TOXICOLOGICAL REVIEW

OF

ACETONITRILE

(CAS No. 75-05-8) 

In Support of Summary Information on the 

Integrated Risk Information System (IRIS) 

January 1999 

U.S. Environmental Protection Agency 

Washington, DC 
DISCLAIMER 

This document has been reviewed in accordance with U.S. Environmental Protection Agency 

policy. Mention of trade names or commercial products does not constitute endorsement or 

recommendation for use. 

Note: This document may undergo revisions in the future. The most up-to-date version will be 

made available electronically via the IRIS Home Page at http://www.epa.gov/iris. 

ii 
CONTENTS--TOXICOLOGICAL REVIEW OF ACETONITRILE (CAS No. 75-05-8) 

FOREWORD ................................................................ iv

AUTHORS, CONTRIBUTORS, AND REVIEWERS .................................v

1. INTRODUCTION ......................................................... 1

2. CHEMICAL AND PHYSICAL INFORMATION RELEVANT TO ASSESSMENTS .....2

3. TOXICOKINETICS RELEVANT TO ASSESSMENTS ............................2

4. HAZARD IDENTIFICATION ................................................4

4.1. STUDIES IN HUMANS .................................................4

4.2. PRE-CHRONIC AND CHRONIC STUDIES AND CANCER BIOASSAYS IN

ANIMALS—ORAL AND INHALATION ..................................5

4.3. REPRODUCTIVE/DEVELOPMENTAL STUDIES - ORAL AND INHALATION . 10

4.4. OTHER STUDIES ....................................................13

4.4.1. Acute Data ....................................................13

4.4.2. Genotoxicity ...................................................14

4.5. SYNTHESIS AND EVALUATION OF MAJOR NONCANCER EFFECTS AND

MODE OF ACTION—ORAL AND INHALATION ..........................15

4.6. WEIGHT OF EVIDENCE EVALUATION AND CANCER CLASSIFICATION ...16

4.7. SUSCEPTIBLE POPULATIONS .........................................17

4.7.1. Possible Childhood Susceptibility ..................................17

4.7.2. Possible Gender Differences .......................................17

5. DOSE RESPONSE ASSESSMENTS ..........................................17

5.1. ORAL REFERENCE DOSE (RfD) .......................................17

5.2. INHALATION REFERENCE CONCENTRATION (RfC) .....................18

5.2.1. Choice of Principal Study and Critical Effect ..........................18

5.2.2. Methods of Analysis .............................................19

5.2.3. RfC Derivation - Including Application of Uncertainty Factors and

Modifying Factors ...............................................19

5.3. Cancer Assessment ....................................................20

6. MAJOR CONCLUSIONS IN THE CHARACTERIZATION OF HAZARD AND DOSE

RESPONSE ..............................................................20

6.1. HUMAN HAZARD POTENTIAL ........................................20

6.2. DOSE RESPONSE ....................................................21

7. REFERENCES ...........................................................22

APPENDIX A: EXTERNAL PEER REVIEW—SUMMARY OF COMMENTS

AND DISPOSITION ..........................................................28

iii 
FOREWORD 

The purpose of this Toxicological Review is to provide scientific support and rationale 

for the hazard and dose-response assessment in IRIS pertaining to chronic exposure to 

acetonitrile. It is not intended to be a comprehensive treatise on the chemical or toxicological 

nature of acetonitrile. 

In Section 6, EPA has characterized its overall confidence in the quantitative and 

qualitative aspects of hazard and dose response. Matters considered in this characterization 

include knowledge gaps, uncertainties, quality of data, and scientific controversies. This 

characterization is presented in an effort to make apparent the limitations of the assessment and 

to aid and guide the risk assessor in the ensuing steps of the risk assessment process. 

For other general information about this assessment or other questions relating to IRIS, 

the reader is referred to EPA’s Risk Information Hotline at 202-566-1676. 

iv 
AUTHORS, CONTRIBUTORS, AND REVIEWERS 

Chemical Manager/Author 

Mark Greenberg

National Center for Environmental Assessment

Office of Research and Development

U.S. Environmental Protection Agency

Research Triangle Park, NC

Reviewers 

This document and summary information on IRIS have received peer review both by EPA 

scientists and by independent scientists external to EPA. Subsequent to external review and 

incorporation of comments, this assessment has undergone an Agency-wide review process 

whereby the IRIS Program Director has achieved a consensus approval with EPA’s Program 

Offices (the Offices for Air and Radiation, Planning and Evaluation, Prevention, Pesticides, and 

Toxic Substances, Research and Development, Solid Waste and Emergency Response, and 

Water) and the ten Regional Offices. 

Internal EPA Reviewers 

Katherine Anitole, Ph.D. 

Risk Assessment Division

OPPTS

David Lai, Ph.D.

Risk Assessment Division 

OPPTS

External Peer Reviewers 

Shayne C. Gad, Ph.D.

Gad Consulting Services

Raleigh, NC

C.C. Conaway, Ph.D.

American Health Foundation

Valhalla, NY

Edward J. Sowinski, Ph.D.

Environmental Health Management and Science, Inc.

v 
AUTHORS, CONTRIBUTORS, AND REVIEWERS (continued) 

Summaries of the external peer reviewers’ comments and the disposition of their 

recommendations are in Appendix A. 

vi
1. INTRODUCTION 

This document presents background and justification for the hazard and dose-response 

assessment summaries in EPA’s Integrated Risk Information System (IRIS). IRIS summaries 

may include an oral reference dose (RfD), inhalation reference concentration (RfC) and a 

carcinogenicity assessment. 

The RfD and RfC provide quantitative information for noncancer dose-response 

assessments. The RfD is based on the assumption that thresholds exist for certain toxic effects 

such as cellular necrosis but may not exist for other toxic effects such as some carcinogenic 

responses. It is expressed in units of mg/kg-day. In general, the RfD is an estimate (with 

uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human population 

(including sensitive subgroups) that is likely to be without an appreciable risk of deleterious 

noncancer effects during a lifetime. The inhalation RfC is analogous to the oral RfD, but 

provides a continuous inhalation exposure estimate. The inhalation RfC considers toxic effects 

for both the respiratory system (portal-of-entry) and for effects peripheral to the respiratory 

system (extrarespiratory or systemic effects). It is generally expressed in units of mg/m3

. 

The carcinogenicity assessment provides information on the carcinogenic hazard potential 

of the substance in question and quantitative estimates of risk from oral exposure and inhalation 

exposure. The information includes a weight-of-evidence judgment of the likelihood that the 

agent is a human carcinogen and the conditions under which the carcinogenic effects may be 

expressed. Quantitative risk estimates are presented in three ways. The slope factor is the result 

of application of a low-dose extrapolation procedure and is presented as the risk per mg/kg/day. 

The unit risk is the quantitative estimate in terms of either risk per μg/L drinking water or risk 

per �g/m3

 air breathed. Another form in which risk is presented is a drinking water or air 

concentration providing cancer risks of 1 in 10,000; 1 in 100,000; or 1 in 1,000,000. 

Development of these hazard identification and dose-response assessments for 

acetonitrile has followed the general guidelines for risk assessment as set forth by the National 

Research Council (1983). EPA guidelines that were used in the development of this assessment 

may include the following: the Guidelines for Carcinogen Risk Assessment (U.S. EPA, 1986a), 

Guidelines for the Health Risk Assessment of Chemical Mixtures (U.S. EPA, 1986b), Guidelines 

for Mutagenicity Risk Assessment (U.S. EPA, 1986c), Guidelines for Developmental Toxicity 

Risk Assessment (U.S. EPA, 1991), Guidelines for Neurotoxicity Risk Assessment (U.S. EPA, 

1998a), Proposed Guidelines for Carcinogen Risk Assessment (1996a), and Reproductive 

Toxicity Risk Assessment Guidelines (U.S. EPA, 1996b); Recommendations for and 

Documentation of Biological Values for Use in Risk Assessment (U.S. EPA, 1988); (proposed) 

Interim Policy for Particle Size and Limit Concentration Issues in Inhalation Toxicity (U.S. EPA, 

1994a); Methods for Derivation of Inhalation Reference Concentrations and Application of 

Inhalation Dosimetry (U.S. EPA, 1994b); Peer Review and Peer Involvement at the U.S. 

Environmental Protection Agency (U.S. EPA, 1994c); Use of the Benchmark Dose Approach in 

Health Risk Assessment (U.S. EPA, 1995); Science Policy Council Handbook: Peer Review 

(U.S. EPA, 1998b); and memorandum from EPA Administrator, Carol Browner, dated March 21, 

1995, Subject: Guidance on Risk Characterization. 

1
Literature search strategy employed for this compound were based on the CASRN and at 

least one common name. At a minimum, the following databases were searched: RTECS, 

HSDB, TSCATS, CCRIS, GENETOX, EMIC, EMICBACK, DART, ETICBACK, TOXLINE, 

CANCERLINE, MEDLINE, and MEDLINE backfiles. Any pertinent scientific information 

submitted by the public to the IRIS Submission Desk was also considered in the development of 

this document. 

2. CHEMICAL AND PHYSICAL INFORMATION RELEVANT TO ASSESSMENTS 

Acetonitrile (ACN) is also known as cyanomethane or methyl cyanide. Some relevant 

physical and chemical properties of ACN are listed below (WHO, 1993): 

CAS Registry number: 75-05-8 

Empirical formula: CH3CN

Molecular weight: 41.05 

Vapor pressure: 74 mmHg at 20�C

Water solubility: infinitely soluble

Log KOW: -0.34

Conversion factor: 1 ppm = 1.68 mg/m3

, 1 mg/m3 = 0.595 ppm (25 �C, 760 mmHg)

At room temperature, ACN is a volatile, colorless liquid with etherlike odor. It is one of 

the most stable nitriles. Acetonitrile has a TLV-TWA of 40 ppm (67 mg/m3), with a short-term 

exposure limit (STEL) of 60 ppm (101 mg/m3), recommended to protect against organic cyanide 

poisoning and injury to the respiratory tract (ACGIH, 1991). 

