Hydrogen sulphide

Endpoint summary

Description of key information

Subchronic Inhalation toxicity studies in laboratory animals (rat and mouse) have identified the olfactory nasal mucosa as the principal target site affected.

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:

adverse effect observed

Dose descriptor:

NOAEC

42 mg/m³

Study duration:

subchronic

Species:

other: rat and mouse

Quality of whole database:

reliable studies

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEC

14 mg/m³

Study duration:

subchronic

Species:

other: rat and mouse

Quality of whole database:

reliable studies

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

The Chemical Industry Institute of Toxicology (1983a) performed a 90-day inhalation toxicity study in B6C3F1/CrlBr mice using H₂S. Animals were individually housed and exposed to 0, 10.1, 30.5, or 80 ppm (0, 14, 42, or 111 mg/m3) H2S for 6 hr/day, 5 days/week for 90 days (10 mice/sex/group).

Histological examination of surviving animals revealed the nasal tract as the only site where lesions were considered compound-related. Male (8/9) and female (7/9) mice exposed to 80 ppm (111 mg/m3) H₂S exhibited minimal to mild inflammation of the anterior portion of the nasal mucosa (section I; section I and II in two mice). The lesion was primarily located in the squamous portion of the nasal mucosa but extended into the respiratory type epithelium (ciliated) in some animals. In one female, the lesion was suppurative and severe involving the entire nasal passage and associated structures. The lesion was also observed in two 80 ppm (111 mg/m3) mice that were exposed in extremis. No other histological findings were considered compound-related.

The critical effect in mice was inflammation of the nasal mucosa. This effect was present in male (8/9) and female (7/9) animals exposed to 80 ppm (111 mg/m3) H2S. The lesions, judged as minimal to mild in severity, were localized primarily in the squamous portion of the nasal mucosa but extended to regions of ciliated respiratory-type epithelium.The reevaluation of the nose slides conducted in 2002–2003 (Dorman et al., 2004) also established the presence of olfactory neuronal loss resulting in atrophy or thinning of the olfactory epithelium. An increased incidence of this lesion was found in male and female B6C3F1 mice following exposure to 30 and 80 ppm H2S. Since H2S is an irritant gas and other researchers (Brenneman et al., 2000; Dorman et al., 2000; Lopez et al., 1987) have reported nasal inflammation following H₂S exposure, the critical effect in mice was considered to be olfactory neuronal loss.The LOAEL for this effect in mice was 30.5 ppm (42 mg/m3) and the NOAEL was 10.1 ppm (14 mg/m3). The LOAEL for the systemic toxicity was 80 ppm (111 mg/m3) based on a decreased body weight gain and the NOAEL was 30.5 ppm (42 mg/m3).

With the following exception, the same methods described in the CIIT (1983a) mouse study were used in a similar study using Fischer 344 rats (CIIT, 1983b). Following a 90-day exposure period in rats, 10 males and females from each group were selected for clinical pathology and histology, while the remaining 5 males and females from each group were used for special neuropathologic studies.

In Fischer 344 rats, no mortality was observed during the 90-day study. Body weights were significantly reduced from weeks 1 to 13 in males and females exposed to 80 ppm (111 mg/m3) H₂S. Feed consumption was also significantly depressed in males and females exposed to 80 ppm (111 mg/m3) H₂S. No abnormalities in ophthalmology, neurological function, serum chemistry, or urinalysis were reported.

The reevaluation of the nose slides conducted in 2002–2003 (Dorman et al., 2004) established the presence of olfactory neuronal loss resulting in atrophy or thinning of the olfactory epithelium in male and female rats following exposure to 30 and 80 ppm H2S. The 2002–2003 reevaluation of the lung slides demonstrated also an increased incidence of bronchiolar epithelial hypertrophy and hyperplasia in male rats following exposure to 80 ppm H2S. Female Fischer-344 rats did not develop this lung lesion following H2S exposure. The epithelial changes were variably associated with peribronchiolar fibrosis and smooth muscle hypertrophy and often mixed inflammation. Other histologic changes noted during the reevaluation were often similar in character to those noted in the original read and were interpreted as likely background findings without clinical significance to the species, strain, and age of animal evaluated. The critical effect was considered to be olfactory neuronal loss.The LOAEL for this effect was 30.5 ppm (42 mg/m3) and the NOAEL was 10.1 ppm (14 mg/m3). The LOAEL for the systemic toxicity was 80 ppm (111 mg/m3) based on a decreased body weight gain and the NOAEL was 30.5 ppm (42 mg/m3).

The same methods described above (CIIT, 1983a,b), including the special neuropathologic studies in CIIT (1983b) were used in the following study which used Sprague-Dawley rats. In the Sprague-Dawley rats, 15 males and 15 females per group, no mortality was observed during the 90-day study.

