Tetradecamethylhexasiloxane

Endpoint summary

Additional information

There are no terrestrial toxicity data available for tetradecamethylhexasiloxane (L6, CAS 107-52-8). Although terrestrial testing is not an Annex VIII requirement, Reach Guidance (REACH R7c, ECHA 2017) indicates that substances considered to be highly hydrophobic (log K_ow >5) require terrestrial assessment even at tonnages below 1000 tpa.

Testing for toxicity to terrestrial organisms has been carried out with earthworms (OECD 222) and soil micro-organisms (OECD 216) with the analogue substance, dodecamethylpentasiloxane (L5, CAS 141-63-9).

READ-ACROSS JUSTIFICATION:

Read-across from L5 to L6:

L6 and the surrogate substance L5 are linear siloxanes with 6 and 5 silicon atoms, respectively, with methyl branches and alternated by oxygen. L6 and L5 possess similar physicochemical properties: high molecular weight (459 and 385, respectively), low water solubility (both insoluble, at 2.3E-06 mg/l and 7.0E-05 mg/l, respectively), high log K_ow (>9.41 and 9.41, respectively) and high log K_oc (>6.0 and 5.16, respectively). Both substances have negligible biodegradability and similar slow hydrolysis rates.

Given the similar properties and structural similarities, it is considered valid to read across terrestrial toxicity data from L5 to L6.

A comparison of the key physico-chemical properties is presented in the table below:

Property	L6 (107-52-8)	L5 (141-63-9)
Molecular weight	459	385
Log Kow(at 25°C)	>9.41 (read across from L5)	9.41
Log Koc(at 23°C)	>6.0	5.16
Water solubility (mg/l)	2.3E-06	7.0E-05
Hydrolysis half- life at pH 7 (h)	6300	>728 (read-across from L4)

 

Terrestrial toxicity data for L5

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). No effects on survival or reproduction were observed.

A 28 day NOEC value of ≥1000 mg/kg dry weight has been determined for the effects of the test substance on growth and survival of adult earthworms, based on nominal concentrations. A 56 day NOEC value of ≥1000 mg/kg dry weight and EC₁₀ value of >1000 mg/kg dry weight have been determined for the effects of the test substance on reproduction of the earthworm, based on nominal concentrations. The nominal concentration of 1000 mg/kg is equivalent to a measured concentration of 850 mg/kg.

 

A 28 day toxicity to soil microorganisms test for effects of L5 on nitrate formation rate of soil microflora has been conducted in accordance with OECD TG 216 (Soil Microorganisms: Nitrogen Transformation Test) at concentrations up to 100 mg/kg soil dry weight. 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.

 

Approach to Chemical Safety Assessment

The terrestrial testing strategy and chemical safety assessment for L6 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.

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 read across to the registration substance, L6, from structural analogues. The data show no acute or chronic effects close to or above the limit of solubility of the substance.

L6 is highly adsorptive (log K_ow/K_oc >5) and is likely to be very persistent in soil, therefore long-term soil test data are required. Because there were no effects observed in studies with the analogue substances with aquatic organisms, there is an absence of a clear indication of selective toxicity. An earthworm reproduction test under OECD TG 222 and a soil microorganism toxicity test are available with the analogue substance, L5. No effects on soil organisms were observed in these tests.

The aquatic and terrestrial data read across to L6 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.

Under the weight-of evidence approach, the guidance states that 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. In the absence of long-term effects with aquatic organisms, the PNECaquatic conclusion was No Hazard Identified. The similar absence of effects with terrestrial organisms means it is not possible to derive a terrestrial PNEC. The PNECsoil conclusion, using the weight-of evidence approach, is therefore No Hazard Identified.