Hexamethylene diisocyanate

Endpoint summary

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Study duration:

chronic

Species:

rat

Quality of whole database:

The study is GLP compliant and is of high quality (Klimisch score=1)

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEC

0.035 mg/m³

Study duration:

chronic

Species:

rat

Quality of whole database:

The study is GLP compliant and is of high quality (Klimisch score=1)

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Additional information

Inhalation exposure is the most appropriate route for assessing occupational risk in humans. Effects from repeated exposure of animals to 1,6-hexamethylene diisocyanate (HDI) are limited to effects on the respiratory tract caused by local irritation.

The most relevant evaluation of repeated dose toxicity comes from a 2-year chronic toxicity and oncogenicity study in rats according to OECD TG 453 (Shiotsuka, 1989). The animals were whole-body exposed to 0, 0.005, 0.025 and 0.164 ppm (0, 0.035, 0.175 and 1.15 mg/m3) of HDI vapour. The exposure regimen was 6 hours/day, 5 days/week for one year (chronic toxicity assessment) or two years (toxicity and oncogenicity assessment). There were no compound-related effects on mortality, ophthalmology, clinical biochemistry, urinalysis and organ weights after 2-year inhalation with vapour concentrations up to and including 0.164 ppm. Those effects determined to be compound-related were transient ocular irritation in males, small but consistent decrease in body weight of females (particularly during the second year of exposure) and slight anemia in females at 0.164 ppm. There were no compound-related gross lesions. Histopathologically, compound-related non-neoplastic changes were limited to the nasal cavity and the lungs. Changes in the nasal cavity were observed in both sexes at 0.005 ppm and above (except males of the lowest dose group) and characterized by a non-specific epithelial tissue reaction to irritation at all exposure concentrations. The following nasal lesions were observed: hyperkeratosis, hyperplasia of the squamous epithelium, chronic active inflammation, squamous metaplasia, mucus secretory cell or goblet cell hyperplasia, hyaline droplet degeneration and minimal degeneration of the olfactory epithelium. Lung lesions were noted as epithelialization, interstitial pneumonia or alveolar macrophage accumulation in both sexes at 0.025 ppm and above after two years of exposure. No evidence of compound-related oncogenicity was found. The highest concentration of 0.164 ppm is regarded as a Maximum Tolerated Dose (MTD) based on a slight decrease in body weight and slight anemia of females and microscopic changes in the nasal cavity of both sexes. The lowest concentration of 0.005 ppm is considered to be a NOEC after one year of exposure since the changes observed occurred only in one sex, were qualitatively similar to those seen in controls and did not show any concentration-dependent increase in degree. After two years of exposure to the lowest concentration (0.005 ppm), indications of a protective response to non-specific irritation was observed (NOAEC).

Analysis of the results from the principal study revealed that compound-related effects were limited to histopathology in the nasal passages. Although some lesions were noted in the nasal tract of animals from all exposure groups, Foureman et al. (1994) concluded that the olfactory epithelial degeneration should be considered as the significant effect in this study, with a NOAEC of 0.005 ppm and a LOAEL of 0.025 ppm, because it followed a concentration-response relationship for both incidence and severity. The data for this lesion show its absence at the lowest concentration with parallel increases in both incidence and severity at the two highest concentrations. For the other lesions, including chronic inflammation, mucus cell hyperplasia, epithelial hyperplasia, hyaline droplet degeneration, and squamous metaplasia no concordance in incidence and severity was found. In response to an irritant, the character of lesions in the nasal tract such as squamous metaplasia, mucus cell hyperplasia, and hyaline droplet formation appears to be more adaptive than adverse.

In a 90-day inhalation study in rats (OECD TG 413; Shiotsuka, 1988) with HDI vapour concentrations of 0, 0.01, 0.04 and 0.14 ppm the only compound-related clinical sign was ocular irritation which was observed at all three concentrations. There were no mortalities and no compound-related effects on the following parameters: body weight, clinical chemistry, hematology, urinalysis, gross pathology, organ weight and organ to body weight ratios. The compound-related histopathologic lesions were observed at all three concentrations and were generally located in the cranial nasal cavity anterior to the nasal papilla. Hyperplasia and/or squamous metaplasia of the respiratory epithelium were considered to be the most important compound-related lesions in both sexes. Keratin was often present covering the metaplastic epithelium in the mid- and high-dose rats. Mucous cell hyperplasia predominantly of the respiratory epithelium and inflammatory cell infiltrate principally within subepithelial tissues were also considered to be compound- related. The concentration of 0.01 ppm was considered to approximate a threshold for respiratory tract lesions, but a clear NOEC was not established for this study.

