure, which is twice the duration of that in the NTP study. It is very unlikely that exposures to low concentrations of HCN for 7 d or less could alter thyroid function. Therefore, ACs for exposures of 7 d or less will not be set on the basis of thyroid toxicity.

The major difficulty in setting exposure limits for HCN is the lack of controlled human inhalation exposure data, or good dose-response inhalation data from animals. It is recommended that experiments be carried out to better elucidate the concentration response of HCN at exposure concentrations at 20 ppm and below. These results will be useful for reevaluating SMACs, TLVs, and the OSHA PEL. The current TLV and PEL for HCN are 10 and 4.7 ppm, respectively.

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