The reevaluation of the nose slides conducted in 2002–2003 also established the presence of olfactory neuronal loss resulting in atrophy or thinning of the olfactory epithelium. An increased incidence of this lesion was found in female rats following exposure to 30 and 80 ppm H2S. Male rats developed olfactory neuronal loss following exposure to 80 ppm H2S only. At the 80 ppm exposure level, olfactory neuronal loss typically affected most or all of the olfactory epithelium present at nose levels 2 and 3, where it lines the dorsal medial meatus. At nose level 4, this lesion had a lesser extent and affected smaller sites within the olfactory epithelium, which lines the complex ethmoid recess. Sites affected bordered the dorsomedial portion of this cavity and most often included the nasal septum and tips of the ethmoid turbinates. The lesion tended to decrease in severity, extent, and sometimes incidence at the 30 ppm exposure level. Other nasal lesions found included abnormalities of the lacrimal duct, lacrimal gland, vomeronasal organs or teeth, sinusitis, or respiratory epithelial hypertrophy and hyperplasia without inflammation. The 2002–2003 reevaluation of the lung slides demonstrated also an increased incidence of bronchiolar epithelial hypertrophy and hyperplasia in female rats following exposure to 30 or 80 ppm H2S, while male rats developed the lesion following exposure to 80 ppm H2S.There was also a moderate to high background incidence of mixed inflammation in the lung of rats that may have prevented detection of treatment-related inflammatory changes. Other histologic changes noted during the reevaluation were often similar in character to those noted in the original read and were interpreted as likely background findings without clinical significance to the species, strain, and age of animal evaluated.

The critical effect was the olfactory neuronal loss in females. The LOAEL for this effect was 30.5 ppm (42 mg/m3) and the NOAEL was 10.1 ppm (14 mg/m3). The LOAEL for the systemic toxicity was 80 ppm (111 mg/m3) based on a decreased body weight gain and the NOAEL was 30.5 ppm (42 mg/m3).

Ten-week-old male Sprague-Dawley CD rats (12/exposure group) were exposed to 0, 10, 30, or 80 ppm (0, 14, 42.7, or 111 mg/m3) H₂S for 6 hr/day, 7 days/week for 10 weeks. At the end of the 10-week exposure period, animals were euthanized with CO₂and the noses of the animals were dissected free. The nasal cavities were sectioned at 6 different levels such that sections 3-6 each allowed for histological evaluation of transitional /respiratory and olfactory epithelium. The lesions noted were limited to olfactory tissue, i.e., olfactory neuron loss and basal cell hyperplasia at the olfactory mucosa, and were graded in severity by a subjective scale where 0 = normal, 1 = mild, 2 = moderate, 3 = marked, and 4 = severe. Table 1 shows that no effects were observed in the control animals or in animals exposed to 10 ppm (14 mg/m3) H₂S. Lesions, limited to the olfactory mucosa, were observed in animals exposed to either 30 (427 mg/m3) or 80 ppm (111 mg/m3) H₂S. These olfactory lesions consisted of multifocal, bilaterally-symmetrical olfactory neuron loss and basal cell hyperplasia affecting the lining of the dorsal medial meatus and dorsal and medial region of the ethmoid recess. The incidence, mean severity, and distribution of the exposure-related lesions increased in a concentration-dependent manner.

 

Table 1. Incidence (Inc) and average severity (Sev) of nasal lesions in male CD rats exposed to H₂S

Lesion	Nasal Cavity Level	0		10 ppm		30 ppm		80 ppm
		Inc	Sev	Inc	Sev	Inc	Sev	Inc	Seva
Olfactory Neuron Loss	3	0/9	—	0/8	—	0/6	—	8/8	2.4
	4	0/12	—	0/12	—	11/12	1.4	12/12	2.4
	5	0/12	—	0/12	—	9/12	1.1	11/12	1.5
	6	0/12	—	0/12	—	0/12	—	5/12	1.2
Basal Cell Hyperplasia	3b	—	—	—	—	—	—	—	—
	4	0/12	—	0/12	—	10/12	1.8	12/12	1.2
	5	0/12	—	0/12	—	7/12	1.3	11/12	1.3
	6	0/12	—	0/12	—	0/12	—	6/12	1.0

a Average severity of the lesion in affected rats in the exposure group: 1 = mild, 2 = moderate, 3 = severe.

b Rats were not evaluated at level 3 because of the common occurrence of basal cell hyperplasia at background levels.

Source: adapted from Brenneman et al.(2000).

Although the olfactory mucosa was widely distributed, lesions in this tissue were found at select sites. At level 5, mild to moderate olfactory neuron loss and mild basal cell hyperplasia mainly affecting the nasal septum, dorsal nasal cavity, and marginal ethmoturbinate were observed in both exposure groups. The nasal septum was not affected in the 30 ppm (42 mg/m3) exposure group. The same pattern and severity of lesions were observed at level 6 in the 80 ppm (111 mg/m3) exposure group. The critical effects in this study are nasal lesions of the olfactory mucosa, 30 ppm (42 mg/m3) is the LOAEC and 10 ppm (14 mg/m3) the NOAEC.

Justification for selection of repeated dose toxicity via oral route - systemic effects endpoint:
No study with acceptable reliability available.

Justification for selection of repeated dose toxicity inhalation - systemic effects endpoint:
None selected, 4 subchronic toxicity studies of equal good quality are available to derive a NOAEC for the systemic toxicity.

Justification for selection of repeated dose toxicity inhalation - local effects endpoint:
Brenneman et al. (2000) was considered to be the most appropriate for the derivation of an inhalation DNEL for several reasons. First, the most critical effect, nasal lesions of the olfactory mucosa, has been also reported by other investigators (Dorman et al., 2000 and 2004; CIIT, 1983a; b and c; Lopez et al., 1988); second, the effect is consistent with the irritant properties of this gas; third, along with the neurological system, the respiratory system has been reported be a target organ of H2S toxicity by numerous researchers; fourth, the LOAEL (42 mg/m3) and NOAEL (14 mg/m3) are at lower concentrations than those in the other subchronic studies.

Justification for classification or non-classification

According to REGULATION (EC) No 1272-2008 and Annex VI of Commission Directive 2001/59/EC/ Council Directive 67/548/EEC, H2S is not classified for repeated dose toxicity.