Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:6:p7
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 6 (pt 7/12)
Character Range: 1068570–1071609

does not base policy specifically on them. 1 in 100,000 is the risk level used when the UK judges that a linear low dose extrapolation is the most appropriate basis for its Index Dose.
Canada          1:100,000                                             Health Canada (2004)                                                                            This value was recommended given the conservative margin associated with slope factors and the negligible impact of a 1:100,000 incremental risk level for contaminated site exposures.

When using non-threshold TRV as doseresponse criteria, the recommended acceptable incremental lifetime risk of developing cancer arising from exposure to single or multiple carcinogens is 1 in 100,000 (10-5) consistent with enHealth (2012a).

6.5              Risk evaluation of mixtures
Contaminated land studies frequently involve assessing the health risks associated with soil where a number of different chemical contaminants are present. In contrast, toxicological studies usually assess the effects of a single chemical. The risk assessor faces a difficulty in determining whether the effects of the mixture might be additive, greater than additive (synergistic) or less (antagonistic) (Priestly 2009). It is possible that such effects are not important at the low doses common in environmental exposure, leading to the concept of an 'interaction threshold' below which the effects of mixtures are insignificant (Hamm et al. 2005). Additive effects are being found to be more common than synergism.

Despite the limitations in the data, risk assessors have been incorporating additivity into their assessments for some time. This is done by adding all risks from all chemicals together to get total risk for a site and also by adding risks from all pathways. There are likely to be situations where the chemicals cause quite different effects by quite different mechanisms and so it is possible that summing risks in this way can overestimate risks in such situations. However, most sites have a mix of chemicals that are similar because they have arisen due to the particular activities on a site.

Priestly (2009) has reviewed all the available approaches to the risk evaluation of mixtures including summing of risk quotients as already described, which are summarised below:
Hazard quotient approach uses the ratio of the estimated exposure to the measure of acceptable exposure (the hazard quotient) for each component of the mixture and adds them to produce a hazard index which is the expression of the likely acceptability of the mix. This approach is widely used in Australian risk assessment. It is recommended for the assessment of petroleum hydrocarbons by the England and Wales Environment Agency (EA 2005).

The approach suffers from a fundamental limitation, which is the inherent assumption that the components summed have a common mode of action, or at least a common end-point. Where this is not the case, the components are theoretically toxicologically independent.

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