Although nitriles are widely used to synthesize amines, amides, ketones, aldehydes, and a 

variety of other compounds (Kirk-Othmer Concise Encyclopedia of Chemical Technology, 

1985), ACN is used primarily as a solvent. Nitriles in general are hydrolyzed in the presence of 

acidic conditions to form amides. Although ACN is one of the more stable nitriles, acidic 

hydrolysis would be expected to yield hydrogen cyanide. Hydrolysis in water has been reported 

as extremely slow (WHO, 1993). Willhite (1983) found that freshly prepared solutions of ACN 

in distilled water did not undergo any significant hydrolysis upon incubation at 37 �C for 2.5 

hours. 

3. TOXICOKINETICS RELEVANT TO ASSESSMENTS 

Inorganic cyanide has long been known to react with trivalent iron of cytochrome oxidase 

in mitochondria and block the reduction of oxygen needed for cellular respiration, thus leading to 

cytotoxic anoxia (Albaum et al., 1946). The toxicity of ACN is believed to be mediated, in part, 

through this mechanism. ACN is metabolized to inorganic cyanide, but the conversion occurs 

slowly compared to other nitriles (which may explain the delay in onset of acute symptoms). 

Freeman and Hayes’ (1988) data suggest that the conversion to cyanide is oxygen- and NADPHdependent, possibly mediated by P450 isozyme (2E1 or P-450j). Some investigators suggest that 

ACN produces cyanohydrin by a P450 reaction, which is then decomposed by catalase to release 

2
cyanide (Ahmed et al., 1992; Feierman and Cederbaum, 1989; Willhite and Smith, 1981). 

Formaldehyde and formic acid are also postulated to be by-products of ACN metabolism 

(Ahmed et al., 1992). Cyanide can be further oxidized to thiocyanate, a less toxic compound that 

is excreted in urine, but one that may interfere with thyroid function (Hartung, 1981). 

Conversion is mediated by rhodanese, a sulfurtransferase found in liver and human nasal 

respiratory mucosa (Lewis et al., 1991). A minor urinary metabolite that has been detected after 

administration of ACN in drinking water to rats is 2-aminothiolazine-4-carboxylic acid (Swenne 

et al., 1996). Cyanide also can be oxidized to cyanate ion with further oxidation to formic acid 

(McMahon and Birnbaum, 1990). 

Like hydrogen cyanide (HCN), ACN is readily absorbed from the lungs and 

gastrointestinal tract, and is distributed throughout the body in both humans and laboratory 

animals. In a group of male and female test subjects (16), 74% of inhaled ACN was absorbed 

when cigarette smoke was held in the mouth for 2 seconds (and not inhaled), and 91% was 

absorbed when smoke was inhaled (Dalhamn et al., 1968a,b). Autopsy of an individual who died 

2 days following inhalation of ACN vapors showed that cyanide reaches the spleen, lungs, and 

kidneys, but was not detected in the liver (WHO, 1993). Oral exposure of animals resulted in 

metabolites found primarily in the spleen, stomach, and skin (U.S. EPA, 1985a). ACN serum 

concentrations were higher than cyanide levels in an individual hospitalized after oral ACN 

ingestion (Michaelis et al., 1991); elimination half-lives were 32 hours for ACN and 15 hours for 

cyanide. Cyanide was not detected in the blood of three human subjects exposed to 

concentrations of 40, 80, or 160 ppm for 4 hours; a slight increase in thiocyanate levels was 

detected in urine (Pozzani et al., 1959). 

Hydrocyanic acid was found in brain, heart, kidney, and spleen of rats after inhalation of 

ACN vapors (Haguenoer et al., 1975). Cyanide and thiocyanate were measured in blood, liver, 

brain, and kidney of hamsters following oral exposure (Willhite, 1983); thiocyanate in blood, 

kidney, and liver was up to 10-fold higher than in the brain, while cyanide in blood and liver was 

generally higher than in the brain and kidney 2.5 hours after exposure to a single dose of 100, 

200, or 400 mg/kg ACN. Exposure of rats to ACN in drinking water indicated that the rat has a 

high capacity for cyanide detoxification on both normal and low-protein diets. On a low-protein 

diet, cyanide detoxification is at the expense of protein catabolism (Swenne et al., 1996). 

Absorption of ACN is rapid in beagle dogs exposed to 16,000 ppm ACN (26,880 mg/m3

) 

vapors for 4 hours, based on blood cyanide concentrations peaking and reaching steady-state 

concentrations of 305-433 μg/100 mL after approximately 3 hours (Pozzani et al., 1959). With 

longer-term exposure in monkeys inhaling 350 ppm for approximately 99 days, blood samples on 

the 35th day of exposure, after a 2-day rest period (i.e., a weekend), did not detect cyanide ion, 

but 7.6-9.2 μg cyanide ion/100 mL was measured after the 39th day of exposure (i.e., following 

5 consecutive days of exposure). Thiocyanate was detected in the urine after the 2-day break, 

and accumulated over the 5-day exposure week (Pozzani et al., 1959). Rats exposed to 166 or 

300 ppm, 7 hours/day, 5 days/week, for 90 days had almost complete urinary excretion of 

thiocyanate after the 2.5-day rest period each week (Pozzani et al., 1959). 

ACN conversion to cyanide appears to be about 10-fold greater in rat nasal 

ethmoturbinate microsomes than in liver microsomes (Dahl and Waruszewski, 1989), possibly 

3
because of high P450 content. The nasal cavity, in particular the olfactory region, has a high 

concentration of rhodanese (Dahl and Waruszewski, 1989; Dahl, 1989). Rhodanese has also 

been found in the respiratory epithelium of human nasal tissue (Lewis et al., 1991); these in vitro 

studies suggest that rhodanese in smokers may have reduced affinity for cyanide. Lewis and 

colleagues also calculate that the capacity of rat nasal tissue is sufficient to metabolize a 

maximum of 2,800 ppm inhaled HCN. The high concentration of rhodanese in rat nasal tissue 

suggests that cyanide may be detoxified more rapidly to thiocyanate than in animal species with 

lower amounts of the enzyme (Dahl and Waruszewski, 1989). Aminlari et al. (1994) 

demonstrated that dogs had a greater rhodanese activity in the nasal cavity than in the lower 

respiratory tract. 

Whole body autoradiography in male mice injected intravenously with ACN radiolabeled 

with 14C in the methyl group indicated that radioactivity was widely distributed throughout the 

body (e.g., liver, thymus, and reproductive organs). Interestingly, nonvolatile radioactivity was 

also observed in nasal secretions, mouth cavity, esophagus, and stomach contents (Ahmed et al., 

1992). One could infer from these observations that ACN could also distribute to the stomach 

upon inhalation exposure. 

4. HAZARD IDENTIFICATION 

4.1. STUDIES IN HUMANS 

Other than one case-referent study, there are no epidemiological studies of effects of 

ACN exposure in humans. Individuals who were acutely exposed to ACN developed effects 

generally attributed to metabolism of ACN to cyanide (U.S. EPA, 1985a). Case reports of acute 

occupational exposure to ACN indicate that workers exhibited nausea, shallow and/or irregular 

respiration, and impaired motor activity. An autopsy of a worker who died shortly after exposure 

revealed cerebral, thyroid, liver, splenic, and renal congestion (WHO, 1993). Gastric erosion has 

been reported in individuals who ingested ACN (Way, 1981; Ballantyne, 1983). 

In a clinical study by Pozzani et al. (1959), two men who inhaled 40 ppm ACN (67 

mg/m3

) for 4 hours did not develop any adverse subjective effects, and blood cyanide and urinary 

thiocyanate were not appreciably increased. A third subject, also exposed to the same conditions, 

experienced slight chest tightness the evening after exposure, and cooling sensation in the lungs 

the following day. Only an increase in thiocyanate in the urine was evident; no cyanide was 

detected in the blood. “Olfactory fatigue” was reported during exposure for all subjects. Nine 

days later, two of the subjects inhaled 160 ppm ACN vapor (269 mg/m3

) for 4 hours. Flushing of 

face and chest tightness were reported 5 days after exposure, but there were no changes in urinary 

thiocyanate and blood cyanide from pre-exposure values. 

WHO (1993) has reported on several cases of children or adults who ingested large 

amounts of ACN (�250 to 4,000 mg/kg); symptoms included vomiting, respiratory distress, 

confusion, convulsions, seizures, and pulmonary edema, and in some instances, death occurred. 

Cyanide was detected in the blood of these individuals. 

4
In a case-referent study of medical records of Finnish women employed in state 

laboratories that was designed to determine if there was an association between spontaneous 

abortion and exposure, the odds ratio for seven cases involving exposure to ACN was close to 

unity (Taskinen et al., 1994). However, the small number of cases and other limitations (e.g., 

exposure was self-reported) of this study preclude any definitive conclusions. 

4.2. PRE-CHRONIC AND CHRONIC STUDIES AND CANCER BIOASSAYS IN 

ANIMALS—ORAL AND INHALATION 

The National Toxicology Program (NTP, 1996) evaluated the toxicity of ACN to the rat 

and mouse in both subchronic and chronic inhalation studies. In the 13-week subchronic study 

that served to set exposure levels for the chronic study, F344/N rats and B6C3F1 mice 

(10/sex/group) were exposed whole-body to ACN concentrations of 0, 100, 200, 400, 800, or 

1,600 ppm (0, 168, 336, 672, 1,343, or 2,686 mg/m3

), 6 hours/day, 5 days/week (durationadjusted concentrations were 0, 30, 60, 120, 240, and 480 mg/m3

). Purity of ACN was 99% or 

greater and actual concentrations were within 10% of target concentrations. Clinical observation 

and body weights were recorded weekly. At necropsy, brain, heart, kidney, liver, lungs, testis, 

and thymus weights were measured, hematology was performed, and thyroid hormone assays 

were conducted. All organs of animals exposed to 0, 800 ppm (males only), and 1,600 ppm were 

examined by histopathology. Selected organs of 800-ppm females (bone marrow, brain, lung, 

lymph node, ovary, spleen, thymus) and 400-ppm males (bone marrow, testes, thymus) were 

subjected to histopathological examination; additional organs were examined if they exhibited 

gross lesions. 

