EPA 749-F-94-011a
CHEMICAL SUMMARY FOR CYCLOHEXANE
prepared by
OFFICE OF POLLUTION PREVENTION AND TOXICS
U.S. ENVIRONMENTAL PROTECTION AGENCY
September 1994
This summary is based on information retrieved from a systematic
search limited to secondary sources (see Appendix A). These sources
include online databases, unpublished EPA information, government
publications, review documents, and standard reference materials. No
attempt has been made to verify information in these databases and
secondary sources.
I. CHEMICAL IDENTITY AND PHYSICAL/CHEMICAL PROPERTIES
The chemical identity and physical/chemical properties of
cyclohexane are summarized in Table 1.
TABLE 1. CHEMICAL IDENTITY AND CHEMICAL/PHYSICAL PROPERTIES
OF CYCLOHEXANE
_______________________________________________________________________
Characteristic/Property Data Reference
_______________________________________________________________________
CAS No. 110-82-7
Common Synonyms hexahydrobenzene,
hexamethylene,
hexanaphthene Budavari et al. 1989
Molecular Formula C6H12
Chemical Structure
Physical State liquid Budavari et al. 1989
Molecular Weight 84.16 Budavari et al. 1989
Melting Point 6.47�C Budavari et al. 1989
Boiling Point 80.7�C @760 mm Hg Budavari et al. 1989
Water Solubility 55 mg/L @ 25�C CHEMFATE 1994
Density d20/4 �C, 0.7781 Budavari et al. 1989
Vapor Density (air = 1) 2.90 Verschueren 1983
KOC 482 (calculated) CHEMFATE 1994
Log KOW 3.44 CHEMFATE 1994
Vapor Pressure 77 mm Hg @ 20�C Verschueren 1983
Reactivity flammable; reacts with
oxidizing materials HSDB 1994
Flash Point 1�F ( 18�C) (closed cup) Budavari et al. 1989
Henry's Law Constant 0.195 atm.m3/mol
@ 25�C (calculated) CHEMFATE 1994
Fish Bioconcentration
Factor 240 (calculated) CHEMFATE 1994
Odor Threshold 300 ppm (in air); mild,
sweet odor (chloro-
form-like) HSDB 1994
Conversion Factors 1 ppm = 3.49 mg/m3
1 mg/m3 = 0.29 ppm Verschueren 1983
_______________________________________________________________________
II. PRODUCTION, USE, AND TRENDS
A. Production
There are four cyclohexane producers in the United States.
Table 2 lists producers, plant locations, and plant capacities.
Annual US capacity is approximately 378 million gallons. In 1992,
an estimated 338 million gallons of cyclohexane were produced in the
US. During that same year, 5 million gallons were imported into the
US and 38 million gallons were exported (Mannsville 1993).
B. Use
Cyclohexane is used in a number of industrial applications. The
primary use of cyclohexane, accounting for approximately 58 percent
of all use, is in the production of adipic acid, a nylon intermediate.
Cyclohexane is also used in the production of caprolactam, another
nylon intermediate. Small amounts are used as a solvent for lacquers
and resins; as a paint and varnish remover; as an intermediate in the
manufacture of benzene, cyclohexanone, and nitrocyclohexane; as fuel
for camp stoves; as an ingredient in fungicidal formulations; and in
the industrial recrystallization of steroids (Mannsville 1993).
Table 3 shows the estimated 1992 US end-use pattern for cyclohexane.
C. Trends
Demand for cyclohexane is expected to increase at a rate of 2 to 2.5
percent per year (Mannsville 1993).
TABLE 2. United States Producers of Cyclohexane
_______________________________________________________________________
Company Plant Location Plant Capacity
(in millions of gallons)
_______________________________________________________________________
Champlin Corpus Christi, TX 30
Chevron Port Arthur, TX 38
Phillips Borger, TX 45
Sweeny, TX 110
Guayama, PR 90
Texaco Port Arthur, TX 65
_______________________________________________________________________
Source: Mannsville 1993.
