Chloro(methyl)silane

Endpoint summary

Description of key information

See Discussion section below

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEL

250 mg/kg bw/day

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEC

2 213.5 mg/m³

Additional information

There are no adequate repeated dose toxicity data on chloro(methyl)silane or its hydrolysis product, methylsilanol, so good quality data for the related substance trimethylsilanol have been used to assess the general systemic toxicity of chloro(methyl)silane. Local effects from the other hydrolysis product, hydrogen chloride are not addressed by these data.

In the absence of measured data for chloro(methyl)silane, it is considered appropriate to use this result in support of the repeated dose toxicity endpoint for chloro(methyl)silane as the registered substance is hydrolysed very rapidly in the presence of moisture to methylsilanol and hydrogen chloride. The tested substance, trimethylsilanol is closely related to methylsilanol (replacement of two -H with two -CH₃) and both substances have similar physicochemical properties (high water solubility and low log K_ow), therefore the toxicological properties are expected to be similar.

Data for analternative read-across substance, methyltrimethoxysilane, is also available. This substance hydrolyses rapidly to methylsilanetriol, which is the substance that is expected to form after hydrolysis of both the Si-Cl and Si-H bonds in methylchlorosilane. However, in the absence of conclusive evidence that the Si-H bond would hydrolyse under relevant conditions, the best read-across for the initial hydrolysis product methylsilanol was sought. The number of -OH groups at Si, rather than the number of -Me groups, is considered the key determinant of the toxicology of the substance. Therefore, the data for trimethylsilanol are selected as key.

In the key 28-day oral study (Ramm & Bomhard, 1986) on trimethylsilanol, adverse effects (reduced body weight gain, reduced alkaline phosphatase, reduced glucose (males), increased liver weights (females), and increased adrenal weights (males), and minor deposits in the bile ducts) were observed at the highest dose of 750 mg/kg bw/day. The NOAEL was 250 mg/kg bw/day, as effects observed at this or the lower dose of 80 mg/kg bw/day, were not dose-dependent and often values were within the normal range for historical controls.

Exposure to methyltrimethoxysilane (MTMS) was associated with organ weight and/or histomorphological changes in males (liver, thymus, thyroid, duodenum, jejunum, and red blood cell) and females (liver, thyroid, duodenum, jejunum, and adrenal gland) at dose levels at or above 250 mg/kg bw/day. A marked increase in prothrombin time was observed for males at 250 and 1000 mg/kg bw/day whereas females were unaffected. Exposure was also associated with increased blood platelet concentration for males and females at 1000 mg/kg bw/day. These data support a NOAEL for the toxicity phase of the study of 50 mg/kg bw/day.

In the key repeated inhalation study (Fleeman, 2008; an OEDCD 422 study) read-across from trimethylsilanol, test substance-related effects were limited to changes in haematology (lower eosinophil and lymphocyte counts for males) and serum chemistry (higher alanine aminotransferase for males and toxicity phase females) at 600 ppm trimethylsilanol. These changes occurred in the absence of correlating histologic changes and were not considered adverse. Therefore, under the conditions of this screening study, an exposure level of 600 ppm (2213.5 mg/m³) was considered to be the NOAEL for trimethylsilanol.

There is also a 90-day inhalation study on methyltrimethoxysilane that can be read-across to chloro(methyl)silane. Based on the increased incidence of grossly observed urinary bladder calculi along with the kidney dilation at the 400 ppm exposure level, the No Observable Adverse Effect Level (NOAEL) for methyltrimethoxysilane vapour administered six hours per day, five days per week for a 90-day interval via whole-body inhalation exposure to male and female Sprague-Dawley rats, was 100 ppm (equivalent to 557.14 mg/m³).

Since chloro(methyl) silane is hydrolysed to methylsilanol and hydrogen chloride, additional local irritation can be expected due to the acidic nature of the hydrogen chloride.

Repeated dose toxicity: inhalation - systemic effects (target organ) respiratory: other

Justification for classification or non-classification

The data do not suggest that chloro(methyl)silane should be classified for adverse effects following repeated exposures. It has been proposed that it be classified for its corrosive effects in Section 7.3.