1,1,1,3,5,5,5-heptamethyltrisiloxane

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.

Terrestrial studies with siloxanes such as 1,1,1,3,5,5,5-heptamethyltrisiloxane (H-L3, CAS 1873-88-7), hexamethyldisiloxane (HMDS, CAS 107-46-0) and octamethyltrisiloxane (L3, CAS 107-51-7) 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 substances for which the air/soil partition coefficient is greater than one, or to substance with vapour pressure (VP) exceeding 300 Pa at 25°C. HMDS and L3 meet both of these criteria (for HMDS, 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 H-L3 is 3.7 at 20°C and vapour pressure 850 Pa at 25°C, therefore the behaviour of this substance in the soil test systems is expected to be similar. Terrestrial toxicity testing with HMDS 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 (HMDS) 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.

H-L3 has a K_air-soil of 3.7, and a vapour pressure of 850 at 25°C. Based on the partitioning properties of H-L3 and the experimental findings for the source substances HMDS 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 H-L3 on the basis that the test substance is too volatile to maintain adequate concentrations in the test system.

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.