TABLE 3. Estimated 1992 United States End-Use Pattern of Cyclohexane
_______________________________________________________________________
Use of Cyclohexane Percentage of US
(typical Standard Industrial Cyclohexane Use
Classification (SIC) Code)
(see end note 1)
_______________________________________________________________________
Adipic acid (production, SIC 2869) 58%
Caprolactam (production, SIC 2865) 35%
Miscellaneous (including solvent)
(no applicable SIC Code(s)) 7%
_______________________________________________________________________
Source: Mannsville 1993.
III. ENVIRONMENTAL FATE
A. Environmental Release
Cyclohexane occurs naturally in crude oil and may be released
to the environment from sites where petroleum products are refined,
stored, and used (HSDB 1994). It is also released into the
atmosphere from volcanos and tobacco smoke. The chemical is
present
in exhaust gases from motor vehicles and in fugitive emissions and
in wastewater from industrial facilities involved in its
production and use (HSDB 1994).
In 1992, environmental releases of the chemical, as reported to the
Toxic Chemical Release Inventory by certain types of U.S.
industries, totaled about 14 million pounds, including 13.6 million
pounds to the atmosphere; 21,039 pounds to surface water; 230,985
pounds by underground injection; and 107,748 pounds to land
(TRI92 1994).
Air monitoring has detected the chemical in the air at various
US locations, including Los Angeles, CA; Houston, TX; Tulsa,
OK; and Jones State Forest, TX. When reported, concentrations
were less than 25 ppb (CHEMFATE 1994). Water samples collected
from the Hudson River Basin (28 sites), the Mississippi River
Basin (Alabama and Texas, 45 sites), and the Gulf of Mexico
contained less than 20 ppb of cyclohexane (CHEMFATE 1994).
The chemical has also been detected in samples of mother's milk in
Baton Rouge, LA; Bridgeville, PA; and Bayonne and Jersey City, NJ
(CHEMFATE 1994).
B. Transport
Cyclohexane is volatile (vapor pressure, 77 mm Hg @ 20�C; Henry's
Law constant, 0.195 atm.m3/mol @ 25�C) and is expected to partition
into the atmosphere from both water and soil (HSDB 1994). The
estimated KOC for cyclohexane, 482 (CHEMFATE 1994), indicates a
moderate potential for soil adsorption. Cyclohexane is slightly
soluble in water (55 ppm) and has the potential to leach through
soil into groundwater (HSDB 1994).
C. Transformation/Persistence
1. Air - In the atmosphere, cyclohexane degrades by reaction with
photochemically produced hydroxyl radicals. One estimate of the
rate constant for the reaction between photochemically-produced
hydroxyl radicals and cyclohexane is 0.795 x 10-11 cm3/molecule-
sec (CHEMFATE 1994); an estimated half-life for this type of
reaction is 52 hours (HSDB 1994). The half-life is shorter in the
presence of photochemical smog; in Los Angeles in sunlight, for
example, 39% of the chemical was degraded in 6 hours (HSDB 1994).
The products of the reaction are cyclohexyl nitrate, and
unidentified carbonyl compounds (HSDB 1994).
2. Soil - Volatilization and leaching are the primary removal
mechanisms for cyclohexane in soil. The chemical is resistant to
biodegradation under most conditions, unless other degradable
hydrocarbons, such as oil and gasoline, are present (CHEMFATE 1994;
HSDB 1994).
3. Water - The primary route for the removal of cyclohexane from the
aquatic environment is volatilization (half-life in a model river,
2 hours) (HSDB 1994).
4. Biota - The estimated fish bioconcentration factor for cyclohexane
of 240 (CHEMFATE 1994) indicates a potential for its limited
bioaccumulation in the aquatic food chain.
IV. HEALTH EFFECTS
A. Pharmacokinetics
1. Absorption - Cyclohexane is absorbed following inhalation (HSDB
1994) and nominally by the skin. Massive applications of the
chemical to the skin of rabbits have produced microscopic changes
in the liver and kidneys (effective doses not given) (Sandmeyer
1981). Systemic toxicity observed in animals exposed orally
to cyclohexane (see section IV. B) indicates that gastro-
intestinal absorption of the chemical also occurs .
In workers exposed to atmospheric cyclohexane, 22.8% of the
total respiratory intake was absorbed, and a "significant amount"
of the absorbed cyclohexane was either retained or metabolized
(Longacre 1987).