In rats, deaths occurred at 800 ppm (1 male) and 1,600 ppm (6 males and 3 females). All 

but one of the deaths occurred during the first 2 weeks of exposure. There was a significant 

decrease in body weight gain and final body weight at 1,600 ppm (81% of control for males; 91% 

for females); no change occurred in the other groups. Clinical signs at the two 

high-concentration groups included hypoactivity and ruffled fur during the first week. Ataxia, 

abnormal posture, and clonic convulsions occurred in the 1,600-ppm males that died. The other 

groups did not exhibit any treatment-related signs. Thymus weights were significantly lower in 

800- and 1,600-ppm rats (both sexes), compared to those of the controls. Significant decreases in 

red blood cell count, hemoglobin concentration, and hematocrit occurred in the 800 and 

1,600-ppm females and 1,600-ppm males. The investigators reported that these alterations were 

suggestive of anemia (characterized as nonresponsive, normocytic, and normochromic) since 

reticulocyte counts, mean cell volume, and mean cell hemoglobin concentration were similar to 

controls. The 1,600-ppm females also exhibited a decrease in triiodothyronine (T3) 

concentration, without changes in thyroxine (T4) and thyroid-stimulating hormone (TSH) 

concentrations. Histopathologic effects were limited to rats that died at 800 and 1,600 ppm; 

effects included congestion, edema, and hemorrhage in alveoli observed in lungs (no incidence 

data were provided). Because of the one death at 800 ppm (in week 1), coupled with mortality in 

the mouse (see below) at a lower concentrations, it is prudent to regard 800 ppm as a FEL 

(FEL[ADJ] = 239 mg/m3

) for the rat. Because only limited histopathology (i.e., bone marrow, 

thymus, and testes) was conducted in animals exposed to 400 ppm (males only), the results are 

insufficient to identify a no-observed-adverse-effect-level (NOAEL). 

5
In mice, deaths were observed at concentrations of 400 ppm and greater (0/10, 0/10, 0/10, 

0/10, 1/10, 10/10 in males; 0/10, 0/10, 0/10, 1/10, 4/10, 10/10 in females). All animals in the 

1,600-ppm groups died by week 4 of the study. Final body weight (92% of control) and body 

weight gain were significantly reduced at 400 ppm in males, but are not considered 

toxicologically significant. Hematological parameters were not evaluated in mice. Males 

exhibited a significant concentration-related increase in absolute (�200 ppm; p � 0.05) and 

relative (�100 ppm; p � 0.01) liver weight. Significant increased relative lung and kidney 

weights in some male groups were also observed, but changes were not concentration-related. 

Females had a significant increase in absolute liver weight at 800 ppm and in relative weight at 

�400 ppm. Histopathology revealed an increased incidence of hepatocellular vacuolation, with 

significance at 400 and 800 ppm (p � 0.01) (0/10, N/A, 0/10, 8/10, 7/9, and 0/10 in males; 0/10, 

N/A, 0/10, 7/10, 6/10, and 0/10 in females; 100 ppm males and females were not examined). No 

other hepatic effects were observed. Vacuolization was considered to represent increased 

glycogen storage by distension of previously existing clear spaces. Severity of vacuolization was 

characterized as moderate in the 800-ppm female group and mild in males. No hepatocellular 

vacuolization was observed in the 1,600-ppm animals that died during the study. The absence of 

this change in the 1,600-ppm animals may be indicative of an increased utilization of glycogen 

stores by the animals that died. Incidences of forestomach squamous epithelial hyperplasia were 

significantly increased in 800-ppm males and in females exposed to �200 ppm. The incidences 

were 0/10, 0/10, 3/10, 6/9, and 1/9 in males (100-ppm males were not examined) and 0/10, 0/10, 

7/10, 8/10, 7/10, and 5/10 in females; severity was not concentration-related. Hyperkeratosis and 

inflammatory cell infiltrate (effects associated with hyperplasia) also occurred in the 

forestomach. A significant increase in focal ulcers of the forestomach was also observed in 

1,600-ppm female mice. There were no effects reported for the lungs. 

The study identified a NOAEL of 200 ppm (NOAEL[ADJ] = 60 mg/m3

) based on 

mortality. The level of 400 ppm is considered a FEL given the early death (week 2) of one 

female mouse in the 400 ppm group and increased mortality (one male and four females) at 800 

ppm. Neither a NOAEL nor lowest-observed-adverse-effect level (LOAEL) can be identified for 

forestomach lesions inasmuch as grooming of contaminated fur and/or mucociliary clearance 

likely was the primary cause of the increased incidence in hyperplasia of the forestomach. 

Hyperplasia is considered adverse because it was associated with infiltration of inflammatory 

cells and, at the highest concentration in females, focal ulcers. 

Based on the findings in the 13-week study, the two-year study was initiated with F344/N 

rats (56/sex/group) exposed to actual ACN concentrations of 0, 100, 200, or 400 ppm (0, 168, 

336, or 672 mg/m3

), 6 hours/day, 5 days/week, for 103 weeks (duration adjusted to 30, 60, and 

120 mg/m3

) and B6C3F1 mice exposed to concentrations of 0, 50, 100, or 200 ppm (0, 84, 168, 

or 336 mg/m3) for 111 weeks (duration adjusted to 15, 30, and 60 mg/m3). An interim necropsy 

at 15 months involved 8 rats (each sex) and 10 mice (each sex). Complete histopathological 

examinations were conducted on all animals at this time and hematological parameters and liver, 

kidney, and lung weights were measured. Clinical signs and body weight were assessed 

throughout the study. After 2 years, animals were necropsied and examined for gross and 

microscopic alterations. 

6
At the 15-month interim necropsy for rats, hematological alterations were observed, but 

were significant (p � 0.05) only at the high concentration. Changes included decreased mean cell 

volume and mean cell hemoglobin (in both sexes), increased red cell count (males), and 

decreased hematocrit and hemoglobin (females); none of these effects was concentration-related. 

At this time, there were no neoplastic or nonneoplastic lesions observed in tissues examined that 

were attributable to exposure. Hematological parameters were not measured at study end. 

No significant changes in survival, body weights, or clinical appearance were observed in 

rats following 103-week exposure to ACN. Histopathological examination revealed no 

neoplastic or nonneoplastic lesions in any organs of exposed females. In male rats, a statistically 

significant increase in the incidence of basophilic hepatic foci was observed in the 200- and 400-

ppm groups (15/48, 22/47, 25/48, and 31/48), but the foci were not atypical in appearance. Thus, 

it is uncertain if these lesions are preneoplastic. The incidences of eosinophilic and mixed cell 

foci were marginally elevated in 400-ppm males, but were not statistically significant. Although 

there was a marginally significant positive trend in the incidence of adenoma, carcinoma, or 

adenoma and carcinoma (combined) in liver of male rats, no significant dose-related trend was 

present after incidences were adjusted for survival using the life table test. The incidences of 

hepatocellular adenomas and carcinomas in male rats were not significantly increased in the 

treated animals based on pairwise comparison with incidences in control animals. Also, the 

tumor incidences at 400 ppm were only slightly higher than the historical control range. Other 

effects observed in male rats included marginal (not concentration-related) increases in tumors in 

the adrenal medulla and pancreatic islets; incidences observed were within historical control 

range. Keratoacanthoma was observed in the skin of 400-ppm males (0/48, 1/47, 0/48, and 

4/48), but was not considered treatment-related; the incidence was within the historical control 

range. Although an increased incidence of basophilic foci is generally considered a possible 

preneoplastic effect and, thus, appropriate for discussion of potential carcinogenicity, the 

incidence is not considered evidence of a hepatotoxic effect nor a precursor to an hepatotoxic 

effect. As support for this view, hydrogen cyanide (U.S. EPA, 1985b) has not been found to 

cause adverse liver effects in rat feeding studies nor has it been associated with liver effects in 

human occupational studies. Thus, a NOAEL of 400 ppm (NOAEL[ADJ] = 120 mg/m3

) was 

identified for the rat. 

In mice, no changes in the survival of the treated animals were observed, compared to the 

survival in control animals. Body weights were similar for all groups, and treatment-related 

clinical signs were not evident. In contrast to the 13-week study, there were no 

concentration-related effects of liver weight, suggesting that the changes observed in the 13-week 

study were adaptive. At the 15-month interim sacrifice, the only nonneoplastic change observed 

was a significant increase in the incidence of squamous hyperplasia in the forestomach of 

200-ppm females (incidences were 0/10, 1/10, 0/10, and 6/10). At terminal sacrifice, the 

incidence of alveolar/bronchiolar adenomas was significantly increased in male mice following 

administration of the high concentration (p = 0.011) (6/50, 9/50, 8/48, and 18/50). Combined 

incidences of alveolar/bronchiolar adenomas or carcinomas were also significantly increased in 

200-ppm males (p = 0.042) (10/50, 14/50, 14/48, and 21/50). The 100-ppm males exhibited a 

statistically significant increased incidence of hepatocellular carcinoma (7/50, 11/50, 13/49, and 

7/50) (p = 0.038) and combined adenoma or carcinoma (19/50, 21/50, 30/49, and 15/50) 

7
(p = 0.013), with incidences greater than those observed in historical controls. Because the 

incidence of this lesion did not increase with increasing concentration, it was considered a 

sporadic finding. Unlike the rat study, basophilic liver cell foci were not observed in the mouse. 

The incidences of hepatocellular adenoma or carcinoma (combined) in females were similar to 

controls. No increases in the incidence of lung tumors were observed in female mice. 

Forestomach squamous hyperplasia was significantly increased in 200-ppm males (3/49, 3/50, 

6/48, and 12/50) and in 100- and 200-ppm females (2/49, 7/50, 9/50, and 19/48); however, 

severity of the effect was not concentration-related. The incidence at 200 ppm equaled the 

highest values observed in historical controls. The incidence of squamous cell papillomas in the 

forestomach was slightly increased after 2 years (incidences were 0/49, 0/50, 1/48, and 2/5 in 

males; 1/49, 0/50, 1/50, and 3/48 in females); however, these increases were not statistically 

significant and were within the range of historical control values. 

Although forestomach hyperplasia in mice is clearly associated with exposure to ACN, 

the role of inhaled concentrations in eliciting these lesions is not known. It is likely that preening 

activities and/or mucociliary clearance, resulting in oral ingestion of ACN, play a central role. 

Thus, it is not possible to identify either a NOAEL or LOAEL attributable to inhalation exposure 

in the chronic study. The absence of these lesions in the rat study is puzzling. In a study by 

Wolff et al. (1982), whole-body exposure versus nose-only exposure of rats to radiolabeled fine 

particles indicated that 60% of the pelt burden was calculated to be ingested following 

whole-body exposure. 

