Hexamethyldisiloxane

Toxicity to soil microorganisms

Description of key information

Toxicity to soil micro-organisms: Hexamethyldisiloxane could not be maintained in the test system (refer to the discussion in Additional Information); no dose-response relationship was apparent. (OECD TG 216).

Additional information

A 28 day EC₅₀ value of >8.5 mg/kg soil dw and an EC₁₀ value of >8.5 mg/kg soil dw (highest nominal concentrations tested) have been reported in an OECD Guideline 216 (Soil Microorganisms: Nitrogen Transformation Test) for the effects of hexamethyldisiloxane (HMDS) on nitrate formation rate of soil microflora.

The test indicates that there were no inhibitory effects on the soil micro-organisms. However, analysis of the test substance concentrations show that test material was lost by day three of the test, with all concentrations below the limit of detection, therefore the micro-organisms would have been exposed to unquantifiable, minimal amounts of the test substance. No dose-response relationship was apparent, and in view of the <LOQ recoveries of test material, it is not appropriate to ascribe the absence of effects to a lack of toxicity of HMDS. The results of this test cannot be used for derivation of PNECs and risk assessment.

Hexamethyldisiloxane has a very high Henry's Law Constant and losses through volatilisation during test media preparation and within the test system were anticipated. Because of this, measures were taken during the soil micro-organism test to minimise test substance losses.

An initial test was conducted using a closed bottle system. During this test, nitrate levels decreased significantly in all concentrations, including the control. The report authors explained that this was most likely due to the test vessels being sealed air tight, thus not supplying enough oxygen to allow the nitrifying bacteria to convert the alfalfa nitrogen source to NO₃. Thus the study was repeated under a separate protocol, using a different test design.

During the definitive test, the following measures were taken to prevent loss of test item, while allowing for air exchange:

-The test vessels were sealed with perforated parafilm, which allowed for air exchange and avoided loss in moisture. 

- All test substance concentrations were dosed using gastight Hamilton syringes in multiple spots underneath the soil.

- Soil was dosed at ~30% above the calculated maximum sorption concentration to allow for some losses through volatility.

Despite these measures, significant loss of test material was observed by day 3. It may therefore be concluded that the soil micro-organisms were not exposed to adequate concentrations of test substance, and the EC₅₀ and EC₁₀ values determined are not representative of actual exposure of the test organisms to the test substance. It is therefore not possible to use these test results for calculation of terrestrial PNECs and risk assessment.