2. Distribution - Following inhalation exposure of Wistar rats to
concentrations of cyclohexane ranging from 300-2000 ppm,
perirenal fat concentrations of the chemical were 23- to 38-fold
greater than brain concentrations after one week of exposure and
50- to 80-fold greater than brain concentrations, after two
weeks. No information was found regarding distribution to other
organs.
3. Metabolism - Cyclohexane is metabolized via the hepatic,
vascular, and renal systems (Sandmeyer 1981). Microsomal
hydroxylases oxidize cyclohexane to cyclohexanol in the presence
of NADPH and oxygen (Longacre 1987). Other metabolites
identified
in mammalian systems include trans-cyclohexane-1,2,-diol, cyclo-
hexanone, and adipic acid (HSDB 1994).
4. Excretion - Shoe factory workers exposed to atmospheric concentra-
tions of cyclohexane ranging from 17 to 2484 mg/m3 excreted
cyclohexanol in the urine at concentrations of 0.27 to 7.18
micrograms/mL and at the rate of 0.05 to 3.23 micrograms/min.
The excretion rates of cyclohexanol correlated well with cyclo-
hexane concentrations in the blood and in alveolar air (Longacre
1987). In a study of alveolar excretion, workers were exposed to
cyclohexane for 4 hours and excretion was measured during a 6-hour
post-exposure period. The post-exposure decline occurred in two
phases: in the first, the half-life was 11.2 minutes, whereas in
the second, about one hour later, the half-life was 115.3 minutes.
The alveolar excretion of cyclohexane was approximately 9.1 mg for
an average alveolar ventilation of 5 L/minute. Only 0.5 to 1.0%
of the dose was excreted in the urine as cyclohexanol and
cyclohexanone (Longacre 1987).
Portions of inhaled cyclohexane are excreted unchanged in the
urine and in exhaled air; the remainder is metabolized to
cyclohexanol and excreted in the urine, mainly as the sulfate or
glucuronide conjugate (HSDB 1994; Sandmeyer 1981). Adult
chinchilla-doe rabbits given a single oral dose of 300-400 mg/kg
14C-cyclohexane excreted 35-45% of the dose in expired air
(10% as CO2 and 25-35% as unchanged cyclohexane), and eliminated
33-56% of the dose in the urine (Longacre 1987). Animals given a
lower dose (0.3 mg/kg) of the radiolabeled cyclohexane excreted
no unchanged chemical in expired air, 5% as CO2, and 98% in the
urine. The only urinary metabolites detected were the
glucuronides of cyclohexanol (30-40% and 60% of the high and
low dose, respectively) and transcyclohexane-1,2-diol (5-8%
and 17% of the high and low dose, respectively). A total of
0.1-0.2% was excreted in the feces, and 2.4-2.6% of the dose
was recovered from the tissues.
B. Acute Effects
Cyclohexane has low acute toxicity, producing eye irritation in
humans and neurological symptoms (see section IV. G), other organ
effects, and death in animals at very high doses.
1. Humans - According to one source, cyclohexane is detectable by
odor and is irritating to the eyes at 300 ppm; another source
suggested 25 ppm as the odor threshold (ACGIH 1991). Undiluted
cyclohexane is also irritating to the skin (Longacre 1987). No
other information was found in the secondary sources searched
for the acute toxicity of cyclohexane to humans.
2. Animals - The oral LD50 for cyclohexane in rats ranges from 8.0
to 39 mL/kg (both greater than 5 g/kg), depending upon the age of
the animals (Sandmeyer 1981). The oral LD50 for mice is 1.3 g/kg;
the minimum lethal oral dose in rabbits is 5.5-6.0 g/kg; and the
dermal LD50 in rabbits is >180 g/kg (Longacre 1987). Within 1 to
1.5 hours, lethal doses to animals produced severe diarrhea,
vascular damage and collapse, hepatocellular degeneration and
toxic
glomerulonephritis (Sandmeyer 1981). Exposure of rabbits to 3330
ppm (duration not given) produced no effect; 18,500 ppm for 8
hours
was non-lethal; and 26,600 ppm for 1 hour was lethal (ACGIH 1991).
Application of 1.55 g/day of cyclohexane to the skin for 2 days
produced minimal irritation (Longacre 1987).
