1,1,1,3,3,3-hexamethyldisilazane

Long-term toxicity to aquatic invertebrates

Link to relevant study record(s)

Reference

Endpoint:

long-term toxicity to aquatic invertebrates

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

other:

Reason / purpose for cross-reference:

data waiving: supporting information

Reason / purpose for cross-reference:

data waiving: supporting information

Reason / purpose for cross-reference:

data waiving: supporting information

Reason / purpose for cross-reference:

data waiving: supporting information

Reason / purpose for cross-reference:

data waiving: supporting information

Reason / purpose for cross-reference:

data waiving: supporting information

Description of key information

The endpoint is waived.

Additional information

Reliable long-term data are available sourced from the Environment Agency Proposed EQS for Water Framework Directive Annex VIII Substances: Ammonia (un-ionised) 2007. A 29-d NOEC of 0.066 mg NH₃/l has been reported for toxicity of ammonia to the aquatic invertebrate Deleatidium sp. (Hickey et al, 1999).

Testing for long-term toxicity to invertebrates is not considered necessary because:

No data are available for the registered substance, which rapidly hydrolyses to trimethylsilanol and ammonia (half-life: <<1 min at 25°C and pH 7). Long-term aquatic toxicity testing for the parent substance is therefore not appropriate. Chemical safety assessment and PNECs are based on these hydrolysis products. Measured short-term aquatic toxicity data are available for the silanol hydrolysis product, trimethylsilanol, which are used to derive PNECs for this assessment entity. Long-term toxicity testing for trimethylsilanol is not required for the following reasons:

In accordance with Column 2 of REACH Annex IX, there is no need to further investigate the effects of this substance in a long-term aquatic toxicity to invertebrates study because, as indicated in guidance R.7.8.4.3 (ECHA 2017), the quantitative chemical safety assessment (conducted according to Annex I of REACH) indicates that the Risk Characterisation Ratio is below 1 and therefore the risk is already adequately controlled and further testing is not justifiable.

The substance is highly water-soluble, has low potential for bioaccumulation (based on log K_ow <3 (1.19)) and there is no reason to expect any specific mechanism of toxicity beyond narcosis. Therefore, the occurrence of toxic effects that were not expressed in the existing short-term aquatic studies (conducted at concentrations up to 1000 mg/l with trimethylsilanol) would be considered unlikely.

Based on the short-term aquatic data set, the most sensitive trophic level is invertebrates. Based on the results from trimethylsilanol, no toxicity was observed below 100 mg/l.

A PNEC has, however, been derived for the purpose of chemical safety assessment using the Daphnia magna 48-hour EC₅₀ value of 124 mg/l as the starting point. An assessment factor of 1000 was applied to derive the freshwater PNEC; this high assessment factor to derive the predicted no-effect level already reflects the typically higher value of a short-term EC₅₀ compared to a long-term EC₁₀. For a narcotic chemical without a specific mode of toxic action, it is unlikely that the aquatic PNEC would be significantly over-estimated using this method.

Based on the highest freshwater RCR available for the silanol hydrolysis product, trimethylsilanol (0.265), the PNECaquatic (freshwater) value would need to be ≤ 0.032 mg/l to result in RCR values > 1. This value is 3.75 times lower than the current PNECaquatic (freshwater), based on the short-term dataset (0.12 mg/l). A PNECaquatic (freshwater) value of ≤ 0.032 mg/l would correspond to a long-term EC₁₀ or NOEC value of ≤ 1.6 mg/l when applying an assessment factor of 50, indicating high toxicity which was not observed in the short-term dataset.

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, CSR Section 7.0, and Chapters 9 and 10 of the Chemical Safety Report, respectively.

Ammonia:

Data and PNEC derivation for ammonia have been taken from published sources.

Reliable chronic data are described in CSR Section 7.0.