1,1,3,3-tetramethyldisiloxane

Endpoint summary

Additional information

Approach to the Terrestrial Chemical Safety Assessment

1,1,3,3-Tetramethyldisiloxane (CAS 3277-26-7) hydrolyses very rapidly (t_½ = 11.3 min at pH 7 and 25°C) to produce the intermediate hydrolysis products dimethylsilanol. Dimethylsilanol hydrolyses further (t_½ = 2.5 days at pH 7 and 25°C) to produce the ultimate hydrolysis products, dimethylsilanediol (CAS 1066-42-8) and hydrogen.

REACH guidance (ECHA 2016, R.16) states that “for substances where hydrolytic DT₅₀ is less than 12 hours, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself”. ECHA Guidance Chapter R.7b (ECHA, 2017) states that where degradation rates fall between >1 hour and <72 hours, testing of parent and/or degradation product(s) should be considered on a case-by-case basis.

Exposure of soil is expected to be via the partitioning of the substance to the wastewater treatment plant (WWTP) sludge. Although the majority of the substance will partition to water, the proportion of the substance that adsorbs to the WWTP sludge will either be burnt or spread on soil, depending on the practices of the WWTP. For sludge that is spread onto soil, the substance will have undergone further hydrolysis by the time it reaches the soil, however given the half-life of the second hydrolysis rate, it is possible that residual intermediate hydrolysis product will be present. The terrestrial chemical safety assessment is therefore based on the properties of the intermediate and ultimate hydrolysis products.

Dimethylsilanol:

In accordance with Column 2 of REACH Annex IX, there is no need to further investigate the effects of this substance in a long or short-term terrestrial toxicity to invertebrates/higher plants study because, as indicated in guidance R.7.11.6 (ECHA 2017), the chemical safety assessment indicates that the risk is already adequately controlled and further testing is not justifiable.

The substance is highly water soluble, has low potential for bioaccumulation adsorption (based on log K_ow <3 (0.6) and Log K_oc1.33, derived using EUSES). Low toxicity was observed in short-term aquatic tests for a structurally analogous substance, 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 were not expressed in the aquatic studies would be considered unlikely.

Dimethylsilanol is classed as hazard category 1 for the terrestrial environment (Table R.7.11-2 of ECHA guidance R7.c, 2017) based on the fact that there is no indication for high adsorption (log K_oc 1.33) or high persistence of the substance in soil (the substance will hydrolyse to dimethylsilanediol) and the substance displays low toxicity to aquatic organisms (EC/LC₅₀ not <1 mg/l). In this situation, the approach for screening assessment is to conduct a PEC/PNEC screen based on the equilibrium partitioning method.

The PNECscreen(EQPM) for dimethylsilanol is derived from the short-term test results with invertebrates and has a value of 0.068 mg/kg dwt. Based on the exposure assessment of dimethylsilanol, the highest PEC/PNECscreen(EQPM) is 1.86E-4. The RCR is therefore <1. Table R.7.11-2 of ECHA guidance R7.c, 2017, concludes that if PEC/PNECscreen < 1: No toxicity testing for soil organisms need to be done,

Overall it is concluded that the risk characterisation conclusion is sufficiently conservative 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.

Dimethylsilanediol:

In accordance with Column 2 of REACH Annex IX, there is no need to further investigate the effects of this substance in a long or short-term terrestrial toxicity to invertebrates/higher plants study 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 below 1 and therefore the risk is already adequately controlled and further testing is not justifiable.

Dimethylsilanediol is classed as hazard category 3 for the terrestrial environment (Table R.7.11-2 of ECHA guidance R7.c, 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 required, in addition to the equilibrium partitioning approach with an extra factor of ten in order to determine whether further full tests are necessary.

A confirmatory test would be conducted with the most sensitive organism group based on existing aquatic data. For this substance, there were no effects observed in the short-term data and the long-term toxicity study with aquatic invertebrates. In this case it would be preferable to conduct a confirmatory test with terrestrial invertebrates.

Only an indicative PNECscreen(EQPM) for dimethylsilanediol can be derived from the long-term aquatic test results because no effects were observed. The PNEC derived in such a way has a value of ≥0.32 mg/kg dwt. For the purpose of the screening assessment comparison only, an extra factor of ten is applied (PECx10/PNECscreen(EQPM)). Based on the exposure assessment of dimethylsilanediol, the highest PECx10/PNECscreen(EQPM) is 0.012x10/0.32 = RCR 0.38.

If only an invertebrate study were required, the definitive terrestrial risk characterisation would use a PNECsoil based on the lower of the test results between invertebrates and the existing soil micro-organisms study with an assessment factor of 50; if a plant study were also conducted, the assessment factor would become 10.

 

To be more conservative than the value of PNECscreen(EQPM), the terrestrial plants or invertebrates study would have to exhibit a dose response with a NOEC/EC₁₀ ≤16 mg/kg dw, and assuming an AF 50.

The substance is highly water soluble, has low potential for bioaccumulation and adsorption (based on log K_ow <3 (-0.38) and Log K_oc 0.82, derived using EUSES) but is not biodegradable. No toxicity was observed in short- and long-term aquatic tests at the highest concentrations tested, 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 were not expressed in the aquatic studies would be considered unlikely.

In the case of dimethylsilanediol, the registrants therefore consider that a long-term terrestrial study is unlikely to affect the outcomes of the chemical safety assessment. As such the registrants propose that further testing (including the confirmatory study) is not 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.

In addition, a soil micro-organism toxicity study, equivalent to OECD 216 and 217, is available with dimethylsilanediol where no effects were reported at the highest concentrations tested (up to 300 ppm, equivalent to 300 mg/kg dry weight).

Finally, it should be noted that a plant toxicity study with dimethylsilanediol is available indicating effects to wheat plants exposed to aqueous solutions of the substance (Cote-Beaulieu et al. 2009). The study assessed the effects of exposure to methyl silicic acids on growth and development of wheat seedlings and on susceptibility of the seedling to infection by powdery mildew. The methods used were not in accordance with any standard guideline and the presence of silica was observed also in the control of the peat-based studies. It is difficult to assess the significance of the results for the standard REACH endpoints. Therefore, the study is considered reliability 4 and the data are not considered to be relevant for the risk assessment.