C. Subchronic/Chronic Effects
Cyclohexane administered subchronically is of low toxicity, producing
neurological effects (see section IV. G), ocular, gastrointestinal,
and
respiratory effects in animals at very high, lethal concentrations.
1. Humans - No information was found for the subchronic/chronic
toxicity of cyclohexane in humans in the secondary sources
searched.
2. Animals - No effects were observed in rabbits exposed to 434 ppm
cyclohexane for fifty 6-hour periods or in rhesus monkeys exposed
to 1234 ppm under identical exposure conditions (Longacre 1987).
Concentrations of �7445 ppm, 6 to 8 hours/day for 2 to 26 weeks
were lethal to rabbits, producing neurological effects (see
section IV. G) as well as closure of the eyes, conjunctival
infection, salivation, labored respiration, cyanosis and diarrhea
prior to death (Longacre 1987). Rats exposed by inhalation to
1500
or 2500 ppm cyclohexane for 9-10 hours/day, 5 days/week for 7, 14,
or 30 weeks exhibited no adverse effects (Longacre 1987).
D. Carcinogenicity
1. Humans - No information was found in the secondary sources
searched regarding the carcinogenicity of cyclohexane in humans.
2. Animals - No information was found in the secondary sources
searched regarding the carcinogenicity of cyclohexane in
animals.
E. Genotoxicity
Cyclohexane was negative for viral enhanced cell transformation in
Syrian hamster embryo (SA7/SHE) cells and for histidine reverse
gene
mutation in Salmonella typhimurium (Ames assay) (GENETOX 1994).
F. Developmental/Reproductive Toxicity
1. Humans - No information was found in the secondary sources
searched regarding the developmental/reproductive toxicity of
cyclohexane in humans.
2. Animals - No information was found in the secondary sources
searched regarding the developmental/reproductive toxicity of
cyclohexane in animals.
G. Neurotoxicity
The central nervous system is a major target organ for the toxicity
of cyclohexane. High concentrations of the chemical produce
various
effects, ranging from trembling to death.
1. Humans - At high concentrations, cyclohexane is a central
nervous
system depressant and may cause dizziness and unconsciousness
(Sandmeyer 1981). No other information was found in the
secondary
sources searched regarding the neurotoxicity of cyclohexane in
humans.
2. Animals - Mice exposed to 50 mg/L (14,500 ppm) for 2 hours
exhibited minimal narcotic effects (Longacre 1987). Exposure to
18,000 ppm produced trembling within 6 minutes, disturbed
equilibrium within 15 minutes, and complete recumbency within
30 minutes (Longacre 1987). Following exposure of Wistar rats
for 2 weeks to concentrations of cyclohexane ranging from
300-2000
ppm, cerebral levels of RNA, glutathione, glutathione peroxidase,
and azoreductase were evaluated. The only effect noted was a
decrease in azoreductase activity (Longacre 1987). The effect
of cyclohexane on the vestibular function of rats was measured by
recording nystagmus induced by accelerated rotation. Cyclohexane
caused an excitation of the vestibulo-oculomotor reflex
(threshold
blood level, 1.1 mmole/L) (HSDB 1994). Concentrations of �7445
ppm, 6 to 8 hours/day for 2 to 26 weeks were lethal to rabbits,
producing convulsions, tremors, narcosis, and paresis of the legs
(Longacre 1987).
V. ENVIRONMENTAL EFFECTS
TLm values for fish range from 32 to 57.7 mg/L, indicating that the
chemical is moderately toxic to aquatic organisms in acute tests.
Cyclohexane is expected to be of low toxicity to terrestrial organisms
and has a smog-forming potential.
A. Toxicity to Aquatic Organisms
TLm values for fish (24-96 hr) are 43-32 mg/L (Pimephales promelas,
fathead minnow), 43-34 mg/L (Lepomis macrochirus, bluegill), 42.3
mg/L (Crassium auratus, goldfish), and 57.7 mg/L (Poecilia
reticulata, guppy) (Verschueren 1983). Mussel larvae (Mytilus
edulis) exposed to 1 to 100 ppm (mg/L) cyclohexane exhibited a 10-20%
increase in growth rate (Verschueren 1983). The threshold concentra-
tion of cyclohexane in the cell multiplication inhibition assay,
measured in the protozoa Uronema parduczi Chatton-Lwoff, was >50 mg/L
(Verschueren 1983).
