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

Toxicity to soil microorganisms

Additional information

Due to the hydrolysis half-life of the substance (<<1 min at pH 7), the exposure of the terrestrial compartment is expected to be predominantly to the hydrolysis products. Exposure of soil is via the partitioning of the substance to the WWTP sludge. The sludge may then be spread onto soil, depending on the practices of the WWTP. The substance will have undergone significant hydrolysis by the time it reaches the soil, therefore exposure and chemical safety assessment of the terrestrial compartment is based on the degradation products trimethylsilanol and ammonia.

Trimethylsilanol: In accordance with Column 2 of REACH Annex IX, there is no need to further investigate the effects of this substance in terrestrial toxicity studies 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 Ratio is below 1, and therefore the risk is already adequately controlled and further testing is not justifiable. The substance is highly water soluble and is not adsorbing (based on log K_ow <3 (1.19) and log K_oc 1.6) and is therefore not expected to partition to the terrestrial compartment. Low or no toxicity was observed in short-term aquatic tests as well as the aquatic microorganism test. Therefore, the occurrence of more severe toxic effects in the terrestrial compartment that were not expressed in the pelagic organism and aquatic microorganism tests would be considered unlikely. Based on the aquatic data set, the most sensitive relevant trophic level is invertebrates, based on data from 1,1,1,3,3,3-hexamethyldisilazane (CAS 999-97-3). A soil hazard category 3 has been derived for the substance. According to the screening assessment for soil hazard category 3, the PNEC_soil has been calculated from PNEC_freshwater on the basis of the equilibrium partitioning method and a confirmatory long-term toxicity to terrestrial organisms is not thought to be necessary for chemical safety (see the terrestrial endpoint summary discussion). The risk characterisation ratio (RCR) based on PNEC_soil derived from the equilibrium partitioning method is <1. It is recognised that the aquatic PNEC used in the EQPM does not take into account any indicator for effects in aquatic microorganisms. However, the short-term fish, invertebrate and algae data suggest that aquatic microorganisms are comparatively less sensitive: the OECD 209 activated sludge respiration inhibition test recorded an EC₅₀ of 6670 mg/l (nominal) and no inhibition to respiration was observed in the test (Muller, 1992, see IUCLID Section 6.1.7), while the lowest EC₅₀ obtained with Daphnia was 80 mg/l (Hüls, 1994). Therefore, it is unlikely that the PNEC_terrestrial based on aquatic ecotoxicity test results would not be protective for terrestrial microorganisms. The chemical safety assessment using EQPM does not suggest any unacceptable risks for the terrestrial compartment. Similarly, PNEC_stp has been calculated to be 66.7 mg/l, whereas PNEC_soil has been calculated via the equilibrium partitioning method to be a more conservative 0.12 mg/kg dwt. Therefore, it is unlikely that the PNEC_terrestrial derived by the equilibrium partitioning method based on the PNEC_aquatic would not be protective for terrestrial microorganisms. The chemical safety assessment using EQPM does not suggest any unacceptable risks for the terrestrial compartment.

Ammonia: Ammonia is assessed separately from dimethylvinylsilanol. Data are available for the toxicity of ammonia to terrestrial plants only. There is no indication that soil micro-organism toxicity from ammonia would need to be assessed through a toxicity study because concentrations of ammonia in soil 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). Soil micro-organism toxicity is therefore not expected.