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Description of key information

An NOAEL of 2,000 ppm  (6,880  mg/m3)  can  be  assumed  for  changes  in  SCOB  after  acute  exposure of rats to cyclohexane.
Acute neurotoxicity by inhalation was assessed in human volunteers, with the NOAEC being at the highest dose tested: 250 ppm (860 mg/m3) for 4-hour exposure period (860 mg/m3).

Key value for chemical safety assessment

Effect on neurotoxicity: via oral route

Endpoint conclusion
Endpoint conclusion:
no study available

Effect on neurotoxicity: via inhalation route

Link to relevant study records
neurotoxicity: acute inhalation
Type of information:
experimental study
Adequacy of study:
supporting study
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: GLP (no data), non-guideline human and animal experimental studies available as unpublished reports, no restrictions, fully adequate for assessment.
Reason / purpose for cross-reference:
reference to same study
no guideline followed
Principles of method if other than guideline:
Following whole-body exposure to cyclohexane for 8 hours/day for 3 days, effects were assessed using a Functional Observation Battery and assessment of automated locomotor activity and visual discrimination performance.
GLP compliance:
not specified
Limit test:
other: WAG/RijCR/BR
Details on test animals or test system and environmental conditions:
- Source: Charles River Wiga, Sulzfeld, Germany
- Age at study initiation: approximately 14 weeks
- Weight at study initiation: 222.2-267.8 g (FOB)
- Weight at study initiation: 180.6-391.7 g (visual discrim, water deprived)
- Weight at study initiation: 215.7-274.7 g (blood & brain concs)
- Housing: Suspended stainless steel cages
- Diet: ad libitum (except during exposure)
- Water: tap water ad libitum (except during exposure)
- Acclimation period: at least 5 days

- Photoperiod: 12 hrs dark / 12 hrs light

Route of administration:
other: air
Analytical verification of doses or concentrations:
Duration of treatment / exposure:
8 hours per day
Frequency of treatment:
3 consecutive days
Doses / Concentrations:
0, 1400 mg/m3, 8000 mg/m3 & 28000 mg/m3
other: target concentrations
Doses / Concentrations:
approx 0, 400, 2300, 8000 ppm
other: target concentrations
No. of animals per sex per dose:
8 males per dose
Control animals:
yes, concurrent no treatment
Neurobehavioural examinations performed and frequency:
To assess the effects of cyclohexane, tests from the Functional Observation Battery together with assessment of automated locomotor activity and visual discrimination performance were conducted.
Clinical signs:
no effects observed
no mortality observed
Body weight and weight changes:
no effects observed
Food consumption and compound intake (if feeding study):
not examined
Food efficiency:
not examined
Water consumption and compound intake (if drinking water study):
not examined
Ophthalmological findings:
not examined
Clinical biochemistry findings:
not examined
Behaviour (functional findings):
effects observed, treatment-related
Gross pathological findings:
not examined
Neuropathological findings:
not examined
Other effects:
not examined
Details on results:
There were minimal systemic effects following exposure to 28000 mg/m3 and small, inconsistent and reversible effects in FOB observations.
Dose descriptor:
Effect level:
8 000 mg/m³ air (nominal)
Basis for effect level:
other: slight reductions in psychomotor speed in the high exposure group but minimal nervous system effects.
Remarks on result:
Cyclohexane exposure up to 28000 mg/m3 produced minimal acute CNS effects in rats, with small effects on gait and a statistically significant reduction in psychomotor speed in visual discrimination tests.
Executive summary:

Statistically significant differences from control in FOB values were seen at 8,000 ppm (28,000 mg/m3) however, there was a lack of consistency between assessments. Effects observed at 2,000 ppm were considered to be the beginning of behavioural toxic effects. The authors concluded that "exposure at 400 ppm or 2,000 ppm of cyclohexane on a group basis did not induce neurobehavioural effects".

Endpoint conclusion
Endpoint conclusion:
no adverse effect observed
Dose descriptor:
860 mg/m³
Study duration:
other: human
Quality of whole database:
Adequate information is available to characterise the effects of cyclohexane on the nervous system.

