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Description of key information

Based on the 14-wk inhalation study, the NOAEL for repeated inhalation toxicity in the rat is 24.8 ppm (122 mg/m3).  These values are based on systemic effects other than thyroid and kidney that are not considered  relevant to humans (IARC, 1998).  

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion
Dose descriptor:
NOAEL
20 mg/kg bw/day
Study duration:
subacute
Species:
rat

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion
Dose descriptor:
NOAEC
122 mg/m³
Study duration:
subchronic
Species:
rat

Additional information

There are no sub-chronic repeat dose studies available by the oral route of exposure. There is a sub-acute 28-day repeated oral dose toxicity test available. Sprague-Dawley rats received doses of 0, 4, 20 and 100 mg/kg/day for 28-d by oral gavage. 

In the 100 mg/kg group females the mean body weight was significantly reduced at dosage termination but not in the recovery group. Food consumption was not affected. Urinalysis revealed an increase in the number of animals with protein-positive urine and a decrease in water consumption in males given 100 mg/kg. These changes were not found at the end of recovery period. In the hematological examination, no effect was observed after administration of ENB. In the blood biochemical examination, 20 and 100 mg/kg group males showed a decrease in alpha-1 globulin level. However, these changes were not found at the end of the recovery period. In high dose males there was increased brain/body wt ratio and increased kidney/body wt ratio; both effects were absent in recovery males. At autopsy, pale discoloration of the kidneys was observed in males given 100 mg/kg, and histopathology showed increased hyaline droplets in renal tubule epithelium of all male rats in the 20 and 100 mg/kg groups. Histopathological examination of the thyroid indicated hypertrophy of follicular epithelium, as well as decrease in colloid or irregular shape of follicles in males given 4 mg/kg or more. Hypertrophy of follicular epithelium and decrease in colloid were also observed in females given 100 mg/kg. There were no effects on the testes. The NOAEL of ENB for repeated dose toxicity was reported to be less than 4 mg/kg/day for males (based on thyroid effects at 4 mg/kg and kidney effects at 20 mg/kg) and 20 mg/kg/day for females (based on thyroid effects at 100 mg/kg). However, the male rat kidney effects are consistent with alpha-2u-globulin nephropathy and are not considered to be relevant to humans (IARC, 1998). The thyroid effects are also likely to be a species specific effect not relevant to humans (IARC, 1998). For these reasons, the oral NOAEL based on systemic effects other than thyroid and kidney is 20 mg/kg/d, based on reduced body weight of females in the 100 mg/kg/d group.

There are no repeat dose studies available by the dermal route of exposure.

There are four inhalation repeated dose toxicity studies of ENB. One study was of less than 2 wk duration and served as a range finding study for a subchronic study. The early studies, with limited monitoring, showed that exposure to high concentrations (e.g., 237 ppm) of ENB vapor produced liver, kidney, and testicular injury in the rat accompanied by a high incidence of mortality, and liver and testicular injury in the dog (Kinkead, E. et al., 1971). In the more recent rat studies where the purity of the ENB was measured and exposure concentrations maintained at or below 149 ppm, there is no biochemical or histologic evidence for testicular effects, minor adaptive and reversible changes in the liver, and male-rat specific kidney effects (Ballantyne, B. et al., US, 1997b). Thyroid effects were evaluated in the rat oral 28-d study (MHLW,, 2001) and the rat inhalation 14-wk study (Ballantyne, B. et al, US, 1997b). These two were selected as key studies and considered to be the most reliable because they were conducted under well-designed protocols, reported analytical purity of the test material, and providedetailed information. An observation common to these two studies are thyroid changes which are considered likely to be a species specific effect not relevant to humans.

The NOAEL for repeated oral dose toxicity in rats is considered to be 20 mg/kg/day for male and female rats, based on the 28-d oral toxicity study. Based on the 14-wk inhalation study, the NOAEL for repeated inhalation toxicity in the rat is 24.8 ppm (122 mg/m3). These values are based on systemic effects other than thyroid and kidney, which are not relevant to humans (IARC, 1998). 

Justification for classification or non-classification

Under regulation 1272/2008, a classification for STOT (RE) category 2 could be considered appropriate as a LOAEL of 0.75mg/l (150ppm) was established in a sub-chronic study. However, the only effects seen were increases in liver and kidney weight without associated changes to histopathology and changes to urinary parameters of unknown toxicological significance. Other effects noted at this or lower concentrations (e.g. on the thyroid) were established as reversible. Therefore no 'significant' toxic effects were seen and therefore classification under 1272/2008 is not warranted based on this study. However, the data from older studies reported adverse findings (LOAEL) of 61ppm (0.305mg/l) in both rats and dogs. These changes were associated with adverse histopathology as well. These findings could be considered 'significant' and therefore, on a precautionary basis, a STOT classification (category 2) would appear warranted.