Hexamethyldisiloxane

Endpoint summary

Additional information

Terrestrial studies with siloxanes such as HMDS 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. 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 substances with vapour pressure exceeding 300 Pa at 25°C. Terrestrial toxicity testing with HMDS and L3 has shown this to be the case (see discussion in IUCLID Section 6.3.1). HMDS meets both of these criteria (Kair-soil = 2.8, VP 5500 Pa); for L3, the vapour pressure criteria is met, and the air/soil partition coefficient is close to one (Kair-soil = 0.9, VP 530 Pa).

A 28-day test of the effects of the registered substance, hexamethyldisiloxane (HMDS, L2, CAS 107-46-0), 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.

Additionally, testing for toxicity to other terrestrial organisms (earthworm reproduction test, OECD TG 222; terrestrial plant test, OECD TG 208) was also requested by ECHA for HMDS. However, a stability/recovery test under OECD TG 222 conditions with the related test substance octamethyltrisiloxane (L3, CAS 107-51-7; EC 203-497-4) 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.

The registration substance, hexamethyldisiloxane (HMDS, L2, CAS 107-46-0), and the source substance, octamethyltrisiloxane (L3, CAS 107-51-7), are members of the Siloxane Category and are structurally similar substances. Please refer to Section 6 of IUCLID and Section 7.0 of the CSR for further discussion and justification of read-across.

The physico-chemical properties of L3 are reasonably similar to those of HMDS, but the former should have greater stability in soil: HMDS has a higher tendency to volatilise from soil compared to L3, based on its higher vapour pressure (5500 Pa versus 530 Pa at 25°C) and lower tendency to partition to organic matter (log K_oc 3.0 versus 4.3) than L3. HMDS also has a faster homogeneous hydrolysis rate (t_½ 116 h for HMDS versus 329 h for L3, at pH 7 and 25°C), and so is expected to degrade faster than L3 in soil. It is also well established that siloxanes undergo clay-catalysed hydrolysis in soil (Xu et al., 1998 and Xu, 1998), with half-lives increasing with increasing molecular size of the siloxane (Xu and Chandra, 1999).

Based on these experimental findings, the registrants believe that it is not technically feasible to proceed with the OECD TG 222 and OECD TG 208 test for HMDS on the basis that the test substance is too volatile to maintain adequate concentrations in the test system.

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