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Report of the Board of Review for Decamethylcyclopentasiloxane (Siloxane D5)

8 Strengths and Uncertainties

297. Risk assessments will, by their very nature, contain some measure of uncertainty. New data provided by Environment Canada and the SEHSC/CCTFA have enabled the Board to conduct a more refined assessment.

298. The probabilistic nature of risk is such that absolute certainty of safety or of danger is not possible. However, the amount of information, scientific and other, now available for Siloxane D5 adds considerable strength to the assessment of risks posed to the environment. The additional data on toxicity and on concentrations in the environment have similarly allowed for better characterisation of these two critical parameters to the assessment of risk.

299. That said, the Board acknowledges that some uncertainties remain. Although the physical and chemical properties of Siloxane D5 strongly suggest that its only mechanism of toxicity is via general narcosis and physical interference with membranes, it could interact with unknown receptors or transport proteins or other chemicals present in the environment (mixture effect). The lack of effect in mammals and all other organisms, even at high doses or concentrations above the threshold for solubility, supports a conclusion that narcosis is the mechanism of action for Siloxane D5. This might not be true for classes of organisms other than those tested, but the Board considers this to be unlikely considering the similarities between species with respect to membranes, structures, and receptors.

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