From CHRONIC TOXICITY SUMMARY HYDROGEN CYANIDE (Formonitrile; hydrocyanic acid; prussic acid) CAS Registry Number: 74-90-8:


'Another retrospective study (Blanc et al., 1985) examined 36 former silver-reclaiming workers with longterm exposure to hydrogen cyanide fumes. The authors found significant trends between the incidence of self-reported CNS symptoms during active employment (headache, dizziness, nausea, and bitter almond taste), the symptoms reported post-exposure, and a qualitative index of exposure retroactively defined by the investigators as low-, moderate-, or high-exposure through work histories. Some symptoms persisted for 7 months or more after exposure. None of the workers had palpable thyroid gland abnormalities, but clinical tests revealed decreases in vitamin B12 absorption and folate levels and statistically significant increases in thyroid-stimulating hormone levels, which in combination with the CNS effects, suggest longterm adverse effects associated with cyanide exposure.'


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CHRONIC TOXICITY SUMMARY
HYDROGEN CYANIDE
(Formonitrile; hydrocyanic acid; prussic acid)
CAS Registry Number: 74-90-8
I. Chronic Toxicity Summary
Inhalation reference exposure level 9 µg/m3 (8 ppb)
Critical effect(s) CNS effects, thyroid enlargement, and hematological
disorders in workers
Hazard index target(s) Nervous system; endocrine system;
cardiovascular system
II. Physical and Chemical Properties (HSDB, 1999 )
Description Colorless liquid/gas
Molecular formula HCN
Molecular weight 27.03
Boiling point 25.6 oC
Melting point -13.4 oC
Vapor pressure 630 torr @ 20oC
Solubility Miscible in water, alcohol; slightly soluble
in ether
Conversion factor 1 ppm = 1.10 mg/m3 @ 25 oC

III. Major Uses or Sources
Hydrogen cyanide is used in a variety of syntheses including the production of adiponitrile (for nylon), methyl methacrylate, sodium cyanide, cyanuric chloride, chelating agents, pharmaceuticals, and other
specialty chemicals. Manufacturing activities releasing hydrogen cyanide include electroplating, metal mining, metallurgy and metal cleaning processes. Additionally, hydrogen cyanide has some insecticide and fungicide applications (ATSDR, 1993). Fires involving some nitrogen-containing polymers, often found in fibers used in fabrics, upholstery covers, and padding, also produce hydrogen cyanide (Tsuchiya and Sumi,
1977).
Another common source of hydrogen cyanide is cigarette smoke. Levels in inhaled mainstream cigarette smoke range from 10 to 400 µg per cigarette (U.S. brands); 0.6% to 27% (w/w) of these mainstream levels
are found in secondary or sidestream smoke (Fiskel et al., 1981). The annual statewide industrial emissions from facilities reporting under the Air Toxics Hot Spots Act in California based on the most recent
inventory were estimated to be 188,665 pounds of hydrogen cyanide (CARB, 1999b).


IV. Effects of Human Exposure
Occupational epidemiological studies of hydrogen cyanide exposure are complicated by the mixed chemical environments, which are created by synthetic and metallurgic processes. However, several reports indicate that chronic low exposure to hydrogen cyanide can cause neurological, respiratory, cardiovascular, and thyroid effects (Blanc et al., 1985; Chandra et al., 1980; El Ghawabi et al., 1975).
Although these studies have limitations, especially with incomplete exposure data, they also indicate that long-term exposure to inhaled cyanide produces CNS and thyroid effects.