Subchronic studies (Pozzani et al., 1959) were performed on Carworth Farms-Wistar rats 

(15/sex/group) exposed to 0, 166, 330, or 655 ppm ACN vapors (0, 278, 554, or 1,100 mg/m3

), 7 

hours/day, 5 days/week, for 90 days (duration-adjusted to 0, 58, 115, and 229 mg/m3). The purity 

of the ACN was not reported. Body, liver, and kidney weights were determined, and 

histopathology was performed on liver and lungs (any effects in these organs resulted in 

examination of brain, pancreas, spleen, trachea, and testis). Hematocrit and hemoglobin values 

were measured in 5 rats in the 655 ppm group and controls 4 days prior to exposure and on the 

53rd, 72nd, and 89th days. The values with exposed animals were no different from controls. 

Exposure to ACN did not affect body or organ weights in exposed rats, and no deaths attributable 

to exposure were reported. Pathological effects were limited primarily to the 655-ppm group; 

alveolar capillary congestion, focal edema, bronchial inflammation, desquamation, and 

hypersecretion of mucus occurred in lungs (10/27; p = 0.001), tubular swelling in kidneys (8/27; 

p = 0.05), and central cloudy swelling in liver (7/27; p = 0.04). No lesions were reported for 

other organs at 655 ppm. In the other groups, histiocyte clumps in alveoli or atelectasis (2/28 at 

166 ppm), as well as bronchitis and pneumonia (3/26 at 330 ppm) were reported. Hematocrit and 

hemoglobin values for five female rats (males not evaluated) were similar to controls at 655 ppm. 

No treatment-related tumors developed in any groups. Interpretation of study results was limited 

by incomplete histopathology (e.g., stomach and thymus) and a lack of details about protocol. 

An unambiguous NOAEL and LOAEL could not be identified. 

The investigators also examined effects of 350 ppm ACN (588 mg/m3

) on three male 

rhesus monkeys and three male dogs (2 Basenji-Cocker hybrid and one Basenji-Chow × Springer 

spaniel hybrid), 7 hours/day, 5 days/week, for 91 days (duration-adjusted to 123 mg/m3

). The 

purity of the ACN was not reported. Controls consisted of two male Basenji-Cocker hybrid 

8
dogs; there was no control group for monkeys. In dogs and monkeys, focal emphysema and 

diffuse proliferation of alveolar septa were exhibited. Monkeys also showed hemosiderin 

accumulation in lungs and swelling of convoluted tubules in kidneys. Interpretation of this 

experiment was limited by inadequate measurement of chamber concentrations and the lack of a 

control group for monkeys. In a separate inhalation study (7 hours/day, 5 days/week) with 4 

rhesus monkeys, the one monkey (female) exposed to 2,510 ppm died on the second day, the two 

female monkeys exposed at 660 ppm died by day 51, and the one male monkey exposed to 330 

ppm was sacrificed at day 99 of exposure. At that time it exhibited considerable excitability. 

Each of three monkeys in the 2,510, 660, and 330 concentration groups was found to have dural 

and subdural hemorrhages. The two monkeys in the 660-ppm group had focal areas of 

emphysema and cloudy swelling of the proximal and convoluted tubules of the kidney. 

An unpublished 90-day inhalation study in the B6C3F1 mouse was conducted by 

Hazelton Laboratories (1983a) for the National Toxicology Program (NTP). In this study, male 

and female mice (10/sex/group) were exposed by inhalation to ACN concentrations 

(purity>99%) of 0, 25, 50, 100, 200, and 400 ppm (0, 42, 84, 168, 336, and 672 mg/m3

) for 6.5 

hours/day, 5 days/week for a total of 65 exposures during a 92-day period. The duration-adjusted 

concentrations were 0, 8.1, 16.2, 32.5, 65, and 130 mg/m3

. Chamber atmospheres were 

monitored every 30 minutes using infrared spectroscopic methods. Actual mean concentrations 

were all within ± 15% of nominal concentrations. Histopathological examination at necropsy 

included all major tissues and organs, including thymus, testes, ovaries, and lungs from controls 

and 400-ppm group mice. Three sections of the nasal turbinates were examined from all animals 

in all groups. Livers were examined from mice in 100- and 200-ppm groups as well. Clinical 

chemistry and hematological parameters were also examined. All animals from control, 100-, 

200-, and 400-ppm groups at terminal necropsy were subjected to examination of sperm motility, 

count, and sperm head staining. Separate groups of females were exposed in the same study to 0, 

100, 200, and 400 ppm ACN for 6.5 hours/day, 5 days/week for a total of 10 exposures and used 

for immunotoxicology studies. 

Three male mice died during the course of the study (one in each of the 50, 200, and 400-

ppm groups). Mortality was not considered to be exposure-related. In contrast to the findings 

from a 14-day study (ImmuQuest Laboratories Inc., 1984; see Section 4.4) in the same strain of 

mice, there were no reported histopathological effects on the thymus. Thymus/body weight 

ratios were somewhat lower in the 200- and 400-ppm groups compared to controls, but were not 

significantly decreased. Other altered terminal organ/body weight and terminal organ/brain 

weight ratios were mentioned, but tabular data were not included with the final report. There 

were no adverse effects on sperm or in the nasal turbinates. There were no adverse liver effects 

observed upon histopathology. Although cytoplasmic vacuolization was observed in all animals, 

including those in the control group, the vacuolization in animals in the 200- and 400-ppm 

groups was only slightly greater than those in animals of the 100-ppm and control groups. 

In the group of females examined for hematologic and immunotoxic responses, all 

exposure groups exhibited significant decreases in hematocrit, hemoglobin, and red blood cell 

counts. They were described as of low magnitude and of questionable biological significance. 

Lymphocyte counts were decreased only in the 200- and 400-ppm groups. IgG was significantly 

decreased in all exposure groups in a concentration-related manner. These decreases in IgG are 

9
consistent with the findings in the ImmuQuest study (see Section 4.4) at these concentrations. 

Other tests of immune function (e.g., lymphocyte proliferation, delayed hypersensitivity, host 

resistance) were unaffected by exposure; thus, the depressed IgG is of uncertain significance. 

The hematological and hepatic effects in mice were used as the basis for deriving an oral RfD 

that was previously placed on IRIS. Inasmuch as (1) these hematological and hepatocellular 

effects were of questionable biological significance, (2) hematological parameters were 

unaffected in the NTP (1996) subchronic and chronic rat study (these parameters were not 

measured in the mouse portion of the study), and (3) hepatic vacuolization was not observed in 

the chronic mouse study, it was recommended that the RfD for ACN be withdrawn from IRIS. 

In a study with the F344 rat using the same exposure and examination protocols, there 

were no adverse gross or histopathological effects (Hazelton Laboratories, 1983b). 

As part of a developmental toxicity study (Mast et al., 1994), nonpregnant female 

Sprague-Dawley rats (10/group) were exposed for 14 consecutive days to 0, 100, 400, or 

1,200 ppm ACN (168, 672, and 2,015 mg/m3

). One animal at the high concentration died; no 

treatment-related clinical signs or body weight changes were evident in exposed animals. Gross 

examination did not reveal any significant effects in the exposed animals. 

Willhite (1981) exposed CD-1 mice to ACN for 60 minutes to determine the lethal 

concentration (LC). The LC50 value was determined to be 2,693 ppm (4,524 mg/m3

). 

4.3. REPRODUCTIVE/DEVELOPMENTAL STUDIES – ORAL AND INHALATION 

Mast et al. (1994) exposed positively mated female Sprague-Dawley rats (33/group) to 0, 

100, 400, or 1,200 ppm (0, 168, 672, and 2,015 mg/m3

) ACN, 6 hours/day, 7 days/week during 

gestation days 6-19, and then sacrificed on gestational day 20. Controls consisted of 10 

nonpregnant females per exposure group. The 1,200-ppm dams exhibited hypoactivity (14/33) 

and appeared emaciated (6/33). Deaths occurred in two 1,200-ppm dams and one 400-ppm dam. 

The death at 400 ppm was suggested by the investigators to be the result of a possible 

spontaneous cerebral hemorrhage. There was no effect on body or organ weights in pregnant 

dams. Fertility did not appear to be affected by ACN exposure; mean pregnancy rate for spermpositive females was 79%. A slight increase in percentage of resorptions per litter (particularly 

late resorptions) was seen at the high concentration, but the effect was not significant or 

concentration-related. The percent of live fetuses per litter was not affected for any group, nor 

were there treatment-related fetal malformations. The only effect on skeletal variations was a 

significant increase in percent of supernumerary ribs per litter at 100 ppm, but the effect did not 

occur for the other groups. While ACN was readily measurable in maternal blood (determined in 

a separate group of nonpregnant animals), cyanide was not detectable except in the 1,200 ppm 

group. Considering that mortality was treatment-related at 1,200 ppm and exposure may also 

have played a role in the one death at 400 ppm, it is prudent to consider 400 ppm (672 mg/m3) as 

an FEL. The NOAEL for developmental effects is 1,200 ppm (2,015 mg/m3

). 

Pregnant Sprague-Dawley rats (20-23/group) were exposed to 0, 1,000, 1,287, 1,592, or 

1,827 ppm ACN (0, 1,679, 2,161, 2,673, or 3,067 mg/m3

), 6 hours/day, on gestation days 6 to 20 

10
(Saillenfait et al., 1993). Dams were sacrificed on gestational day 21. At 1,827 ppm, mortality 

occurred in 8/20 dams, and maternal body weight gain was significantly reduced from gestation 

days 6 to 21. There was no mortality at other concentrations. Acetonitrile did not affect fertility 

(i.e., no differences in number of pregnancies). A markedly increased percentage of nonlive 

implants per litter (resorptions and dead fetuses) and early resorptions per litter were observed at 

1,827 ppm, along with a decrease (not significant) in the mean number of live fetuses per litter. 

One litter was completely resorbed at 1,827 ppm. No differences were observed between the 

other exposed groups and the controls. Acetonitrile exposure had no significant effect on the 

mean number of implantation sites per litter, fetal sex ratio, or fetal weights per litter. Incidences 

of any visceral or skeletal anomalies were not significantly different between exposed and control 

groups. A NOAEL of 1,592 ppm (2,673 mg/m3

) was determined for maternal and developmental 

toxicity and a FEL of 1,827 ppm (3,067 mg/m3

). Based on increased percentage of nonlive 

implants per litter, a LOAEL of 1,827 ppm was identified for developmental toxicity. 

