Registration Dossier

Ecotoxicological information

Endpoint summary

Administrative data

Description of key information

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 substances with high potential to volatilise during terrestrial testing, it is proposed to waive terrestrial toxicity testing due to the test being technically not feasible.

Terrestrial studies with siloxanes such as Vi4D4, 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, Kair-soil=3.3 at 20°C (temperature for terrestrial testing), VP 5500 Pa at 25°C; for L3, Kair-soil=1.3 at 20°C, VP 530 Pa at 25°C). The air/soil partition coefficient for Vi4D4 is 2.2 at 20°C. Although the vapour pressure for this substance does not exceed the threshold (93.5 Pa at 25°C), the substance is poorly water soluble (7.3E-03 – 8.8E-03 (at 23°C) mg/l). Therefore, the behaviour of this substance in the soil test systems is expected to be similar to L2 and L3, i.e. losses due to volatilisation would be expected. 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.

Vi4D4 has a Kair-soilof 2.2. Based on the partitioning properties of Vi4D4 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 Vi4D4 on the basis that the test substance is too volatile to maintain adequate concentrations in the test system.

PNECsoilfor Vi4D4 has therefore been calculated from PNECfreshwateron the basis of the equilibrium partitioning method; the risk characterisation ratios (RCR) based on PNECsoilare <1.