1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane

Endpoint summary

Additional information

The low water solubility, high log K_ow and high log K_oc of 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3) indicate that it will adsorb to organic matter and partition to sludges in a waste water treatment plant. No terrestrial toxicity test data are available with the registered substance.

 

Screening Assessment:

The registration substance falls within soil hazard category 3 as defined in REACH R.7; high absorption (log K_ow >5) but there is no indication that the substance is very toxic to aquatic organisms. Aquatic toxicity data show no effects at concentrations that are close to or above the limit of water solubility of the substance. As discussed in Section 7.1.2.1, a true PNEC_aquatic cannot be calculated from the test data because the LC₅₀/EC₅₀ and NOEC values that have been determined are limit values. However, in order to conduct the screening assessment for terrestrial testing, an indicative PNEC_aqua is derived solely for this purpose. A PNEC_{aqua(freshwater)} value of ≥0.0000047 mg/l can be used to calculate a PNEC_{soil(screening)} using the equilibrium partitioning approach.

PNECsoil	=	Ksoil-water	*  PNECaquatic(freshwater)  *  1000
		RHOsoil

For 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane this is:

PNECsoil(screening)	=	3.03E+04	*   ≥0.0000047 *  1000
		1700

                                               =     ≥0.084 mg/kg wwt

This is equivalent to ≥0.095 mg/kg dwt.

Using this PNEC_{soil(screening)}, the PEC_soil x 10 / PNEC_soil ratio is <1 (calculated value 1.11E-03). It should also be noted, that the soil toxicity data available with decamethylcyclopentasiloxane (D5, CAS 541-02-6) indicate that there is a good correlation between measured data and equilibrium partitioning derived PNECs. Therefore, even though the log K_ow of the substance is >5 there is no indication that an additional assessment factor of 10 is needed to take into account possible uptake via soil ingestion.

In order to complete the screening assessment, a confirmatory long-term soil toxicity test is required. Terrestrial toxicity studies with siloxanes are considered to be difficult to conduct due to their high volatilisation potential (high Henry’s Law Constant and low octanol-air partition coefficient) and the potential for degradation in soil. This is based both on chemical properties, and challenges encountered in studies of fate and ecotoxicity conducted to date with substances within this Category. Soil testing according to guideline methods does not allow for a renewal of the substrate and hence re-application of test substance. Therefore, there is potential for the organisms to not be exposed to the test material for a sufficiently long period of time for effects to be expressed, as well as the difficulty of quantifying actual exposure concentrations.

Although test substance losses are expected during terrestrial testing with the registration substance, it is considered that it is possible that measurable concentrations will remain in the soil at the end of the eight-week test period for an OECD TG 222 study. An OECD TG 222 study is therefore proposed with the registration substance 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3). The need for further terrestrial toxicity testing (OECD TG 208) will be reviewed once the results of the OECD TG 222 study with the registration substance are available.

 

Effects on soil microorganisms are not anticipated. In aquatic toxicity testing, activated sludge (sewage treatment plant micro-organisms) do not appear to be affected by siloxanes. However, a toxicity to soil microorganisms study is proposed for the registration substance.

 

Until these data become available, there are no unacceptable risks for the soil compartment if the operational conditions and risk management measures described in Section 9 are applied.

PNEC_soil for the registration substance has been derived based on read across from D5, which has similar physicochemical properties; the risk characterisation ratio (RCR) based on PNEC_soil is <1.

Justification for interim read-across is given in Section 6 of IUCLID and Section 7.0 of the CSR. The read-across data are considered reliable and suitable for deriving an interim PNEC_soil for the registered substance. The testing with D5 has been carried out with species that are representative of three taxonomic groups; earthworms, springtails and plants.

·        A 28-day LC₅₀ value of >4074 mg/kg dry weight and a 56-day NOEC of ≥4074 mg/kg dry weight have been determined for the effects of the test substance on mortality and reproduction and growth respectively of the earthworm, Eisenia andrei.

·        A 28-day LC₅₀ value of 813 mg/kg dry weight and a 28 day IC₅₀ value of 767 mg/kg dry weight have been determined for the effects of the test substance on mortality and reproduction of the springtail, Folsomia candida. A NOEC of 377 mg/kg dry weight has been determined by the reviewer on the basis of a visual examination of the data for both mortality and reproduction.

·        A 14-day IC₅₀ value of 209 mg/kg dry weight has been determined for the effects of the test substance on root dry mass of Barley, Hordeum vulgare. IC₅₀/EC₅₀ values for effects on seedling emergence, root and shoot length and shoot dry mass determined in the same test were ≥248 mg/kg dry weight.

·        14-day EC₅₀ values of >4054 mg/kg dry weight have been determined for the effects of the test substance on seedling emergence, root and shoot length and root and shoot dry mass of Red Clover, Trifolium pratense. NOECs were not determined in the tests.

The D5 terrestrial studies are considered to be non-standard; they have been conducted by spiking surrogate biosolids mixed into natural soil rather than by direct spiking of soil. The registered substance and the surrogate substance share similar physicochemical properties but are not close structural analogues (linear and cyclic siloxanes respectively). However the studies are considered to be suitable to derive an interim hazard and risk assessment under REACH.