El Ghawabi et al. (1975) studied 36 male electroplating workers in three Egyptian factories exposed to plating bath containing 3% copper cyanide, 3% sodium cyanide, and 1% sodium carbonate. Breathing zone cyanide concentrations ranged from 4.2 to 12.4 ppm (4.6 to 13.7 mg/m3), with means from 6.4 to 10.4 ppm (7.1 to 11.5 mg/m3), in the three factories at the time of this cross-sectional study. The men were exposed for a duration of 5 to 10 years, except for one man with 15 years exposure. Twenty non-exposed male volunteers were used as controls. None of the subjects, controls or workers, currently smoked cigarettes. Complete medical histories were taken, and medical exams were performed. Urinary levels of
thiocyanate (a metabolite of cyanide) were utilized as a biological index of exposure. Thyroid function was measured as the uptake of radiolabeled iodine, since thiocyanate may block the uptake of iodine by the
thyroid leading to iodine-deficiency goiters. Frequently reported symptoms in the exposed workers included headache, weakness, and altered sense of taste or smell. Lacrimation, abdominal colic, and lower stomach pain, salivation, and nervous instability occurred less frequently. Increased blood hemoglobin and lymphocyte counts were present in the exposed workers. Additionally, punctate basophilia were found in 78% (28/36) of the exposed subjects. Twenty of the thirty six exposed workers had thyroid enlargements, although there was no correlation between the duration of exposure with either the incidence or the degree of enlargement. Thyroid function test indicated significant differences in uptake between controls and exposed individuals after 4 and 24 hours. Urinary excretion of thiocyanates correlated with the breathing zone concentrations of cyanides. Symptoms persisted in 50% of the dyspneic workers in a 10-month
nonexposure follow up period. This study reported a LOAEL of 6.4 ppm (7.1 mg/m3) for the CNS
symptoms and thyroid effects.


