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No terrestrial toxicity data are available with the registered substance.

The substance contains many constituents, each of which will behave differently in the environment depending upon their physicochemical and fate properties. It is therefore not possible to determine a single definitive PNECsoil from a single PNECaquatic(freshwater) by the equilibrium partitioning method. Multiple PNECsoil values are instead determined for the constituents or for groups of the constituents which can then be used in conjunction with comparable PECsoil values for risk characterisation.

The constituents of the substance hydrolyse rapidly, therefore the environmental aspects of chemical safety assessment are based on the hydrolysis products Silanol HP-W, HP-X, HP-Y and HP-Z. However, due to the uncertainty around hydrolysis rates of some of the constituents, the parent substance, divided into Blocks, is also assessed.

For assessment of the parent substance constituents, the data relevant to the corresponding silanol hydrolysis product are used.

Parent substance

Constituents of Blocks A and B will rapidly hydrolyse and are not subject to significant uncertainty in relation to their hydrolysis rates. Chemical Safety assessment for these Blocks therefore focuses on the hydrolysis products only.

  

For the rest of the constituents of Blocks C-G, there is greater uncertainty in the rate of hydrolysis, therefore the parent substances are considered.

  

Aquatic toxicity data have been read across from data relevant to the corresponding hydrolysis product. The data indicate no or low aquatic toxicity. PNECs for soil have been calculated on the basis of this read-across data using the Equilibrium Partitioning Method (EQPM).

  

All parent substances are soluble in water, have low log Koc and low log Kow and are susceptible to degradation by hydrolysis. Despite the fact that some of the constituents contain ionisable amine groups, the low log Koc values indicate that they have low potential for adsorption. The parent substances of the constituents in Blocks C-G are therefore assigned to Soil Hazard Category 1 in accordance with ECHA guidance Chapter R.7c, 2017: no indication of high adsorption or high persistence and no indication that the substance is very toxic. Therefore, the approach for screening assessment for the parent constituents is to conduct a PEC/PNECscreen based on the Equilibrium Partitioning Method. The PNECs calculated by the Equilibrium Partitioning Method for the parent constituents generate risk characterisation ratios that are below 1. Terrestrial testing is therefore not required with the parent constituents.

 

In accordance with Column 2 of REACH Annex IX, there is no need to further investigate the effects of this substance in long-term terrestrial toxicity studies because, as indicated in guidance R.7.11.6 (ECHA 2016), the quantitative chemical safety assessment (conducted according to Annex I of REACH) indicates that the Risk Characterisation Ratio is well below 1, and therefore the risk is already adequately controlled and further testing is not justifiable.

It is recognised that the aquatic PNEC used in the EQPM does not take into account any indicator for effects in aquatic microorganisms. However, according to REACH guidance, Chapter R.7c (ECHA, 2017), where inhibition of sewage sludge microbial activity has been observed, a test on soil microbial activity will additionally be necessary for a valid PNEC to be derived. A sewage treatment plant microorganisms toxicity test has been conducted with the registered substance as a whole. No inhibition of microbial activity was observed at concentrations up to 1000 mg/l. Toxicity to soil microorganisms is therefore not expected and it is unlikely that the PNECterrestrial based on aquatic ecotoxicity test results would not be protective for terrestrial microorganisms. Toxicity testing with terrestrial microorganisms does not, therefore, need to be conducted.

Silanol Hydrolysis Products 

The silanol hydrolysis products are highly water soluble and have low log Kow values (-2.4 to -4) and therefore, exposure of the soil compartment is expected to be low. 

The hydrolysis products are not adsorbing (based on low log Koc (-0.23 to -0.6) and have low potential for bioaccumulation (based on log Kow <3 (-2.4 to -4)).

Aquatic toxicity data are available for the hydrolysis products and indicate low or no toxicity. The occurrence of more severe toxic effects in the terrestrial compartment that were not expressed in the aquatic studies would be considered unlikely.

The hydrolysis products are assigned to Soil Hazard Category 3 for the terrestrial environment (Table R.7.11-2 of ECHA guidance R.7c, 2017) based on potential for high persistence (DT50 >180 days), lack of ready biodegradability and low toxicity to aquatic organisms (EC/LC50 not <1 mg/l).  

In this situation, a screening approach is applied: a confirmatory long-term terrestrial test is usually appropriate, in addition to the equilibrium partitioning approach with an extra factor of ten, in order to determine whether further full tests are necessary.

The PNECscreen(EQPM) for the registered substance is based on data for its four silanol hydrolysis products, HP-W, HP-X, HP-Y and HP-Z.

The PNECs calculated by the Equilibrium Partitioning Method have been determined for the purpose of chemical safety assessment for the four silanol hydrolysis product assessment entities, HP-W, HP-X, HP-Y and HP-Z. The risk characterisation ratios for all four silanols are well below 1.

Due to the high water solubility of the substances and their low potential for adsorption to the soil, the compartment of concern is the aquatic compartment rather than terrestrial. The aquatic PNEC and aquatic RCRs are therefore likely to be protective of the terrestrial environment.  

If one confirmatory long-term terrestrial test were to be conducted with each silanol hydrolysis product, an assessment factor of 100 would be applied to the results of each test to derive PNECsoil for the assessment entity.

For HP-W, the terrestrial organisms would have to exhibit a dose response of <0.37 mg/kg dw to be more conservative than the current PNECsoil of 0.0037 mg/kg dw derived using the equilibrium partitioning method.

For HP-X, the terrestrial organisms would have to exhibit a dose response of <3.0 mg/kg dw to be more conservative than the current PNECsoil of 0.03 mg/kg dw derived using the equilibrium partitioning method.

For HP-Y, the terrestrial organisms would have to exhibit a dose response of <0.36 mg/kg dw to be more conservative than the current PNECsoil of 0.0036 mg/kg dw derived using the equilibrium partitioning method.

For HP-Z, the terrestrial organisms would have to exhibit a dose response of <6.9 mg/kg dw to be more conservative than the current PNECsoil of 0.069 mg/kg dw derived using the equilibrium partitioning method.

 

There is no basis to expect such toxicity for these silanols based on the absence of significant toxicity observed in aquatic tests and their low potential for adsorption and bioaccumulation.  

In the case of the registered substance, the Registrant therefore considers that long-term terrestrial studies are unlikely to affect the outcomes of the chemical safety assessment. As such, the Registrant proposes that further testing (including the confirmatory study) is not necessary.

In addition, methylsilanetriol (Silanol HP-X) makes up 50 to 70% of the whole substance. A chronic toxicity to earthworms test under OECD TG 222 conditions is currently in progress with methylsilanetriol.

Overall it is concluded that the risk characterisation conclusion is sufficiently conservative in respect of any uncertainties and therefore further 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.

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