In a 21-day inhalation study in rats (Sangha, 1984) with HDI vapour concentrations up to 0.3 ppm no mortality was observed during the study at all concentrations. Signs of toxicity were concentration-related and were observed at concentrations of 0.0175 ppm and above. The signs included ocular and nasal irritation and were observed on the exposure days only. Body weights, feed consumption, blood chemistry, urinalysis and hematology showed no biologically significant differences from the control group. Only liver and kidney weights (absolute and/or relative) showed a concentration-related effect. No compound-related gross lesions were observed. Microscopic examination of the tissues demonstrated concentration-related effects on the respiratory mucosa and nasal cavity at 0.15 and 0.30 ppm concentration levels. The effects noted at 0.0175 ppm were equivocal, and there was no effect at 0.005 ppm. The changes were characterized by squamous metaplasia. In the larynx and trachea the changes included accumulation of mixed inflammatory cells in the submucosa and a minimal to mild hyperplasia of the epithelium. Recovery at 0.0175 and 0.15 ppm concentrations was suggested. The NOEC was 0.005 ppm.

The earlier HDI studies (performed between 1980 and 1985) may have underestimated actual exposure concentrations in the presence of overestimation of portal-of-entry-related toxicity due to phase-specific alterations of the vapor retention-deposition relationships. Therefore, a further study was conducted to assess previous results in the light of technical improvement of exposure technique (e.g. nose-only vs. whole body exposure, real-time R-NCO specific FT-IR vs. in situ nitroreagent derivatization sampling with subsequent HPLC analysis, inluence of trigeminal sensory irritation on irritation-related findings in the nasal cavities).

In this 1-week inhalation study male rats were nose-only exposed for 6 hours/day to analytical concentrations of 0, 0.027, 0.1, 0.46 and 1.97 ppm (0, 0.19, 0.7, 3.2 and 13.8 mg/m³ air) HDI vapour for 5 consecutive days based on the procedures of OECD TG 412 and OECD GD 39 (Kopf, 2015) . This study did not reveal evidence of pulmonary or thoracic airways irritation. Histopathology revealed and reproduced the typical anterior-posterior gradient of irritation-related injury in the nasal cavities at 0.46 and 1.97 ppm (3.2 and 13.8 mg/m³). At 0.1 ppm (0.7 mg/m³) a borderline (if any) transitional response from adaption to adversity was identified within the very proximal nasal passages. Reflexively-induced changes in breathing patterns originate from stimulation of nociceptive trigeminal nerve located in the nasal cavity. Functionality of nerves is evidenced by reflexively-induced bradypnea. Attenuation of reflexes ("tolerance") is taken as robust, integrated physiological evidence of nerve function. Collectively, physiological measurements did reveal increased responsiveness at 0.46 ppm (3.2 mg/m³) and unchanged responsiveness at 1.97 ppm (13.8 mg/m³) from measurements made on exposure days 0 and 4. Histopathology showed neurodegeneration in conjunction with inflammation and subepithelial edema in the nasal cavities especially at 0.46 and 1.97 ppm (3.2 and 13.8 mg/m³). The integrated quantitative physiological measurement takes precedence over the semi-quantitative pathological analyses at selected anatomical sites of the nasal cavities. Taking all findings into account, 0.1 ppm (0.7 mg/m³) constitute the borderline NOAEL based on the effects observed in upper respiratory tract.

Justification for classification or non-classification

Repeated dose toxicity

Not classified under Annex I of Directive 67/548/EEC. According to Annex I of Regulation (EC) No 1272/2008 no classification is required for repeated dose toxicity.

Remark: A classification with STOT-RE is not justified due to lack of cumulative toxicity. This is in accordance with the ECHA Guidance on the Application of the CLP Criteria (2009), which notes on page 365: Where the same target organ toxicity of similar severity is observed after single and repeated exposure to a similar dose, it may be concluded that the toxicity is essentially an acute (i.e. single exposure) effect with no accumulation or exacerbation of the toxicity with repeated exposure. In such a case classification with STOT-SE only would be appropriate.