Another retrospective study (Blanc et al., 1985) examined 36 former silver-reclaiming workers with longterm exposure to hydrogen cyanide fumes. The authors found significant trends between the incidence of
self-reported CNS symptoms during active employment (headache, dizziness, nausea, and bitter almond taste), the symptoms reported post-exposure, and a qualitative index of exposure retroactively defined by
the investigators as low-, moderate-, or high-exposure through work histories. Some symptoms persisted for 7 months or more after exposure. None of the workers had palpable thyroid gland abnormalities, but
clinical tests revealed decreases in vitamin B12 absorption and folate levels and statistically significant increases in thyroid-stimulating hormone levels, which in combination with the CNS effects, suggest longterm adverse effects associated with cyanide exposure. Due to the systemic nature of the lesions produced by cyanide, orally ingested cyanide will likely result in
injuries similar to that seen by inhalation exposure. Cassava root, a dietary staple in many tropical regions, contains cyanogenic glycosides, such as linamarin, which release cyanide (CN-) when metabolized endogenously (Sharma, 1993; Kamalu, 1995). Consumption of insufficiently processed cassava roots over a period of time in combination with a protein deficient diet has been implicated in neurotoxic effects. One such neuropathy known as konzo results in nerve cell degeneration leading to a permanent but nonprogressive spastic weakness of the legs and degeneration of corresponding corticospinal pathways (Tylleskar et al., 1992; Tor-Agbidye et al., 1999). The development of this sydrome is hypothesized to depend on (a) the amount and duration of exposure to dietary cyanide, and (b) the ability of the body to detoxify cyanide, a function that may vary with nutritional status. The endogenous conversion of cyanide to cyanate (OCN-) is thought to be a contributor to the neurotoxic symptoms, but other substances found in cassava flour have been implicated (Obidoa and Obasi, 1991; Tor-Agbidye et al., 1999; Kamalu, 1995).
Tylleskar et al. (1992) determined daily cassava flour consumption at above 0.5 kg per adult in a konzoaffected, albeit malnourished, African population. Thus, the potential daily cyanide exposure was estimated to be 0.5-1 mmol (13-26 mg), which correlated well with urinary concentrations of the
metabolite, thiocyanate. A similar daily cyanide intake via cassava ingestion was estimated at 15-31.5 mg (approximately 0.2-0.45 mg/kg) following a major outbreak of konzo in Mozambique (Casadei et al.,1984;
Cliff et al., 1984). Other effects associated with cassava consumption include pancreatic diabetes, vitamin B12 deficiency and decreased iodine uptake (Sharma, 1993; Jansz and Uluwaduge, 1997). Cretinism in children, associated with a deficiency of dietary iodine, is worsened by eating cassava (Miller, 1974). Excess thiocyanate due to cyanide metabolism results in a depressed uptake of iodine by the thyroid gland that may lead to symptoms of iodine deficiency, including goiter. A comparison of three villages in Ethiopia observed increased total goiter rate with increasing rate of cassava consumption (Abuye et al., 1998). Goiter was also more prevalent in females and in individuals under 20 years of age. In one village, the incidence of goiter increased following the introduction of cassava, indicating that cassava exacerbated pre-existing
iodine deficiency. Urinary iodine levels of school children revealed marginal dietary consumption of iodine, but were within the normal range. However, low T4 and high TSH levels indicated insufficient
iodine uptake by the thyroid gland due to cassava consumption.
V. Effects of Animal Exposures There is little animal data for chronic inhalation exposure to hydrogen cyanide; only two subchronic studies
were noted by U.S. EPA, one in rabbits (Hugod, 1979, 1981) and the other in dogs (Valade, 1952). Continuous exposure of rabbits to 0.5 ppm HCN (0.55 mg/m3) for either 1 or 4 weeks produced no microscopically detectable morphological changes of the lungs, pulmonary arteries, coronary arteries or aorta. This study observed a subacute inhalation NOAEL for HCN in rabbits of 0.5 ppm (Hugod, 1979, 1981). Four dogs exposed to 50 mg/m3 (45 ppm) hydrogen cyanide in a series of 30-minute inhalation periods conducted at 2-day intervals demonstrated extensive CNS toxicity, including dyspnea and vomiting, with vascular and cellular CNS lesions identified post-mortem (Valade, 1952). Male Sprague-Dawley rats were administered potassium cyanide (0, 40, 80, or 160 mg KCN/kg bw-day) in the drinking water for 13 weeks (Leuschner et al., 1991). At the highest dose, blood cyanide concentrations were between 16 and 26 mmol CN-/ml blood and thiocyanate ranged between 341 and 877
mmmol SCN-/ml plasma. The high dose group exposure was reduced to 140 mg/kg-day after 12 weeks because of decreased body weight gain, reduced drinking water consumption, and mortality in this group.
Male New Zealand white rabbits (6 per group) were administered potassium cyanide in the diet over a 40 week experiment (Okolie and Osagie, 1999). The average cyanide intake was 36.5 mg/day. Based on the
