Summary of Public Comments received on Trisiloxane, octamethyl- (MDM) [Chemical Abstracts Service Registry Number (CAS RN) 107-51-7] Updated Draft Screening Assessment

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Comments on the updated draft Screening Assessment Report for MDM to be addressed as part of the Chemicals Management Plan Challenge Batch 12

A summary of comments and responses is included below, organized by topic:

Clearly articulate to the international community criteria for persistence, bioaccumulation, and inherent toxicity (PBiT) as hazard characteristics that are metrics to be considered in the context of exposure.The characteristics of PBiT are relevant lines of evidence in determining if a substance meets the criteria under section 64 of the Canadian Environmental Protection Act 1999 (CEPA 1999).  The consideration of greater potential for harm from substances demonstrating high hazard aligns with approaches used by regulators internationally.
There is a need to enhance international dialogue regarding weight-of-evidence in assessing environmental risks by considering changing scientific perceptions of PBiT criteria. Specifically when assessing bioaccumulation, the increase in concentration of a substance in one organism does not constitute bioaccumulation.Accumulation in a single species may still lead to internal toxicity thresholds being exceeded. Accordingly, bioconcentration or bioaccumulation in an organism, which represents a particular trophic level, would still be relevant in the evaluation of chemical fate, bioaccumulation, critical body residues and ultimately as a line of evidence in the determination of risk posed by the substance in the environment. 
More detail is requested to support the risk assessment conclusion that MDM is not CEPA toxic [(a) or (b)]. In particular, more clarification is requested on how evidence of bioaccumulation was used in the weight-of-evidence approach to support the conclusion.The lines of evidence used to conclude that MDM is not toxic under paragraphs 64(a) or (b) of CEPA 1999 and their significance are discussed in the “Characterization of Ecological Risk” section of the draft Screening Assessment. Factors considered include import and use patterns, environmental releases and distribution, potential for environmental persistence, bioaccumulation potential, ecological toxicity and hazard potential (including intrinsic properties in many of these), environmental monitoring results and the results of quantitative risk quotient analyses for MDM. Also, the section titled “Potential for Bioaccumulation” provides the individual lines of evidence (including specific bioaccumulation metrics) that were used to establish the lack of biomagnification potential and significant bioaccumulation potential for MDM.

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