Hexamethylcyclotrisiloxane

Endpoint summary

Additional information

No toxicity to terrestrial organisms data are currently available with the registration substance, hexamethylcyclotrisiloxane (D3, CAS 541-05-9; EC No. 208-765-4).

 

D3 rapidly hydrolyses (23 min at pH 7 and 25°C) to the intermediate hydrolysis products 1,1,3,3,5,5-hexamethyltrisiloxane-1,5-diol (CAS 3663-50-1; EC No. 222-920-3; L3-diol) and 1,1,3,3-tetramethyldisiloxane-1,3-diol (CAS 1118-15-6; EC No. 214-258-9; L2-diol), with the final product being dimethylsilanediol (DMSD, CAS 1066-42-8; EC No. 213-915-7). The L3-diol is the initial hydrolysis product. The half-life for the hydrolysis of the L3-diol and L2-diol is approximately 200 hours at pH 7 and 25°C.

 

The environmental safety assessment focuses on L3-diol and DMSD.

 

L3-diol is soluble in water (1700 mg/l limited to 1000 mg/l by condensation reactions), has a log K_ow of 3.2 and is susceptible to degradation by hydrolysis. No effects were observed in short-term aquatic toxicity tests with the parent substance, but which reflect exposure of the organisms to L3-diol.

 

The intermediate hydrolysis product, L3-diol, is 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 L3-diol is to conduct a PEC/PNECscreen based on the Equilibrium Partitioning Method (EQPM).  

 

No aquatic PNECs have been derived due to the absence of effects in the aquatic studies. Indicative aquatic PNECs have been determined for use in the equilibrium partitioning calculation to derive sediment and soil PNECs.

 

In accordance with Column 2 of REACH Annex XI, 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. No inhibition of microbial activity was observed at concentrations up to 100 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.

 

A long-term toxicity to aquatic invertebrates study is proposed with the registered substance. Terrestrial testing requirements and PNECsoil will be updated once the results of the test are available.

 

Dimethylsilanediol:

In accordance with Column 2 of REACH Annex XI, there is no need to further investigate the effects of this substance in a long-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 soil 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 or 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 0.3.

  

However, if only an invertebrate study were conducted, 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 (AF) 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 Kow <3 (-0.38) and log Koc 0.8) 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 7, 9 and 10 of the Chemical Safety Report.

  

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.

References

EC (2003). Technical Guidance Document on Risk Assessment in support of Commission Directive 93/67/EEC on Risk Assessment for new notified substances, Commission Regulation (EC) No 1488/94 on Risk Assessment for existing substances, Directive 98/8/EC of the European Parliament and of the Council concerning the placing of biocidal products on the market. Part IV. EUR 20418 EN/4

 

ECHA (2016). REACH Guidance on Information Requirements and Chemical Safety Assessment Chapter R16: Environmental Exposure Assessment Version: 3.0. February 2016.

 

ECHA (2017). Guidance on Information Requirements and Chemical Safety Assessment. Chapter R.7b: Endpoint specific guidance. Version 4.0 June 2017.