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Terrestrial testing is not an Annex VIII requirement for 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2). However, the low water solubility, high log Kowand high log Kocof the substance indicate that it will adsorb to organic matter and partition to sludges in a waste water treatment plant. As aquatic toxicity testing is not feasible, it may be more appropriate to use terrestrial tests in order to look into potential toxicity of the substance in the environment.

No terrestrial toxicity data are available for the registration substance; a category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017at). Testing for toxicity to terrestrial organisms has been carried out with earthworms (OECD 222) and soil micro-organisms (OECD 216) with the analogue substance, phenyl silsesquioxanes (EC 939-487-8).

Read-across justification

The read-across substance, phenyl silsesquioxanes, is a multiconstituent substance, and is a reaction mass of the monomer, dimer and oligomers of phenyltris(trimethylsiloxy)silane. The linear oligomers of phenyl silsesquioxanes contain a siloxane chain, where the terminal Si atoms are fully methyl substituted, and the repeating unit contains a Si atom substituted with a phenyl and a trimethylsiloxy group. The major constituents comprise 70-80% of the linear oligomers n=1-5; the minor constituents comprise 10-20% cyclic (n=3-7) and linear (n=6-7) oligomers. The combined purity of both linear and cyclic oligomers is about 85-95%. The remaining impurities are higher polymerised material and alkoxy substituted oligomers. The submission substance 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane is also a mono-constituent substance consisting of >95% of 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxaneand <5% identified impurities (phenyl-siloxane). In 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane, each terminal silicon atom is substituted with one methyl and two phenyl groups, whilst the central silicon atom is substituted with one methyl and one phenyl group.

Both substances share structurally similar ultimate silanol hydrolysis products: 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane hydrolyses to form methyl(diphenyl)silanol and methyl(phenyl)silanediol, and phenyl silsesquioxanes hydrolyses to form phenylsilanetriol and trimethylsilanol. Both phenyl silsesquioxanes and 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane are high molecular weight, phenyl-substituted siloxanes, which have a low solubility, high log Kow and high log Koc. Both substances have negligible biodegradability and hydrolyse very slowly. Full details of the read-across justification are presented in Section 7.0 of the CSR (IUCLID Section 6.0). A comparison of the key physico-chemical properties is presented in the table below:

Table7.2.1Physicochemical parameters for the registration and surrogate substances

CAS Number


EC 939-487-8

Chemical Name


Phenyl silsesquioxanes

Si hydrolysis product

methyl(diphenyl)silanol (CAS 778-25-6) and methyl(phenyl)silanediol

Phenylsilanetriol and trimethylsilanol

Molecular weight (parent)


372.8 to 1635.1

log Kow(parent)



Water sol (parent)

9.0E-11 mg/l

0.0066 mg/l to 4.94E-51 mg/l

Vapour pressure (parent)

1.3E-08 Pa

0.23 Pa at 20°C

Hydrolysis t1/2at pH 7 and 25°C

>329 hours

630 – >63 000 h


Terrestrial toxicity data for phenyl silsesquioxanes

An OECD TG 222 earthworm reproduction test is available for the analogue substance, phenyl; silsesquioxanes. 56-day NOEC, EC10and EC50values of ≥1000, >1000 and >1000 mg/kg dwt respectively, have been determined for the effects on the reproduction ofEisenia fetida, based on nominal concentrations. 28-day NOEC and LC50values of ≥1000 and >1000 mg/kg dwt have been determined for the survival and growth (as weight) ofEisenia fetida, based on nominal concentrations. These results are equivalent to ≥556, >556 and >556 mg/kg dwt respectively, when normalised to 2% organic carbon content.

An OECD TG 216 soil microorganisms (nitrogen transformation) is available for the analogue substance. 28-day EC25and EC50values of >100 mg/kg dwt were reported for the effects on the nitrate formation rate of soil microorganisms, based on nominal concentrations. Nitrate formation rates were significantly reduced in all treatment groups compared to the control, which gives a 28-day LOEC of 1.2 mg/kg dwt. However, a dose response relationship was not observed, suggesting the effects to be due to a non-toxic mechanism. The EC25, EC50and LOEC values are equivalent to >400, >400 and 4.8 mg/kg dwt when normalised to 2% organic carbon.

Approach to Chemical Safety Assessment

The terrestrial testing strategy and chemical safety assessment for 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2) is based on a Weight-of-Evidence (WoE) approach.

REACH Guidance Chapter R.7c, Section R., 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’.

REACH Guidance Chapter R.7c, Section R., further states that ‘where there is no toxicity L(E)C50in 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 Kow/Koc>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.’

On the basis that the registration substance is highly insoluble, data waivers are in place for short-term toxicity to fish, aquatic invertebrates and algae; it is therefore not possible to determine any indication of selective toxicity. An earthworm reproduction test under OECD TG 222 has therefore been read across, in compliance with the guidance. In addition, a soil microorganism toxicity test under OECD TG 216 has also been read across

No effects on soil organisms were observed in these tests at the highest concentrations tested (1000 mg/kg dwt and 100 mg/kg dwt, respectively). 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. As the substance is highly insoluble in water, aquatic toxicity is unlikely to occur (and testing is considered to be not technically feasible), therefore 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.

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