Dimethoxymethylsilane

Endpoint summary

Description of key information

Testing for toxicity to terrestrial organisms is not considered necessary because:

- The substance hydrolyses rapidly and its hydrolysis products have very low log Kow values (< 3.0). Therefore, partitioning to the terrestrial compartment is expected to be minimal.

- The substance is used under highly controlled conditions and, therefore, exposure of soil is low.

- PNECsoil has been calculated from PNECaquatic (freshwater) by means of the equilibrium partitioning method. The obtained risk characterisation ratio (RCR) based on PNECsoil is << 1.

- An earthworm reproduction test according to OECD Guideline 222 is ongoing (ECHA decision number: TPE-D-2119517436-40-00/F) for the silanol hydrolysis product methylsilanetriol (CAS No. 2445-53-6). This dossier will be updated once the final study report is available.

Additional information

Testing for toxicity to terrestrial organisms is not considered necessary because:

- The substance hydrolyses rapidly and its hydrolysis products have very low log Kow values (< 3.0). Therefore, partitioning to the terrestrial compartment is expected to be minimal.

- The substance is used under highly controlled conditions and, therefore, exposure of soil is low.

- PNECsoil has been calculated from PNECaquatic (freshwater) by means of the equilibrium partitioning method. The obtained risk characterisation ratio (RCR) based on PNECsoil is << 1.

- An earthworm reproduction test according to OECD Guideline 222 is ongoing (ECHA decision number: TPE-D-2119517436-40-00/F) for the silanol hydrolysis product methylsilanetriol (CAS No. 2445-53-6). This dossier will be updated once the final study report is available.

 

 

The registered substance dimethoxy(methyl)silane (CAS No. 16881-77-9) hydrolyses rapidly (DT50 = 0.3 h) to the silanol hydrolysis products methylsilanediol and methylsilanetriol and the alcohol hydrolysis product methanol.

The ECHA guidance R.16 states that “for substances where hydrolytic DT50 is less than 12 h, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself” (ECHA, 2016). The TGD (EC2003) and ECHA guidance R.16 (ECHA 2010) also suggests that in case the hydrolysis half-life is less than 12 h, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity.

It is anticipated that the second silanol hydrolysis product, methylsilanetriol, is the final product of the

hydrolysis reactions and is thus considered the relevant assessment entity for the present Chemical Safety Assessment according to REACH.

Therefore, the Chemical Safety Assessment is based on the hydrolysis products methylsilanetriol and methanol rather than the parent substance.

Methanol is known for its absence of environmental toxicity at the concentrations relevant to this dossier (OECD SIDS, 2004), and is therefore not considered contributory to the overall aquatic toxicity of the substance and thus not relevant for the assessment of aquatic toxicity.

 

 

Testing considerations for toxicity to terrestrial organisms:

 

PNECsoil has been calculated from the aquatic data using the Equilibrium Partitioning Method. The risk characterisation ratios (RCR) based on the PNECsoil are << 1. The quantitative Chemical Safety Assessment (conducted according to Annex I of REACH) indicates that the Risk Characterisation Ratio (RCR) is well below 1, even with due consideration of contributing uncertainties, and therefore the risk is adequately controlled.

 

The hydrolysis product, methylsilanetriol, is not readily biodegradable but has low potential for bioaccumulation, low potential for adsorption and has low bioavailability (based on log K_ow < 3.0). Therefore, partitioning to the terrestrial compartment is expected to be minimal. No toxicity was observed in aquatic tests conducted at high concentrations, and there is no reason to expect any specific mechanism of toxicity beyond narcosis. Therefore, the occurrence of more severe toxic effects in the terrestrial compartment that may not have been expressed in the aquatic toxicity studies is considered unlikely. 

 

Methylsilanetriol meets the criteria of the Soil Hazard Category 3 (ECHA 2017, guidance part R7(c) Table R.7.11—2) due to its potential persistence. In accordance with the screening assessment for Soil Hazard Category 3 substances, a PNECsoil was calculated from the aquatic data by means of the equilibrium partitioning method for the purpose of the chemical safety assessment. The obtained risk characterisation ratios are below 1. In addition, a confirmatory long-term soil toxicity test should be conducted.

 

No effects on aquatic microorganisms in a study according to OECD Guideline 209 were observed, therefore testing with soil microorganisms is not considered necessary.

 

Overall it is concluded that the risk characterisation conclusion is sufficiently conservative with respect to any uncertainties and therefore further in vivo testing is not considered necessary.  However, in order to comply with ECHA testing requirements for Soil Hazard Category 3 substances, a chronic earthworm reproduction test with methylsilanetriol is ongoing (ECHA decision number: TPE-D-2119517436-40-00/F).

 

Details on how the PNEC and the risk characterisation ratio have been derived can be found in IUCLID Section 6.0 and Chapters 9 and 10 of the Chemical Safety Report, respectively.