growth data presented in the report, cyanide intake was estimated at approximately 20 mg/kg-day. The cyanide-exposed group had higher feed consumption with reduced weight gain, and focal necrosis was
noted in the liver and kidney. Male weanling rats (strain not identified, 10 animals per group) were administered potassium cyanide
(1500 ppm) in the diet for 11.5 months (Philbrick et al., 1979). There were no deaths or overt signs of toxicity. There was a reduction in body weight gain in the exposed group. Myelin degeneration was noted
in the spinal cord white matter of cyanide exposed animals.
Kamalu (1993) fed groups of dogs (6/group; strain not specified) either a control diet containing rice as the carbohydrate source, a diet with cassava as a carbohydrate source, or a control diet containing NaCN, for
14 weeks. Both the cassava and NaCN diets were adjusted to release 10.8 mg HCN/kg cooked food.
Growth was depressed only in the dogs fed rice + NaCN. Plasma thiocyanate was significantly lower in dogs fed cassava compared to dogs fed rice + NaCN. These effects indicate that all the intact cyanogenic
glycosides absorbed from cassava, primarily linamarin, was not hydrolyzed to HCN. However, evidence of liver inflammation and hemorrhage were observed only in the cassava fed dogs. Kidney, adrenal, myocardial, and testicular lesions were noted in both treated groups, but were considered more severe in the cassava fed dogs. It was concluded that the lesions, observed in the cassava fed dogs, were not entirely due to cyanide.
No information was found regarding developmental and reproductive effects in humans for any route of hydrogen cyanide exposure. No animal studies utilizing dermal exposure have been reported for either hydrogen cyanide or cyanide salts. Dietary studies of the high cyanogenic glycoside cassava diet have shown adverse effects, increased runting and decreased ossification in hamsters (Frakes et al., 1986), but
not in rats fed cassava alone, or supplemented with potassium cyanide (Tewe and Maner, 1981). Hamsters with gestational cassava exposure did not display reproductive effects (Frakes et al., 1986).
VI. Derivation of Chronic Reference Exposure Level
Study El Ghawabi et al. (1975); U.S. EPA (1994)
Study population 36 male electroplating workers
Exposure method Discontinuous occupational inhalation exposures
Critical effects CNS effects, thyroid enlargement, and
hematological disorders
LOAEL 7.1 mg/m3
NOAEL Not observed
Exposure continuity 8 hr/day (10 m3/day/20 m3/day), 5 days/week
Average occupational exposure 2.5 mg/m3 for LOAEL group
Human equivalent concentration 2.5 mg/m3 for LOAEL group
Exposure duration 5 to 10 years (except one man for 15 years)
LOAEL uncertainty factor 10
Subchronic uncertainty factor 3
Interspecies uncertainty factor 1
Intraspecies uncertainty factor 10
Cumulative uncertainty factor 300
Inhalation reference exposure level 0.008 ppm (8 ppb, 0.009 mg/m3, 9 µg/m3)
The USEPA based its RfC of 3 mg/m3 on the same study but included a Modifying Factor (MF) of 3 for
lack of chronic and multigenerational reproduction studies. The criteria for use of modifying factors are
not well specified by U.S. EPA. Such modifying factors were not used by OEHHA. OEHHA used a 3-fold
subchronic uncertainty factor because most workers were exposed for less than ten years (78%) and many
were exposed for less than 5 years (39%)..
An alternative analysis was conducted using data from an animal ingestion study reporting effects at low
cyanide concentrations:
Study Jackson (1988)
Study population Miniature swine
Exposure method Daily oral administration of aqueous potassium cyanide
Critical effects Behavioral effects; decreased blood T3 and T4
LOAEL 0.4 mg/kg-day
NOAEL Not observed
Exposure continuity Apparently 7 days per week
Average exposure 0.4 mg/kg-day (1.4 mg/m3 for LOAEL group assuming 20
m3/day inhalation by a 70 kg person)
Human equivalent concentration Not derived due to lack of species-specific data
Exposure duration 24 weeks
LOAEL uncertainty factor 3 (minimal effects at lowest dose)
Subchronic uncertainty factor 10 (based on assumed 27 year lifespan)
Interspecies uncertainty factor 10
Intraspecies uncertainty factor 10
Cumulative uncertainty factor 3,000
Inhalation reference exposure level 0.0005 mg/m3 (0.5 µg/m3; 0.0004 ppm; 0.4 ppb)
This study reported neurobehavioural and thyroid effects at cyanide exposure levels (equivalent to 1.4 to
4.2 mg/m3) similar to that reported by El Ghawabi (2.5 mg/m3). However, as greater uncertainty factors are
required for use of the animal study, a lower REL was derived. Use of a cross-route extrapolation also
introduces uncertainty. Therefore the REL derived from the human data is more appropriate.
VII. Data Strengths and Limitations for Development of the REL
The major strength of the RfC for hydrogen cyanide is the use of human health effects data. The major
uncertainties are the lack of a NOAEL observation in the key study, the difficulty in estimating exposures,
and the discontinuous and variable nature of the exposures.

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hydrocyanic acid.] Bull. Acad. Nat. Med. (Paris) 136:280-285.[/SIZE=1]