1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane

Endpoint summary

Additional information

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. For low molecular weight, volatile siloxanes, it is proposed to waive terrestrial toxicity testing due to the test being technically not feasible.

A 7 day LC₅₀value of >1000 mg/kg has been determined for the effects of1,1,1,3,5,5,5 -Heptamethyl-3-[(trimethylsilyl) oxy]trisiloxane (M3T; CAS 17928 -28 -8)on the mortality of the earthworm E. foetida. It cannot thus be excluded that most of the test material evaporated out of the test media by day 7 when the soil was inspected for earthworms survival, therefore a reliability cannot be assigned to the study.

Terrestrial studies with siloxanes such as M3T, L2 and L3 are 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. OECD TG 222 acknowledges that the test method may not be applicable to substance for which the air/soil partition coefficient is greater than one, or to substance with vapour pressure exceeding 300 Pa at 25°C. L2 and L3 meet both of these criteria (for L2, K_air-soil=3.3 at 20°C (temperature for terrestrial testing), VP 5500 Pa at 25°C; for L3, K_air-soil=1.3 at 20°C, VP 530 Pa at 25°C). The air/soil partition coefficient for M3T is 1.3 at 20°C and vapour pressure 210 Pa at 25°C, therefore the behaviour of this substance in the soil test systems is expected to be similar. Terrestrial toxicity testing with L2 and L3 have shown that maintaining adequate concentrations in the test system is not feasible for these compounds.

A 28-day test of the effects of the source substance hexamethyldisiloxane (L2) 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, as demonstrated by the analytical evidence: measurements in all concentrations were below the Limit of Quantification (LOQ) by day 3 of the test.

A stability/recovery test under OECD TG 222 conditions with the structurally related test substance octamethyltrisiloxane (L3) demonstrated significant loss of test item from the test system, ascribed to volatilisation losses. The definitive test was therefore not carried out. A nitrogen transformation test (OECD TG 216) carried out with L3 also demonstrated significant loss of test material.

M3T has a K_air-soil of 1.3, and a vapour pressure of 210 at 25°C. Whilst the vapour pressure falls below the threshold identified in the OECD TG 222, based on the partitioning properties of M3T and the experimental findings for the source substances L2 and L3, the registrants believe that it is not technically feasible to conduct terrestrial testing (OECD TG 222, OECD TG 216 and OECD TG 208) for M3T on the basis that the test substance is too volatile to maintain adequate concentrations in the test system.

 

PNEC_soil for M3T has therefore been calculated from PNECfreshwater on the basis of the equilibrium partitioning method; the risk characterisation ratios (RCR) based on PNECsoil are <1.