B. Toxicity to Terrestrial Organisms
Based on the low toxicity of cyclohexane to laboratory animals, the
toxicity of the chemical to terrestrial animals is expected to be
low.
C. Abiotic Effects
Limited information indicates cyclohexane may have potential to
contribute to the formation of photochemical smog. U.S. EPA has
denied a petition to delist cyclohexane from the Toxic Release
Inventory on this basis and on the lack of adequate health effects
information. The ozone-forming potential for cyclohexane has been
measured as 2 on a scale of 5 (HSDB 1994). Ozone-forming potential
is an indicator of the smog-forming potential of a chemical.
VI. EPA/OTHER FEDERAL AND OTHER GROUP ACTIVITY
The Clean Air Act Amendments of 1990 list cyclohexane as a hazardous
air pollutant. Occupational exposure to cyclohexane is regulated by
the Occupational Safety and Health Administration (OSHA). The
permissible exposure limit (PEL) is 300 parts per million parts of air
(ppm) as an 8-hour time-weighted average (TWA) (29 CFR 1910.1000).
Federal agencies and other groups that can provide additional
information on cyclohexane are summarized in Tables 4 and 5.
TABLE 4. EPA OFFICES AND CONTACT
NUMBERS FOR INFORMATION ON CYCLOHEXANE
________________________________________________________________________
EPA OFFICE LAW PHONE NUMBER
________________________________________________________________________
Pollution Prevention Toxic Substances Control Act
& Toxics (Sec. 4/8A/8D/8E) (202) 554-1404
Emergency Planning and Community
Right-to-Know Act (EPCRA)
Regulations (Sec. 313) (800) 535-0202
Toxics Release Inventory data (202) 260-1531
Air Clean Air Act (919) 541-0888
Solid Waste & Comprehensive Environmental
Emergency Response Response, Compensation, and
Liability Act (Superfund)/
Resource Conservation and Recovery
Act / EPCRA (Sec. 304/311/312) (800) 535-0202
Water Clean Water Act (202) 260-7588
________________________________________________________________________
TABLE 5. OTHER FEDERAL OFFICE/OTHER GROUP CONTACT NUMBERS
FOR INFORMATION ON CYCLOHEXANE
_______________________________________________________________________
Other Agency/Department/Group Contact Number
_______________________________________________________________________
American Conference of Governmental
Industrial Hygienists
Recommended TLV-TWA (see end note 2): 300 ppm
(ACGIH 1993-1994)) (513) 742-2020
Consumer Product Safety Commission (301) 504-0994
Food & Drug Administration (301) 443-3170
National Institute for Environmental Health Sciences
(EnviroHealth Clearinghouse) (800) 643-4794
National Institute for Occupational Safety & Health
(Recommended TWA (see end note 3): 300 ppm
(NIOSH 1990)) (800) 356-4674
Occupational Safety & Health Administration
(Permissible TWA (see end note 2): 300 ppm
(OSHA 1993))
(Check local phone book for phone number under Department of Labor)
_______________________________________________________________________
VII. END NOTES
1. Standard Industrial Classification code is the statistical
classification standard for all Federal economic statistics. The code
provides a convenient way to reference economic data on industries of
interest to the researcher. SIC codes presented here are not intended
to be an exhaustive listing; rather, the codes listed should provide an
indication of where a chemical may be most likely to be found in
commerce.
2. TLV-TWA, Threshold Limit Value-Time Weighted Average
3. TWA, Time-Weighted Average
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and Physical Agents and Biological Exposure Indices. ACGIH, Cincinnati,
OH.
ACGIH. 1991. American Conference of Governmental Industrial
Hygienists. Cyclohexane. In: Documentation of the Threshold Limit
Values and Biological Exposure Indices, 6th ed. ACGIH, Cincinnati, OH,
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Budavari S, O'Neil MJ, Smith A, Heckelman PE (Eds.). 1989. The Merck
Index, 11th ed. Merck & Co., Inc., Rahway, NJ, p. 426.
CHEMFATE. 1994. Syracuse Research Corporation's Environmental Fate
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GENETOX. 1994. U.S. EPA GENETOX Program, computerized database.
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