Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:6:p8
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 6 (pt 8/12)
Character Range: 1071350–1074202

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.

This HQ approach is recommended as a screening tool for most Tier 2 risk assessments. It is particularly useful for the assessment of petroleum hydrocarbon mixtures, and other mixtures where the assumption that substances are likely to have similar toxicological effects can be justified.

Where the HI for the mixture is less than unity (HI<1), a conclusion that exposure is likely to be within acceptable bounds may be made. When HI>1, it does not necessarily indicate that a site poses an unacceptable risk but it does indicate that either further consideration should be applied, or risk management actions should be recommended. An HI >1 does not necessarily indicate unacceptable risk.

Further consideration would normally include assessment of the modes of action that the mixture components might exhibit. This Schedule provides guidance on how to apply the HQ approach.

Summation of non-threshold risk estimates (ICLRs) applies similar logic to the HQ approach. It is common to assess risk from genotoxic carcinogens in terms of a numerical estimate of increased probability of developing cancer over a lifetime. It is possible to sum these risks from components of a mixture to develop a combined estimate of total risk. The approach suffers from exactly the same limitation as the HQ approach, in that it assumes a similar mode of action or end-point, which may be unjustified.

This approach is recommended as a screening approach for all genotoxic carcinogens in a mixture. Where the sum exceeds the acceptable risk, the components should be assessed in more detail to look at whether the mode of action or end point justifies summing the risk. Where modes of action and/or end-points are clearly different, carcinogens should not be summed.

Assessment of representative mixtures by direct in vivo or in vitro experiments. This approach is clearly limited by the number of variations in relative concentration that could be tested, and also by other complexities of environmental contamination such as weathering and degradation. It is not likely that this approach will be widely applicable to contaminated land health risk assessment.

Toxicity equivalence factor (TEF) approach in which the potential effects of a group of similar substances are estimated relative to a single member of the group. The components of the mixture are 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