The lack of mortality in all but the animals in the highest concentration of the Saillenfait 

et al. (1993) study is in sharp contrast with mortality observed in the Mast et al. (1994) study, 

inasmuch as both studies used Sprague-Dawley rats. There is no clear explanation for these 

divergent effects. 

There were no effects on pup birth weight, litter size, or pup viability when ACN was 

administered by intubation to pregnant Long-Evans Hooded female rats (Smith et al., 1987). In 

this teratology screen, ACN, dissolved in tricaprylin oil, was administered at 0, 50, 150, 300, and 

600 mg/kg from days 7-21 of gestation. Principal parameters evaluated were pregnancy rate, 

litter resorption, and early neonatal death. The two highest dosing levels were maternally toxic. 

Pregnant Sprague-Dawley rats (25/group) were administered gavage doses of 0, 125, 190, 

or 275 mg/kg-day of ACN (dissolved in distilled water) on gestation days 6-19 (Johannsen et al., 

1986; IRDC, 1981; Berteau et al., 1982). Animals were sacrificed on gestation day 20. Maternal 

effects were seen with 275 mg/kg-day; two dams died and four dams appeared emaciated. Body 

weight gain from days 6 to 20 was slightly reduced (96% of control) in the 275 mg/kg-day dams. 

Fertility was not affected. An increase in postimplantation loss per dam occurred at the high 

dose; however, effect was not significant or dose-related. The number of live fetuses per dam 

was slightly less than the control value for the 275 mg/kg-day group (not significant), but the 

effect was not measured for each litter. No differences in total implantation, corpora lutea, fetal 

sex ratio, or fetal body weight were observed in any treated groups compared to controls. There 

was a slight, but not significant, increase in the number of fetuses with unossified sternebrae (5th 

and 6th) in the high-dose group; however, the number of litters with unossified sternebrae was 

not dose-related. No significant effects on fetal anomalies occurred. This study identifies a 

LOAEL of 275 mg/kg/day for maternal and developmental effects and a NOAEL of 190 mg/kgday. 

Inhalation and oral developmental studies were performed on pregnant hamsters exposed 

to ACN (Willhite, 1983). Pregnant Syrian golden hamsters were exposed via inhalation to 0, 

1,800 (6 animals), 3,800 (6 animals), 5,000 (6 animals), or 8,000 (12 animals) ppm ACN (0, 

3,022, 6,380, 8,395, and 13,432 mg/m3

) for 1 hour on gestation day 8 and then sacrificed on 

gestation day 14. Parameters of maternal and developmental toxicity were evaluated. Only those 

11
dams that died were examined for histopathological effects. No hamsters exposed to 1,800 ppm 

exhibited any signs of intoxication and all offspring were normal. Histopathology was not 

performed on this group. One dam at 3,800 ppm died 3 hours after exposure after exhibiting 

dyspnea, tremors, hypersalivation, ataxia, and hypothermia. All offspring from this group were 

normal. At 5,000 ppm, all animals exhibited irritation and excessive salivation; one dam in this 

group died after displaying dyspnea, hypothermia, and tremors. Six abnormal offspring from two 

litters of this group exhibited exencephaly and rib fusions. In the 8,000-ppm group, clinical 

effects included respiratory difficulty, lethargy, ataxia, hypothermia, irritation, and gasping (4/12 

dams), followed by tremors, deep coma, and death (3/12 dams). Histopathological examination 

of the liver, kidneys, and lungs from the dams that died in all groups did not reveal any 

significant treatment-related effects. Fetotoxicity occurred in offspring of dams exposed to 8,000 

ppm, as evidenced by decreased fetal body weight compared to controls (not concentrationrelated). Five of the nine surviving litters at 8,000 ppm developed severe axial skeletal dysraphic 

disorders; one fetus exhibited extrathoracic ectopia cordis with accompanying defects in the 

sternum. This study identifies a maternal NOAEL of 1,800 ppm (3,022 mg/m3

) and an FEL of 

3,800 ppm (6,380 mg/m3). The NOAEL (3,800 ppm [6,380 mg/m3]) and LOAEL (5,000 ppm 

[8,395 mg/m3]) identified for developmental effects occurred at or exceeded the maternal FEL. 

The effects of ingested ACN on pregnant hamsters and their fetuses were examined 

(Willhite, 1983). Pregnant Syrian golden hamsters (6-12/group) received gavage doses of 0, 100, 

200, 300, or 400 mg/kg ACN (in distilled water) on gestation day 8, and were sacrificed on 

gestation day 15. Parameters of maternal and developmental toxicity were evaluated. Maternal 

effects (not specified) were evident in dams given 300 and 400 mg/kg, and death occurred in 

1/6 and 4/12 dams in these groups, respectively. There was a significant reduction in body 

weight gain in dams (gestation days 8-15) given up to 300 mg/kg ACN compared to controls (no 

incidence data provided); however, this effect was not seen at 400 mg/kg. Slight, but significant, 

decrease in fetal body weight (p < 0.05) was seen in all exposed groups; however, the effect was 

not dose-related. A significant increase in the number of resorptions per group (not dose-related) 

was noted in dams exposed to 200 or 400 mg/kg (incidences were 0, 6%, 12%, 0, and 22%). 

A significant increase in the number of malformed offspring per group occurred in dams exposed 

to 300 or 400 mg/kg ACN (0, 10%, 0, 19%, and 18%). The most common effect was rib fusions. 

The reason for the nonlinear effects was not explained by the investigators. A concurrent group 

was administered 300 mg/kg thiosulfate by intraperitoneal injection 20 minutes prior to 

inhalation, and repeated every 2 hours for the next 10 hours. Treatment with thiosulfate 

prevented both maternal and developmental toxicity, indicating a causal connection between 

cyanide and the effects observed. This study identifies a NOAEL of 200 mg/kg and LOAEL of 

300 mg/kg for maternal toxicity. The NOAEL for fetotoxicity could not be established on the 

basis of the nonlinear effects in the fetuses. 

Pregnant New Zealand White SPF rabbits (25/group) were administered 0, 2, 15, or 

30 mg/kg-day of ACN by gavage during gestation days 6-18, and sacrificed on gestation day 29 

(Argus Research Laboratories, Inc., 1984). Dams were examined for clinical signs, body weight 

changes, feed consumption, and changes in absolute liver weight. Examination was performed to 

identify any gross lesions, which were then retained for histopathology. In the high-dose group, 

5 dams died (p = 0.025) between days 12-19 and 2 aborted on either gestation day 23 or 27 (both 

effects attributed to treatment). Clinical signs in these affected rabbits included ataxia, anorexia, 

12
decreased motor activity, bradypnea, dyspnea, impaired righting reflex, and colored exudate in 

cage pan. A decrease in body weight was observed in 30-mg/kg-day dams between gestation 

days 15 and 19 (p � 0.01), but the weight increased significantly after end of administration (i.e., 

gestation days 19-24) and was higher than control weights on days 24-29. Necropsy revealed 

thin stomach walls in the cardiac region in those rabbits that died, and this observation was 

considered treatment-related. Exposure to ACN did not appear to cause reproductive 

dysfunction, although a slight, but not significant, increase in resorptions per litter was observed. 

Developmental effects were seen at the high dose. The average number of live fetuses per litter 

was significantly decreased (p = 0.011) at 30 mg/kg-day. There was no significant increase in the 

incidence of fetal malformations or anomalies with ACN exposure. Based on maternal toxicity, 

this study identifies 30 mg/kg/day as a FEL (mortality). A NOAEL of 15 mg/kg/day is given for 

maternal toxicity. A NOAEL of 15 mg/kg/day is identified for developmental toxicity (decreased 

live fetuses per litter). 

Morrissey et al. (1988) evaluated reproductive endpoints in the male rat and mouse 

exposed to nominal concentrations of 100, 200, and 400 ppm in the Hazelton (1983a,b) studies. 

No treatment effects were observed on the weights of right cauda epididymis, right epididymis, 

and right testis, nor were any effects found on sperm motility in the mouse. Similarly, a lack of 

effect on these endpoints were found in the rat; the weight of the right epididymis was not 

evaluated. In neither species were effects on sperm density or percent of abnormal sperm 

evaluated. 

4.4. OTHER STUDIES 

4.4.1. Acute Data 

There are limited acute data on the oral and inhalation exposure of ACN to humans and 

animals. In humans, qualitative information is based primarily on case reports (see Section 4.1). 

In animals, oral LD50s have been reported for the mouse (269-453 mg/kg) and the rat 

(2,230-4,050 mg/kg), inhalation LC50s (1-2 hours) for the mouse (2,300-5,700 ppm), and LC50s 

(4 hours) for the rat (16,000 ppm) (WHO, 1993). 

In an unpublished subacute study (ImmuQuest Laboratories, Inc., 1984), B6C3F1 female 

mice were exposed to 0, 100, 200, or 400 ppm ACN, 6 hours/day, 5 days/week, for 10 days 

during a 14-day period. Gas chromatographic analysis indicated the test compound had a purity 

exceeding 99%. Chamber concentrations were monitored by infrared spectroscopy every 

30 minutes during each 6-hour exposure. No treatment-related clinical signs were evident. 

Statistically significant decreases (p < 0.05) in red and white blood cell counts, hematocrit, and 

hemoglobin at the two highest concentrations were reported. However, mean values at these two 

concentrations seemed to be marginally lower than controls and may be of questionable 

biological significance. Necropsy revealed thymic atrophy in the 200- and 400-ppm groups 

(incidence not stated); the effect corresponded to reduction (not significant) in thymus/brain 

weight ratio (absolute organ weight and thymus-to-body-weight ratio were not reported). The 

number of mice examined histopathologically was not stated and appears to be no greater than 

six per exposure group, based on information presented for other endpoints. Serum IgG levels 

13
were significantly decreased in a concentration-related manner (26%, 33%, and 48% decrease, 

respectively, of controls), but linear regression analysis indicated no concentration-related trends 

with IgM and IgA. Tests of B-cell function were unchanged from controls. 

