Trichloro(ethyl)silane

Endpoint summary

Description of key information

No adequate repeated dose toxicity data are available for trichloro(ethyl)silane, therefore data for the inhalation and oral routes have been read-across from the related substance, trimethoxy(methyl)silane, CAS 1185-55-3. Exposure to trimethoxy(methyl)silane (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.
There is also a 90-day inhalation study on trimethoxy(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 trimethoxy(methyl)silane  vapor 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).
The key study (DCC, 2007) for repeated dose toxicity via the inhalation route, is a 90-day whole-body inhalation study, in which methyltrimethoxysilane was administered to rats six hours per day, five days per week. The NOAEC of 100 ppm  (0.56 mg/l) was based on an increased incidence of grossly observed urinary bladder calculi along with kidney dilation at the 400 ppm exposure concentration. 
An oral OECD 422 screening test in rats is available for the substance methyltrimethoxysilane. The NOAEL for systemic effects was determined to be 50 mg/kg/day, based on findings in a number of organs including the liver and thymus gland. 

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEL

50 mg/kg bw/day

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Dose descriptor:

NOAEC

560 mg/m³

Additional information

There are no adequate repeated dose toxicity data on trichloro(ethyl)silane or its hydrolysis product, ethylsilanetriol, so good quality data for the related substance methyltrimethoxysilane (MTMS) has been used to assess the general systemic toxicity of trichloro(ethyl)silane. Local effects from the other hydrolysis product, hydrogen chloride (HCl) are not addressed by these data.

Trichloro(ethyl)silane hydrolyses rapidly in contact with water (half-life <1 minute at pH 7), generating HCl and ethylsilanetriol. Methyltrimethoxysilane (MTMS, CAS 1185 -55 -3) hydrolyses more slowly at pH 7 (half-life ca. 2 hours), but under acidic conditions such as in the stomach following ingestion, much more rapid hydrolysis can be expected based on experience with other methoxysilanes. The relevant hydrolysis products are methanol and methylsilanetriol. The silanol hydrolysis products are therefore very similar in chemical structure, the only difference being the replacement of -C₂H₅ with -CH₃. Both have high water solubility and very low log K_ow (log K_ow -1.9 and -2.4, respectively). They are therefore expected to have similar toxicological behaviour. Data obtained via the oral route for MTMS are therefore considered appropriate for read-across to trichloro(ethyl)silane with respect to systemic effects.

For the inhalation route, the hydrolysis rate of MTMS in the respiratory tract and lungs is unknown, but is likely to be slower than that of trichloro(ethyl)silane. For trichloro(ethyl)silane, the species absorbed following inhalation exposure are mainly hydrolysis products, whereas for MTMS, absorption of the parent substance may be more significant. MTMS has a higher log K_ow value (0.7) therefore the proportion of inhaled material which is systemically absorbed is likely to be greater for MTMS than for trichloro(ethyl)silane. Nevertheless, in the absence of other data, the inhalatory NOAEL for MTMS is considered to represent a reasonable worst-case for read-across to trichloro(ethyl)silane.

It is of note that the oral NOAEL in rats for methanol is greater (500 mg/kg bw/day) than the dose that is expected to be generated from hydrolysis of trimethoxy(vinyl)silane in the stomach. Therefore the effects of trimethoxy(vinyl)silane following a dose of 62.5 mg/kg bw/day are not thought to be attributable to methanol. Similarly, following inhalation of methanol the NOAEC is an order of magnitude greater than the NOAEC for trimethoxy(vinyl)silane, and it is therefore unlikely that systemic effects observed following inhalation of trimethoxy(vinyl)silane are due to methanol.

Justification for classification or non-classification

The available read-across data on methyltrimethoxysilane do not suggest that there is a need to classify trichloro(ethyl)silane for adverse effects following repeated exposure.