Octamethyltrisiloxane

Endpoint summary

Additional information

Testing for toxicity to terrestrial organisms has been carried out with soil micro-organisms. A stability/recovery test under OECD 222 (earthworm reproduction test) conditions has also been carried out.

Terrestrial studies with siloxanes such as L3 are considered to be technically difficult to conduct due to their high volatilisation potential (as indicated by high Henry’s Law Constant and low octanol-air partition coefficient) and the potential for degradation in soil. 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 28 day test of the effects of octamethyltrisiloxane on nitrate formation rate of soil microflora has been conducted in accordance with OECD TG 216. However, the substance could not be maintained in the test system even during the substrate preparation phase prior to the start of the test, as demonstrated by the analytical evidence: measurements at day 0 in all concentrations were below the Limit of Quantification (LOQ). LOQ ranged from 0.198 to 0.202 mg/kg, and measured concentrations remained <LOQ throughout the rest of the test. The soil micro-organisms, therefore, would not have been exposed to the test item for long enough or at high enough concentrations to assess its toxicity. Statistically significant effects (positive, non-inhibitory) on nitrate concentration and nitrate transformation rate were observed in this study: replicates at equivalent test concentration showed good repeatability. No dose-response relationship was apparent, and in view of the <LOQ recoveries of test material, it is not appropriate to ascribe the observed effects to L3. The results of this test cannot be used for derivation of PNECs and risk assessment.

The stability/recovery test under OECD 222 conditions also demonstrated significant loss of test item from the test system, ascribed to volatilisation losses. Based on these experimental findings, the registrants believe that it is not technically feasible to proceed with the definitive OECD 222 test on the basis that the test substance is too volatile to maintain adequate concentrations in the test system. The proposed terrestrial plant test (OECD 208) is also considered to be not technically feasible on this basis.

Terrestrial testing will be waived as maintaining the volatile test substance in the system is not feasible. This is based on experience with the soil stability and terrestrial tests conducted with L2 and L3, and using a threshold K _air-soil >1 in order to determine the feasibility of testing.

Due to absence of chronic or long-term effects in aquatic organisms up to the substance solubility limit, no aquatic PNEC has been derived. Consequently, the Equilibrium Partitioning Method (EPM) is not applicable.

Because an aquatic PNEC has not been derived and the substance cannot be maintained in the terrestrial environment, no terrestrial hazard is identified and a PNECsoil has not been derived.