In the same report, reference was made to a concurrent 90-day study in which a separate 

group of B6C3F1 mice were exposed to the same concentrations. No data per se were presented 

for this study. However, thymic atrophy was mentioned for the 200 and 400 ppm groups of the 

90-day study as had been for the 14-day study. No incidence or severity data were reported. 

A statement referring to slight vacuolization of hepatocytes and hydropic degeneration was the 

only information presented on histopathological findings in liver; alanine aminotransferase 

(AST) and aspartate aminotransferase (ALT) levels were within the normal range. It is likely that 

the 90-day study referred to is the Hazelton (1983a), since the principal investigator was a 

co-author of the ImmuQuest report. Of significance is the lack of effect of ACN on thymus in 

the Hazelton (1983a) report, in contrast to the statement in the ImmuQuest report. Thus, the 

significance of an effect of ACN on the thymus without further corroboration is doubtful. 

4.4.2. Genotoxicity 

The overall data indicate that ACN is not a point mutagen, but does interfere with 

chromosome segregation. Acetonitrile at concentrations up to 10,000 μg/plate was negative in 

Salmonella typhimurium strains TA1535, TA1537, TA97, TA98, and TA100 in the absence of 

S9 and in the presence of rat or hamster S9 induced with Aroclor 1254 (Mortelmans et al., 1986; 

NTP, 1996). Negative results were also obtained in a preincubation S. typhimurium assay and a 

reverse mutation assay in Saccharomyces cerevisiae D7, conducted in the presence and absence 

of S9 from rats induced with ACN or phenobarbitone (Schlegelmilch et al., 1988), although the 

bacterial assay was limited by the use of stationary cultures. 

Sister chromatid exchanges (SCEs) in Chinese hamster ovary (CHO) cells were 

significantly increased in the absence of S9 (only at 5,000 μg/mL), but there was no effect in the 

presence of S9 (Galloway et al., 1987; NTP, 1996). Gene conversions in S. cerevisiae were 

increased in the absence of S9, but not in the presence of S9 (Schlegelmilch et al., 1988). Both 

of these assays measure repair of DNA damage, rather than persistent DNA damage. 

Chromosome aberrations were significantly elevated in CHO cells at 5,000 μg/mL in the 

presence of rat S9 but the trend test was borderline (p = 0.016); there was no effect in the absence 

of S9 (Galloway et al., 1987; NTP, 1996). Although the types of chromosome aberrations were 

not reported, Galloway et al. (1987) reported that both simple and complex aberrations were 

elevated at the high dose. The ability of ACN to induce mutations at the HGPRT gene locus in 

CHO cells in vitro was investigated both in the presence and absence of rat liver S9 (Bioassay 

Systems Corporation, 1984). There were no significant differences between treated and control 

cells over a concentration range of 0.1 to 30 mg/mL. Micronucleated normochromatic 

erythrocytes (NCEs) were increased in the peripheral blood of female mice, but not in males, in a 

micronucleus assay conducted in conjunction with a 13-week inhalation toxicity experiment 

(NTP, 1996). Although the micronucleus assay is usually conducted in polychromatic 

erythrocytes (PCEs), MacGregor et al. (1990) showed that micronucleated peripheral blood PCEs 

and NCEs are at steady state following dosing for 45-90 days. Schlegelmilch et al. (1988) found 

14
that intraperitoneal injection of mice with ACN at 60% of the oral LD50 was weakly clastogenic, 

with micronucleated polychromatic erythrocytes significantly increased at 24 hours. When the 

mice were induced by injection of low doses of ACN for 7 days, and then challenged with 60% 

of the oral LD50, an increase in micronucleated PCEs was not observed until 72 hours after the 

challenge dose. Positive micronucleus assays can indicate either clastogenic activity or 

interference with chromosome segregation. ACN (131 ppm for up to 70 minutes) also induced 

aneuploidy (both chromosome gain and chromosome loss) in treated mature oocytes of 

Drosophila melanogaster females exposed either as larvae or as adults (Osgood et al., 1991a,b). 

Toxicity and sterility were induced by the 70-minute exposure. When S. cerevisiae was exposed 

to 5% ACN, it induced mitotic aneuploidy (Zimmerman et al., 1985); the investigators suggested 

that the induction of aneuploidy by ACN in the absence of point mutations or recombination 

resulted from interference with tubulin assembly and the formation of microtubules in the spindle 

apparatus. More recently, Sehgal et al. (1990) obtained in vitro evidence that ACN does inhibit 

microtubule assembly in taxol-purified Drosophila or mouse microtubules, further indicating that 

ACN has potential to induce aneuploidy. 

Although ACN is largely negative in gene mutation assays and produces only marginal 

effects in chromosome aberration assays, the potential of ACN to interfere with chromosome 

segregation both in vivo and in vitro has been demonstrated in D. melanogaster. 

4.5. SYNTHESIS AND EVALUATION OF MAJOR NONCANCER EFFECTS AND 

MODE OF ACTION—ORAL AND INHALATION 

The database for ACN lacks subchronic and chronic oral toxicity studies in animals. 

Human data are limited to case reports of acute ACN exposure with little information on 

exposure level. 

The noncancer inhalation toxicity of ACN in the rat and mouse is overtly demonstrated in 

subchronic studies at levels of 400 ppm and higher, at which lethality takes place (NTP, 1996). 

The two-year studies (NTP, 1996), conducted at lower concentrations, did not demonstrate 

adverse effects, clinically or by histopathology, in any of the organ systems examined in either 

the rat or mouse. 

The lethal effects of exposure to ACN are believed to be associated with the production 

of cyanide leading to respiratory paralysis and inhibition of CNS processes. At these lethal 

levels, clinical signs included ataxia, convulsions, and abnormal posture (NTP, 1996). 

Disturbances in blood chemistry and decreased thymus weight (rats only) were associated with 

levels of 800 ppm and higher. (Concentration-related decreases in hemocrits and red blood cell 

counts were, however, reported in both the ImmuQuest and Hazelton mouse studies at lower 

concentrations). Abnormal histopathology was largely relegated to the lungs and included 

congestion, hemorrhage, and edema (NTP, 1996; Pozzani et al., 1959). 

The absence of thymic atrophy in the mouse in both the NTP (1996) study and the 

Hazelton (1983a) study are in direct contrast to a statement (incidence/severity not reported) in 

the unpublished study by ImmuQuest Laboratories (1984) that thymic atrophy was observed in 

15
B6C3F1 mice exposed to 200 and 400 ppm ACN for either 14 or 90 days. The reference to the 

latter 90-day study is most likely to the Hazelton (1983a) study, since one of the authors of the 

ImmuQuest report was the principal investigator for the Hazelton study. 

Evidence of abnormal histopathology in other organs at levels below those at which 

lethality occurred was not apparent in the NTP study. Thus, the concentration-response 

relationship is quite steep. Lethality in the rat may be strain-dependent. In the subchronic study 

by the NTP (1996), 5/6 F344 rats exposed to 1,600 ppm died during weeks 1 and 2, whereas 

there were no deaths in pregnant Sprague-Dawley rats exposed to 1,592 ppm during a similar 

time period (Saillenfait et al., 1993). In addition, no deaths were reported in Carworth FarmsWistar rats at 655 ppm (Pozzani et al., 1959). Developmental endpoints also have been 

associated with concentrations higher than those that caused lethality (Mast et al., 1994; 

Saillenfait et al., 1993; Willhite, 1983). The effects of ACN exposure on reproductive endpoints 

in either species prior to mating and through parturition has not been examined. 

Mice may be more sensitive than rats to ACN toxicity, evidenced by the increase in 

forestomach hyperplasia and ulcers, effects not observed in the rat (NTP, 1996). The 

forestomach lesions observed in the subchronic and chronic studies conducted by the NTP are 

critical noncancer effects given the case reports of gastric erosion in humans who ingested ACN 

(Way, 1981; Ballantyne, 1983). However, the role that inhalation exposure plays in the 

occurrence of the lesions is unknown, and may be minor compared to ingestion as a result of 

grooming of contaminated fur and/or mucociliary clearance. Moreover, a potential role of 

inhalation can be envisioned given the detection of label as early as 5 minutes post-injection in 

nasal secretions, esophagus, and stomach contents after intravenous administration of C14-ACN 

to mice (Ahmed et al., 1992). Because of the uncertainties surrounding the contribution of oral 

ingestion versus inhalation, quantifiable levels of ACN via inhalation exposure cannot be 

ascertained for this endpoint. 

The subchronic data (NTP, 1996) for the rat and mouse do indicate a NOAEL of 200 ppm 

(NOAEL[ADJ] = 60 mg/m3

) and an FEL of 400 ppm (FEL[ADJ] = 120 mg/m3) for lethality. 

However, an unambiguous NOAEL pertaining to ACN inhalation cannot be determined because 

of the possible role of inhalation in causing forestomach lesions. 

4.6. WEIGHT-OF-EVIDENCE EVALUATION AND CANCER CLASSIFICATION 

The National Toxicology Program (1996) concluded that the evidence for carcinogenicity 

via inhalation of ACN in F344/N rats was equivocal: “a causal relationship between ACN 

exposure and liver neoplasia in male rats is uncertain.” Although there was a statistically 

significant positive trend in the incidence of hepatocellular adenomas, carcinomas, and adenomas 

and carcinomas (combined) in male rats only, the incidences were not statistically significant by 

pairwise comparison or by life table analysis. In addition, the incidence of adenomas and 

carcinomas combined in the 400 ppm group was only slightly higher than the historical range for 

inhalation study controls. Male rats exhibited an increase incidence of basophilic foci in liver 

that was statistically significant in the 200- and 400-ppm groups. Although the appearance of 

these foci was not atypical, as those more closely related to the carcinogenic process (Harada 

16
et al., 1989), altered hepatocellular foci are generally considered to be preneoplastic (Williams 

and Enzman, 1998; Pitot, 1990). There was no evidence of carcinogenicity in female rats or in 

either male or female B6C3F1 mice. 

