Registration Dossier

Ecotoxicological information

Endpoint summary

Administrative data

Description of key information

Short-term or long-term terrestrial toxicity studies are not required for Annex VIII substances under the REACH Regulation. However, due to the high STP sludge adsorption of the substance and the potential for persistence in the terrestrial environment, it is appropriate to assess the potential significance of this compartment as part of the chemical safety assessment. Further testing has already been proposed for a structurally-similar substance within the Siloxanes Category, Silsesquioxanes, Phenyl (CAS 70131-69-0). The approach to chemical safety assessment for octaphenylcyclotetrasiloxane will be reconsidered once further data becomes available. Until further data becomes available, existing terrestrial effects data have been read-across from another analogous substance, decamethylcyclopentasiloxane (D5, CAS 541-02-6) within the Siloxanes Category for the purpose of deriving PNEC and conducting a quantitative environmental risk characterisation as an interim approach.

Additional information

Terrestrial testing is not an Annex VIII requirement. However, the low water solubility, high log Kow and high log Koc of the substance indicate that it will adsorb to organic matter and partition to sludges in a waste water treatment plant. As the substance is poorly water soluble and there are no aquatic data available, it may be more appropriate to use terrestrial data to assess the 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, 2017).

There is a limited amount of terrestrial toxicity data available with siloxanes. An integrated testing strategy for the category is proposed. In view of the high potential to adsorb to soil for siloxane substances, and the lack of terrestrial toxicity testing across the Category, it is concluded that further testing is required within the Category.

Screening Assessment and Testing Strategy for Silsesquioxanes, Phenyl

The registration substance falls within soil hazard category 3 as defined in REACH R.7; high absorption (log Kow>5) but 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. A true PNECaquaticcannot be calculated from the test data because the LC50/EC50and NOEC values that have been determined are limit values. However, in order to conduct the screening assessment for terrestrial testing, an indicative PNECaqua is derived solely for this purpose. A PNECaqua(freshwater) value of ≥0.000079 mg/l can be used to calculate a PNECsoil(screening) using the equilibrium partitioning approach.

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

For octaphenylcyclotetrasiloxane, this is:

PNECsoil(screening) = (3.0E+04/1700) * ≥7.9E-05 * 1000

           =  ≥1.39 mg/kg wwt

This is equivalent to ≥1.58 mg/kg dwt.

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.

A stability/recovery test under OECD TG 222 conditions performed with the structurally-related substance (L4) demonstrated significant loss of test item from the test system over a five-week period (37% remaining radioactivity after 35 days), ascribed to volatilisation losses. However, 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 the definitive OECD TG 222 study. Phenyl substituted siloxanes are expected to be more stable in soil, therefore an OECD TG 222 study is proposed for the analogous siloxane, Silsesquioxanes, Phenyl (CAS 70131-69-0). The need for further terrestrial toxicity tests (OECD TG 208) will be reviewed once the results of this study are available.

An OECD TG 216 toxicity to soil microorganisms study is also proposed for Silsesquioxanes, Phenyl (CAS 70131-69-0).

Until these data become available, there are no unacceptable risks for the soil compartment. PNECsoil has been derived based on read across from decamethylcyclopentasiloxane D5; the risk characterisation ratio (RCR) based on PNECsoil is <1.

Terrestrial toxicity data are read-across from decamethylcyclopentasiloxane (D5, CAS: 541-02-6), a siloxane with similar physico-chemical properties to the registered substance. The read-across is considered to be reliability 2 and the data are considered to be suitable for deriving an interim PNECsoil for the registered substance. The testing 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 IC50value 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 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 physico-chemical properties but are not close structural analogues (linear and cyclic siloxanes).

Read-across of the terrestrial toxicity data for D5 to Octaphenylcyclotetrasiloxane is considered to be suitable to derive an interim hazard and risk assessment under REACH.