Dodecamethylcyclohexasiloxane

Endpoint summary

Additional information

Testing for toxicity to terrestrial organisms has been carried out with earthworms (OECD 222) and soil micro-organisms (OECD 216) with the registered substance, dodecamethylcyclohexasiloxane (D6, CAS 540-97-6).

 

A 56-day earthworm reproduction test, at concentrations up to 1000 mg/kg soil dry weight, has been conducted in accordance with OECD TG 222 (earthworm reproduction test) and in compliance with GLP. No effects on survival or reproduction were observed.

A 28-day LC₅₀ value of >1000 mg/kg dry weight and a NOEC value of ≥1000 mg/kg dry weight (highest concentration tested) have been determined for the effects of the test substance on survival of adult earthworms, based on nominal concentrations.

A 28-day NOEC value of ≥1000 mg/kg dry weight (highest concentration tested) has been determined for the effects of the test substance on growth of adult earthworms, based on nominal concentrations.

A 56-day NOEC value of ≥1000 mg/kg dry weight (highest concentration tested) has been determined for the effects of the test substance on reproduction of the earthworm, based on nominal concentrations (Smithers 2021).

 

A 28-day toxicity to soil microorganisms study for effects of D6 on nitrate formation rate in soil microflora, tested in a series of concentrations up to 100 mg/kg soil dry weight, has been conducted in accordance with OECD TG 216 (Soil Microorganisms: Nitrogen Transformation Test) and in compliance with GLP. No effects on nitrate formation were observed:

A 28-day EC₁₀ value of >100 mg/kg dry weight has been determined for the effects of the test substance on nitrate formation rate, based on nominal concentrations (Smithers 2019).

 

Approach to Chemical Safety Assessment

The terrestrial testing strategy and chemical safety assessment for D6 is based on a Weight-of-Evidence (WoE) approach.

 

REACH Guidance Chapter R.7c, Section R.7.11.5.3 states that it will normally not be possible to derive a robust PNEC for the purposes of a soil screening assessment from acute aquatic toxicity testing showing no effect. This is particularly true for poorly soluble substances. Where the water solubility is <1 mg/l, the absence of acute toxicity can be discounted as reliable indicator for potential effects on soil organisms due to the low exposures in the test. The absence of chronic or long-term effects in aquatic organisms up to the substance solubility limit, or of acute effects within the solubility range above 10 mg/l can be used as part of a Weight-of-Evidence argument to modify/waive the data requirements of Annex IX and X.

 

Also, according to the ECHA guidance, Chapter R.7c, Section R.7.11.5.3 (ECHA 2017), where there is no toxicity L(E)C₅₀ in the standard acute aquatic toxicity tests at >10 mg/l, or no effects in chronic toxicity at the limit of water solubility, or the screening assessment based on EPM shows no concern, then a single short-term soil test on a suitable species would be adequate to meet the requirements of Annex IX. The soil PNEC would be derived by application of appropriate assessment factors to the aquatic data and the soil short-term data, and the lowest value taken. Where the substance is highly adsorptive, e.g. where the log K_ow/K_oc >5, and/or the substance is very persistent in soil, this single test should be a long-term test. Substances with a half-life >180 days are considered to be very persistent in soil. This persistence would be assumed in the absence of specific soil data, unless the substance is readily degradable. The choice of test (invertebrate / plant / micro-organism) would be based on all the information available, but in the absence of a clear indication of selective toxicity, an invertebrate (earthworm or collembolan) test is preferred.

 

Acute and chronic aquatic toxicity data are available with D6 or are read across from structural analogues. The data show no acute or chronic effects close to or above the limit of solubility of the substance.

 

D6 is highly adsorptive (log K_ow/K_oc >5) and is likely to be very persistent in soil, therefore a long-term soil test has been conducted. Because there were no effects observed with aquatic organisms, there is an absence of a clear indication of selective toxicity. An earthworm reproduction test under OECD TG 222 has therefore been carried out, in compliance with the guidance. In addition, a soil microorganism toxicity test was carried out.

 

No effects on soil organisms were observed in these tests.

 

The aquatic and terrestrial data available for D6 all show no effects at the highest concentrations tested. In accordance with ECHA guidance, these tests are adequate for terrestrial risk characterisation and no further terrestrial toxicity tests are required. The PNECsoil conclusion is therefore No Hazard Identified.