The data from the NTP study coupled with the overall lack of genotoxicity potential 

strongly suggest that the carcinogenic potential of ACN in the rat and mouse is low. However, 

because of uncertainty as to the significance of (1) positive trend test of hepatocellular 

carcinomas/adenomas (combined) in male rats compared to controls, (2) the relevance of the 

basophilic cell foci, and (3) the lack of effect in the mouse, the proposed Guidelines for 

Carcinogen Risk Assessment (U.S. EPA, 1996) are clear in stating that the carcinogenic potential 

of a substance (e.g., ACN) for humans following inhalation, oral or dermal exposure “cannot be 

determined because the existing evidence is composed of conflicting data (e.g., some evidence is 

suggestive of carcinogenic effects, but other equally pertinent evidence does not confirm any 

concern).” Similarly, when these data are assessed under the existing Guidelines for Carcinogen 

Risk Assessment (U.S. EPA, 1987), ACN would be classified as Group D - Not Classifiable as to 

Human Carcinogenicity. There is an absence of human evidence and the animal evidence is 

equivocal. 

4.7. SUSCEPTIBLE POPULATIONS 

Sensitive populations may include individuals who generate increased concentrations of 

cyanide because of induction of the cytochrome P450 isoform, CYP2E1. 

4.7.1. Possible Childhood Susceptibility 

There are a few case reports of accidental ingestion of products containing ACN by 

children or infants (WHO, 1993). The symptoms in these cases are similar to those experienced 

by adults in accidental or intentional ingestion of ACN-containing products. The few 

developmental effects seen in oral and inhalation exposures of laboratory animals below levels 

that cause maternal toxicity suggest that children are not likely to be susceptible to 

developmental effects induced by ACN during prenatal exposure. 

4.7.2. Possible Gender Differences 

The World Health Organization (1993) reported two cases in which adult females 

accidentally ingested products containing ACN. The symptoms reported were similar to those 

reported for males. 

5. DOSE-RESPONSE ASSESSMENTS 

5.1. ORAL REFERENCE DOSE (RfD) 

The oral reference dose and supporting information previously on IRIS have been 

withdrawn. The oral reference dose (RfD), derived via a route-to-route extrapolation, had been 

17
based on the judgement that the observation of decreased red blood cells and hepatic lesions (i.e., 

vacuolization) in the unpublished Hazelton Laboratories (1983) 90-day inhalation study were 

adverse. The decreases in red blood cells are not considered adverse in the present U.S. EPA 

assessment. Although blood chemistry was not evaluated in mice in the current NTP studies 

(1996) and thus represents a shortcoming in the protocol, the Hazelton investigators had 

described these effects as being of “low magnitude and questionable biological significance.” 

The vacuolization noted by these investigators was described as “slightly more pronounced ... as 

compared to the control mice.” Similar findings were noted in the NTP (1996) study. The 

vacuolization is not judged adverse. 

Although the available information was inadequate for developing an oral RfD, the 

derivation of a developmental toxicity RfD (RfDDT) was considered due to developmental 

toxicity reported in oral developmental studies reported in hamsters, rats, and rabbits (Argus 

Research Laboratories, Inc., 1984; IRDC, 1981; Johannsen et al., 1986; Willhite, 1983). Based 

on the available developmental studies, the most sensitive developmental endpoint (decreased 

average number of live fetuses per litter), was reported for rabbits administered ACN by gavage 

during gestational days 6-18 (Argus Research Laboratories, 1984). This effect occurred at the 

highest dose tested in the study (LOAEL = 30 mg/kg-day); however, the high mortality (20%) in 

dams also indicated a FEL for this dose level. Because the deaths occurred with a short duration 

of exposure and dosing errors were not reported, it is likely that the observed effects were 

chemical-related. The other oral developmental studies in hamsters and rats were not appropriate 

for the development of an RfDDT. No developmental toxicity was seen at any dose up to 275 

mg/kg/day in the rat study (Johannsen et al., 1986), and the dose-response data in the hamster 

study (Willhite, 1983) were too inconsistent to determine whether there was an effect, and if so, 

at what level. In light of these inconsistencies, and because no study identified a LOAEL in the 

absence of a maternal FEL, the derivation of an RfDDT was not attempted. 

The NTP (1996) 2-year inhalation study in the mouse was not used for route-to-route 

extrapolation to an oral scenario because the quantitative contribution of inhalation and ingestion 

of ACN to the occurrence of forestomach lesions could not be delineated. Therefore, no oral 

dose-response assessment was performed for this compound. 

5.2. INHALATION REFERENCE CONCENTRATION (RfC) 

5.2.1. Choice of Principal Study and Critical Effect 

Inhalation data on subchronic and chronic toxicity of ACN are available from several 

studies: (1) the 1996 NTP study on F344 rats and B6C3F1 mice, (2) the 1984 ImmuQuest 

Laboratories study on mice, and (3) the 1983 Hazelton studies on the rat and mouse. The 

subchronic inhalation study by Pozzani et al. (1959) reported respiratory effects in dogs and mice 

at higher concentrations, and interpretation was limited because of inadequate study details. The 

developmental toxicity endpoints (Mast et al., 1994; Saillenfait et al., 1993; Willhite, 1983) also 

occurred at much higher inhalation concentrations (LOAELs �1,200 ppm) than those of the 

subchronic and chronic studies. 

18
The NTP subchronic study in the mouse, supported by the results of the chronic study, 

was chosen as the principal study, with mortality of 1/10 females exposed to 400 ppm 

(duration-adjusted concentration = 120 mg/m3

) as the critical effect. The increases in absolute 

and relative liver weights in males and the incidence of hepatic vacuolization are not considered 

biologically significant hepatocellular adverse findings. Hepatic vacuolization also was not 

considered a biologically significant finding in either the ImmuQuest study or in the Hazelton 

studies. Because the incidence of forestomach squamous hyperplasia in females was increased 

above controls at 100 and 200 ppm (chronic) and at 200 ppm and above (subchronic) coupled 

with uncertainty as to the role of inhalation in the development of this lesion, there is no 

unambiguous NOAEL in either inhalation study conducted by the NTP. The absence of 

forestomach lesions in the rat cannot be explained. There is no information to suggest that 

differences between species in grooming behavior account for the forestomach lesions. Wolff et 

al. (1982) found that rats exposed whole-body ingested 60% of the pelt burden of a radiolabeled 

aerosol through preening. 

Although an FEL would ordinarily not be chosen as a point of departure for RfC 

derivation, the choice of mortality in this instance appears appropriate. In both sexes of two 

laboratory animal species, there were no observed nonneoplastic adverse effects clearly 

associated with inhalation exposure at three exposure levels (chronic study) below those 

(400 ppm) that resulted in mortality. This steep exposure-response relationship is consistent with 

exposure-response data for other cyanide-containing chemicals. In most cases, abnormal lung 

and brain pathology and clinical signs of respiratory distress are features concomitant with 

mortality. 

Thymic atrophy cited in the ImmuQuest Laboratories (1984) report was not selected as a 

critical effect because it was not corroborated in any of the other studies with the mouse. 

5.2.2. Methods of Analysis 

The RfC was derived according to procedures identified in U.S. EPA (1994b) using the 

regional deposited gas ratio (RDGR). Acetonitrile is considered to be a category 2 gas because 

(1) it has high water solubility, (2) is metabolized to reactive cyanide in the liver, but may be 

rapidly detoxified to thiocyanate, and (3) does not react directly with respiratory tract tissues. 

The RDGR (extrarespiratory effects) for category 2 gases is equivalent to 1 (U.S. EPA, 1994b). 

The RDGR, when multiplied with the NOAEL for mortality adjusted for duration, served to 

derive a human equivalent concentration (HEC). A benchmark concentration analysis (U.S. 

EPA, 1995) was not applied because there were only two data points. 

5.2.3. RfC Derivation - Including Application of Uncertainty Factors (UF) and Modifying 

Factors (MF) 

The following uncertainty factors (UFs) are applied to the NOAEL(ADJ) of 60 mg/m3

: 

3 for interspecies variation, because dosimetric adjustments were applied; 10 for intraspecies 

variation; and 3 for database insufficiencies. The total UF = 3 × 3 × 10 = 100. No uncertainty 

factor was applied to the use of a subchronic study because lethality did not occur in the 

longer-term mouse or rat study at lower levels. Therefore, although this endpoint is of concern 

19
based on the subchronic study, increased exposure would not be expected to increase the 

sensitivity to this endpoint given the known metabolism of cyanide-containing compounds. 

A partial UF of 101/2

 (3), instead of a full factor of 10, was used for database 

insufficiencies because of the lack of data on reproductive endpoints involving exposure of 

laboratory animals before and during mating through parturition. A full factor of 10 was not 

considered necessary because (1) there is no evidence to suggest that ACN accumulates in the 

body, (2) the developmental effects observed seem to be marginal, and (3) these effects occur at 

concentrations lethal to dams. However, a modifying factor (MF) of 10 was applied to account 

for the uncertain role that inhalation may play in causing forestomach lesions. These lesions 

were found in mice subchronically and chronically exposed via inhalation and are likely to be a 

result of grooming of contaminated fur although inhalation cannot be ruled out. 

RfC = 60 mg/m3

 × RDGR (1) = 60 mg/m3 (HEC) 

60 mg/m3

 ÷ [UF × MF] = 60 ÷ [100 × 10] = 6E-2 mg/m3. 

5.3. CANCER ASSESSMENT 

Because the evidence of carcinogenic potential of ACN is equivocal in laboratory animals 

(see Section 4.6) and there are no data for humans, a quantitative dose-response assessment 

cannot be made. 

6. MAJOR CONCLUSIONS IN THE CHARACTERIZATION OF HAZARD 

AND DOSE RESPONSE 

6.1. HUMAN HAZARD POTENTIAL 

ACN is principally used in manufacturing operations as an intermediate in closed systems 

to produce other organic chemicals. Because it is sometimes used in noncaptive processes, it 

does have the potential to reach ambient air and water. In water, it is only slowly hydrolyzed at 

neutral pH. At acidic pH, it would be expected to yield hydrogen cyanide. Its half-life in air 

depends on the extent of its reaction with hydroxyl radicals, the principal scavenging mechanism 

(NTP, 1996). It also is one component of tobacco smoke. 

ACN is readily absorbed in the respiratory tract and distributes throughout the body, 

where it is metabolized in the liver. Metabolism is known to result in formation of cyanide and 

thiocyanate, with formaldehyde and formic acid as additional postulated metabolites. 

Hydrocyanic acid has been detected in various organs (e.g., brain and heart) of the rat upon 

inhalation. Cyanide and thiocyanate have been detected in various organs and the blood of 

hamsters upon oral exposure. However, in clinical studies of three individuals who inhaled ACN 

for 4 hours, no cyanide was detected in blood. 

