Registration Dossier

Ecotoxicological information

Endpoint summary

Administrative data

Description of key information

Additional information

No terrestrial toxicity data are available for the registration substanceoctamethylcyclotetrasiloxane(D4, CAS 556-67-2); 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 analogue group, it is concluded that further testing is required.

The registration substance, D4, falls within soil hazard category 4 as defined in REACH R.7; the registrants therefore propose to attempt terrestrial toxicity testing with the registration substance.

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.

Terrestrial toxicity testing with D4 in response to ECHA decision number TPE-D-2114422498-44-01/F has been delayed due to difficulties in obtaining radiolabelled test material. 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. Based on the air-soil partition coefficient, D4 was expected to be less stable in soil (K­air-soil= 0.39 and 0.22 at test temperature 20°C for D4 and L4 respectively).Therefore, stability/recovery testing under OECD TG 222 conditions was considered necessary with D4 prior to any terrestrial testing. A preliminary stability study has been conducted using non-radiolabelled material in order to minimise further delays and to ensure that sufficient material is available for the definitive studies. The study indicates that terrestrial testing is feasible, and therefore an OECD TG 222 study is contracted and will be carried out in accordance with ECHA decision number TPE-D-2114422498-44-01/F. 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 is also contracted. 

Interim approach to terrestrial hazard assessment

Until these data become available, PNECsoil has been calculated from PNECfreshwater on the basis of the equilibrium partitioning method (EPM), using aquatic toxicity data for D4. Effects were seen during the aquatic tests with D4, whereas no effects were seen in terrestrial tests with D5. Therefore, deriving the PNECsoil for D4 based on EPM represents a worst case, as soil toxicity data are available for D5 which indicate that EPM could be over-precautionary.