Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:6:p9
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 6 (pt 9/12)
Character Range: 1073953–1076954

assumed to contribute to the toxicity in a similar way, and their relative effect is calculated in proportion to their concentration in the mixture by adjustment using a relative potency factor. The approach has limitations in that the assumption of a similar mode of action may be unjustified in a variety of ways. The key limitation in a practical sense is that it is only available where a well-established set of relative potency factors exists for the components of the mixture. Currently its use is limited to carcinogenic PAHs, dioxins, PCBs and some endocrine-disrupting chemicals. This approach is recommended for carcinogenic PAHs, and guidance on how to apply it is provided in the toxicity profile for benzo(a)pyrene in Schedule B7 Appendix A2.

Component elimination or simplification approach may be used in circumstances where it is not reasonable to assume a common mode of action for components in a mixture. In this case, the components are assumed to be toxicologically independent. The outcome of this approach is to assume that the overall risk is no greater than the risk posed by the riskiest component of the mixture. This method should be applied where it can be shown that adding HQs or ILCRs is not reasonable.

6.6              Uncertainty and sensitivity analysis

    6.6.1          Uncertainty analysis
Uncertainty in health risk assessment is the lack of knowledge about the correct value such as a specific exposure measure or estimate (enHealth 2012a). Uncertainty is distinguished from variability, which refers to true differences in attributes due to diversity or heterogeneity. Variability cannot be reduced by further measurement or study, although it can be better characterised (NRC 2008).

Both uncertainty and variability contribute to uncertainty in the estimation of risk and should be adequately assessed in a risk assessment. Such consideration needs to be done transparently so that all users of a risk assessment can understand the approach taken.

An analysis of the uncertainty in the risk assessment is important because:
    * Information from different sources carries different kinds of uncertainty and knowledge of these differences is important when uncertainties are combined for characterising risk.
    * The risk assessment process, with management input, involves decisions regarding the collection of additional data (versus living with uncertainty). In the risk characterisation, a discussion of the uncertainties will help to identify where additional information/data could contribute significantly to reducing uncertainties in risk assessment.
    * A clear and explicit statement of the strengths and limitations of a risk assessment requires a clear and explicit statement of related uncertainties (US EPA 1995c).
    * Characterising uncertainty in risk informs the stakeholders about the range of possible risks from an exposure. Risk estimates may sometimes diverge widely (NRC 2008).
    * Characterising the uncertainty in risk