There is little direct information concerning the nature of effects that ACN causes in 

humans. Case reports of adults or children who ingested ACN indicate that symptoms included 

20
respiratory distress (e.g., pulmonary edema), vomiting, confusion, convulsions, gastric erosion, 

and seizures. Case reports involving occupational exposure involved a similar spectrum of 

symptoms. Autopsy findings from an occupationally exposed individual revealed brain, kidney, 

thyroid, and liver involvement. From these reports alone, it is clear that the respiratory tract, the 

central nervous system, and other organs can be adversely affected. Some of the effects observed 

in the human case reports have also been noted in studies with laboratory animals. Inhalation 

exposure of rats over 13 weeks indicated that ACN at high concentration causes pulmonary 

edema, congestion, and hemorrhage leading to death. At these concentrations, ataxia and clonic 

convulsions were observed as well as decreases in hemoglobin and thymus weight. When 

exposed over 2 years to lower concentrations, male rats (only) exhibited a positive trend in liver 

tumors, but the incidence was not significant when compared pairwise to controls or after 

adjustment for survival. In female mice exposed to high concentrations for 13 weeks, the 

principal finding was focal ulcers in the forestomach. Forestomach hyperplasia was observed in 

males. Other than increases in liver weight, there were no histopathological effects in the liver, 

nor were there adverse lung effects. Effects on hematological parameters were not studied. In a 

lifetime study in the mouse at lower concentrations, there were no clinical effects or effects in the 

liver. Forestomach hyperplasia was increased in incidence, but severity of the lesion was not 

concentration-related. The incidence of pulmonary adenomas/carcinomas observed in males was 

not considered to be treatment-related. 

Oral and inhalation exposure of pregnant rats and hamsters during gestation to ACN 

revealed that developmental effects occurred only at levels at or exceeding those at which 

maternal adverse effects were observed. In one study that examined the effect of inhalation 

exposure on male rat and mouse reproductive parameters, there were no observed effects on 

sperm motility or sperm density. Although two-generation studies have not been performed, and 

this represents a database deficiency, there is no indication that ACN at levels expected in the 

environment poses a risk of developmental effects. 

Because the carcinogenic potential in both the rat and mouse is low, coupled with an 

overall lack of genotoxicity potential, the carcinogenic potential in humans is expected to be low. 

However, there is uncertainty in this area inasmuch as there was a positive trend for 

hepatocellular adenomas/carcinomas (combined) in the male rat as well as an increase in the 

incidence of basophilic liver foci, of which the latter may represent a preneoplastic effect. 

6.2. DOSE RESPONSE 

Quantitative estimates of human risk as a result of exposure to low levels of ACN are 

based on exposures of laboratory animals because no human data are available. The human 

chronic concentration of inhaled ACN considered to be safe (the RfC) is 6E-2 mg/m3

. The RfC 

is based on subchronic and chronic inhalation studies in the rat (NTP, 1996) in which thorough 

histopathological analyses were performed. Because only limited evaluation of hematological 

endpoints was carried out in the rat and none in the mouse, coupled with no examination of the 

effects on ventilatory parameters or CNS endpoints, the scientific quality of these studies is 

considered medium. The confidence in the overall database also is medium inasmuch as twogeneration studies were not performed. Although acceptable developmental studies were carried 

out (via inhalation) in two species, rat and rabbit, with adverse effects occurring at levels at or 

21
exceeding the level that caused severe maternal effects, reproductive endpoints have not been 

thoroughly evaluated. This represents a database deficiency. In addition, the potential of ACN 

inhalation to induce gastric lesions is unknown, but is a consideration given the possible role of 

inhaled ACN in causing forestomach lesions in the mouse. The neurological symptoms and 

respiratory distress in humans after acute high-level exposures suggest that effects observed in 

laboratory animals are consistent with those observed in humans. 

No dose-response assessment was performed for oral exposure to ACN. The database 

lacked subchronic and chronic studies for laboratory animals and there was only limited 

information from case reports of human ingestion. The NTP (1996) inhalation study was not 

used in a route-to-route extrapolation because the contributions of inhalation and ingestion to the 

occurrence of forestomach lesions could not be quantified. Similarly, a route-to-route 

extrapolation from inhalation to other endpoints could not be conducted because of lack of 

mechanistic data. 

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26
APPENDIX A. EXTERNAL PEER REVIEW--SUMMARY OF COMMENTS AND 

DISPOSITION 

The support document and IRIS summary for Acetonitrile have undergone both internal 

peer review performed by scientists within EPA and a more formal external peer review 

performed by scientists performed accordance with EPA guidance on peer review (U.S. EPA, 

1992). Comments made by the internal reviewers were addressed prior to submitting the 

documents for external peer review and are not part of this appendix. The external peer 

reviewers were tasked with providing written answers to general questions on the overall 

assessment and on chemical-specific questions in areas of scientific controversy or uncertainty. 

A summary of significant comments made by the external reviewers and EPA’s response to these 

comments follows. 

(1) General Comments 

A. The carcinogenic potential of ACN 

Comment: All three reviewers agreed that the characterization of carcinogenic potential is 

appropriate. All reviewers agreed that “equivocal” is the appropriate designation. Two of the 

reviewers were of the opinion that the significance of liver tumors in rodents is of uncertain 

relevance to human risk. One reviewer intimated that a better discussion of the significance of 

basophilic liver foci could be presented and cited a recent article by Williams and Enzman (1998) 

as support for an association of these foci with carcinogenicity. 

Response to Comment: Information pertaining to this reference and to the publication of 

Harada et al. (1989) was added to both the Toxicological Review and cancer summary to indicate 

that basophilic liver foci are generally considered to be preneoplastic (Williams and Enzman, 

1998), but that the foci in the NTP mouse study are not atypical in appearance, a characteristic 

that is often associated with progression to cancer (Harada et al., 1989). 

B. Selection of the most appropriate critical effect for RfC calculation 

Comment: All three reviewers agreed that mortality is the most appropriate choice for the 

critical effect given the data presented. 

C. Discussion pertaining to “Supporting/Additional or Other” studies 

Comment: Two reviewers thought the Hazelton (1983a) and ImmuQuest Laboratories (1984) 

should be discussed more fully. In particular, the Hazelton (1983a) should not be discounted 

without presenting a clear rationale. One reviewer suggested that a discussion of this study 

appear in the summary document under I.B.4. 

Response to Comment: A full discussion of the unpublished Hazelton (1983a) 90-day 

inhalation study was included and the results, in part, were contrasted with those of the 

unpublished 14-day ImmuQuest Laboratories (1984) study, presented under “Other Studies.” 

27

The shortcomings of the ImmuQuest study were presented. A discussion of Hazelton (1983a) 

was also presented in Section I.B.4 of the summary document. 

D. Data that should be considered in developing uncertainty and modifying factors 

Comment: Two reviewers agreed that the selection of uncertainty and modifying factors was 

appropriate. One reviewer was uncomfortable with the identification of 400 ppm as an FEL 

considering there was only one mouse death at this concentration. It was proposed that 800 ppm 

be identified as the FEL for risk assessment purposes. 

Response to Comment: It is possible that the one death at 400 ppm was unrelated to 

treatment. However, given the steep concentration-response characteristics of ACN and the 

sudden onset of mortality in both the rat and mouse, it was considered appropriate to regard 

400 ppm as treatment-related. 

E. The rationale for the weight-of-evidence and confidence statements 

Comment: One reviewer suggested that the relevance of liver tumors in the rat to humans 

should be presented here and that the significance of the preneoplastic lesions should be 

addressed more fully. A second reviewer indicated that genotoxicity data should be included. 

The third reviewer stated that the underlying assumptions and limitations are sufficiently 

apparent. 

Response to Comment: EPA recognizes the ongoing scientific debate regarding the 

relevance of rodent tumors to humans. However, in this case the overall weight of evidence is 

inadequate for determining the human cancer potential for acetonitrile, given the lack of evidence 

seen in the rat and the marginal effects seen in the mouse. Therefore, EPA believes that an 

expanded discussion of the significance of preneoplastic basophilic foci is unneeded. 

(2) Comments on Chemical-Specific Questions 

Question 1: Is it appropriate to use mortality as a point-of-departure in the derivation of the 

RfC? 

All reviewers agreed that there are no more sensitive endpoints to use in RfC derivation. 

Question 2: Is the rationale given for discounting the Hazelton (1983a) study sufficient? 

All reviewers agreed that a complete discussion of this study should be presented. 

Response to Comment: In the draft presented to the reviewers a discussion of the results 

of this unpublished study was not presented because NTP had elected not to use or cite the study, 

but repeated it. Therefore, a comment was included to suggest that the study did not meet NTP 

acceptance criteria. In view of the fact that the reasons NTP did not choose to utilize this study 

were not expressed, EPA adopted the reviewers’ suggestion to include a discussion of the 

conduct of the study and its results. 

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Question 3: Do you agree that forestomach lesions in mice cannot be used as the critical effect 

because of uncertainties pertaining to oral ingestion? 

One reviewer stated that “the discussion provided is a good example of the use of 

scientific knowledge in the risk assessment process.” This reviewer indicated that the lesions 

have little relevance to human effects reported after inhalation exposure. The other two 

reviewers agreed that the discussion presented on forestomach lesions clearly indicates why these 

lesions cannot be used in derivation of the RfC. 

Question 4: Was it appropriate to discount the ImmuQuest Laboratories (1984) results in the 

choice of the principal study? 

All reviewers were of the opinion that a more complete discussion of the strengths and 

limitations of this study should be presented. One indicated more weight should be given. 

Response to Comment: A full discussion of this study is now presented in the 

Toxicological Review and in the RfC summary document. In these discussions, it is indicated 

that the lack of study results to support the statement of thymic atrophy in the mouse and the lack 

of such histopathological results in either the subchronic NTP (1996) or Hazelton (1983a) studies 

limit the significance of this undocumented and uncorroborated finding. 

Question 5: Would you agree that the NTP chronic study cannot be used to derive an oral RfD? 

All reviewers were in agreement that an oral RfD cannot be derived because of the 

uncertainties in the role of inhalation and oral ingestion in causing forestomach lesions. 

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