Effect on neurotoxicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no study available

Additional information

Non-human information

The neurotoxicity of acute exposure to cyclohexane was studied in a schedule-controlled operant behaviour (SCOB) study (Christoph et al, 2000).

Adult male rats (trained in operant performance) received an acute inhalation exposure to cyclohexane vapour (0 ppm, 500 ppm. 2000 ppm, or 7000 ppm) for 6 hr. During the operant session (30 min after termination of exposure), fixed ratio (FR) running rate for the 7000 ppm group decreased; FR post-reinforcement pause duration and the rate and pattern of Fl performance were unaffected. Cyclohexane exposures of 500 or 2000 ppm had no detectable effects. No enduring effects of cyclohexane occurred up to 2 weeks after exposure. Positive control animals dosed with CPZ or AMPH confirmed the validity of the test method. The NOAEC was concluded to be 2,000 ppm (6,880 mg/m3).

As part of a wider programme, a study (conducted with 2 experiments) attempted to evaluate the behavioural effects of exposure to cyclohexane in animals and to determine internal levels of exposure at which effects occur (Lammers et al, 2009). Functional observation battery data revealed minimal CNS effects (small, non-statistically significant effects of gait and tremor and statistically significant effect on approach).

Since these were limited effects on the CNS at exposure levels up to 8,000 ppm (28,000 mg/m3) and there was a lack of consistency between assessments (notably between the first and third exposure days), this indicates that the relationship of these small changes to treatment was uncertain. it is concluded that "exposure at 400 ppm or 2,000 ppm of cyclohexane on a group basis did not induce neurobehavioral effects" .

Since other animal data (Christoph et al, 2000; Malley et al, 2000) are available together with a human volunteer study also conducted at TNO (Lammers et al, 2009), the results from this study are not taken forward for consideration in the hazard and risk assessment.

Human information

A study performed by TNO (Lammers et al, 2009) was conducted in order to establish the neurobehavioral effects in human volunteers exposed to 250 ppm (860 mg/m3) of cyclohexane for 4 hours or to placebo and to 25 ppm (86 mg/m3) only. Twelve human male volunteers (ages ranging from 20 to 39 years old) were exposed to the experimental conditions using a double blind, two-way cross-over design. The two tests were spaced 7 days apart. Subjects were tested using automated neurobehavioral tests and questionnaires prior to exposure, 45 and 165 minutes after the beginning and about 60 minutes after the exposure.

The only effects observed were a significant improvement in performance under both treatment conditions the Hand-Eye Coordination Test: sinus condition; the Colour Word Vigilance Test and the Digit Memory Span Test during the first test day. There was no compelling evidence for treatment related effects at 860 mg/m3and the reports of headache, throat and eye irritation were 'self reported' and subjective. Evidence of only minimal effects in rats exposed to 28,000 mg/m3suggests that effects in humans exposed to much lower levels would be unlikely. The subjective, 'self-reported', effects reported in the human volunteer study are therefore not taken into account for the determination of a NOAEC and an NOAEC of 250 ppm (860 mg/m3) is assumed for human neurotoxicity.

Based on the TNO studies (Lammers et al, 2009; Hissink et al, 2009), it was concluded that with the validated human PBPK model it was possible to calculate an exposure level which would lead to neurobehavioral effects in human. This dose would be 3,900 ppm (13,400 mg/m3) leading to a NOAEC of 1,200 ppm (4,125 mg/m3). Since these values are derived from a model (not actual tested values), it seems difficult to take them into account for the risk characterisation instead of the animal data.

When assessing these and other human and animal data, SCOEL (1991) concluded that these results supported a human NOAEC of about 250 ppm which, when compared with animal data indicating a NOAEC of 500 ppm and a LOAEC of 2000 ppm for narcotic effects in rats and mice, resulted in an IOELV of 200 ppm (8-hr TWA).

Justification for selection of effect on neurotoxicity via inhalation route endpoint:
Results from a human volunteer study show no compelling evidence for treatment related effects following 4 hr exposure to 860 mg/m3

Justification for classification or non-classification

These effects are considered to warrant labelling under CLP: Specific Target Organ Toxicity - Single exposure, Category 3 following the criteria for narcotic effects and assign with H336.