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Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.

The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.

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Sediment toxicity

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Description of key information

Key value for chemical safety assessment

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Due to the rapid hydrolysis of the substance, the chemical safely assessment is based on the silanol hydrolysis product trimethylsilanol and ammonia.

Ammonia: given the chemical structure of ammonia, the octanol–water partition coefficient (log Kow) is likely to be low. This is confirmed by the predicted log Kow value of 0.23. The concentrations in sediment resulting from use of 1,1,1,3,3,3-hexamethyldisilazane are minimal compared to the natural background and other anthropogenic sources (see Section 9 of the CSR). Ammonia does not accumulate in sediments and further testing of ammonia to sediment organisms for the derivation of sediment PNECs for the protection of benthic organisms is not required (EA 2007).

Trimethylsilanol: Testing for toxicity to sediment dwelling organisms is not considered necessary because:

PNECsediment has been calculated from the aquatic data using the Equilibrium Partitioning Method. The risk characterisation ratios (RCRs) based on the PNECsediment are <1.

In accordance with Column 2 of REACH Annex X, there is no need to further investigate the effects of this substance in a sediment toxicity study because, as indicated in guidance R.7.11.6 (ECHA 2017), the quantitative chemical safety assessment (conducted according to Annex I of REACH) indicates that the Risk Characterisation Ratios (RCRs) are well below 1, even with due consideration of contributing uncertainties, and therefore the risk is already adequately controlled and further testing is not justifiable.

The substance is not readily biodegradable but has low potential for bioaccumulation, low potential for adsorption and has low bioavailability (based on log Kow <3 (1.19) and log Koc 1.6); therefore, partitioning to the sediment compartment is expected to be minimal. Toxicity of trimethylsilanol was observed in aquatic tests but at concentrations > 100 mg/l (lowest EC50 was 124 mg/l). There is no reason to expect any specific mechanism of toxicity beyond narcosis. Therefore, the occurrence of more severe toxic effects in the sediment compartment that were not expressed in the aquatic studies would be considered unlikely. 

Overall it is concluded that the risk characterisation conclusion is sufficiently conservative in respect of any uncertainties and therefore further in vivo testing is not considered necessary.      

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.