Octamethylcyclotetrasiloxane

Endpoint summary

Additional information

Reliable terrestrial toxicity studies with the registered substance, octamethylcyclotetrasiloxane (D4, CAS 556-67-2, EC No. 209-136-7), for toxicity to soil organisms except arthropods (OECD TG 222) and toxicity to soil microorganisms (OECD TG 216), have been conducted in accordance with ECHA decision number TPE-D-2114422498-44-01/F.

 

The key values for chemical safety assessment are:

OECD TG 222 Earthworm reproduction test:

28-day EC₅₀ >130 mg/kg dw (mean measured concentration), survival and weight of Eisenia fetida;

28-day NOEC ≥130 mg/kg dw (mean measured concentration), survival and weight of Eisenia fetida;

56-day NOEC 75 mg/kg dw (mean measured concentration), reproduction of Eisenia fetida, normalised to 2% organic carbon = NOEC 42 mg/kg dw.

These results are based on mean measured test substance concentrations because significant test substance losses occurred and concentrations were not maintained within 80% of nominal concentrations.

 

OECD TG 216 Soil nitrogen transformation test:

28-day EC₅₀ >100 mg/kg soil dry weight;

28-day EC₁₀ >100 mg/kg soil dry weight;

28-day NOEC ≥100 mg/kg soil dry weight, for the effects of D4 on nitrogen formation rate of soil microflora.

These results are based on nominal test substance concentrations. Although analytical verification of test substance concentrations was not carried out during this test, test concentrations have been measured in preliminary stability studies conducted with the registered substance, where similar methods for application of the test substance to soil were used. The measured concentrations indicated that the substance is not stable in soil throughout the test duration and test substance losses are expected. The test organisms are therefore likely to have been exposed to concentrations of the test substance that are not within 80% of the nominal concentrations. 

 

Octamethylcyclotetrasiloxane (D4, CAS 556-67-2) falls under soil Hazard Category 3 (indication for high adsorption based on log K_ow >3, but no indication of aquatic toxicity (no effects in aquatic studies)). For Hazard Category 3 substances, a confirmatory long-term study is required: the above reported OECD TG 222 study was conducted. ECHA guidance Chapter R.7c (ECHA 2017) states the following: “If PEC/PNECscreen >1 or indication of risk from confirmatory long-term soil toxicity test: Conduct long-term toxicity tests according to the standard information requirements Annex X (invertebrates and plants), choose lowest value for derivation of PNECsoil”.

In accordance with the ECHA guidance and with ECHA decision number TPE-D-2114422498-44-01/F, because effects were observed in the OECD TG 222 earthworm study as well as the soil microorganisms study, a long-term toxicity to terrestrial plants study is required. This study was requested as part of the ECHA decision, but the requirement for the study was dependent on the outcome of the earthworm study. Due to unavoidable delays with the earthworm study, the plant study has subsequently been delayed. Now that the earthworm study is complete, the long-term toxicity to terrestrial plants study in accordance with OECD TG 208 has been contracted and a timeline for completion of the study is included in IUCLID Section 6.3.3 Toxicity to Terrestrial Plants endpoint study record.