Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p2
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 2/5)
Character Range: 2958824–2961844

approach has been adopted (where data is available); however, for carcinogens that are non-genotoxic, a threshold approach has been adopted.

4.3              Toxicity approach for dermal exposure
Where specific dermal TRVs are available, these were used for the assessment of dermal contaminant toxicity; in their absence, oral TRVs have been used for the dermal hazard assessment. Oral TRVs almost invariably relate to applied dose rather than absorbed dose. Hence, where there is suitable chemical-specific data available, the TRV has been adjusted by a gastrointestinal absorption factor (GAF) to produce a reference value relating to absorbed dose (US EPA 2004b).

The equations applied in this adjustment (relevant on a chemical-specific basis) are outlined as follows:
Threshold TRVDermal = Threshold TRVOral x GAF
Non-threshold TRVDermal = Non-threshold TRVOral /GAF
where
TRVOral   = oral toxicity reference value
TRVDermal  = dermal toxicity reference value
GAF   = gastrointestinal absorption factor.

4.4              Background exposure and contribution of soil to total exposure
Background levels of contamination are the chemical concentrations present in the environment as a result of everyday activities (for example, emissions from motor vehicles, industry or efflux from the ground surface in the case of volatiles) or natural sources (for example, dissolution of mineral deposits). Chemicals present in food, air, water and consumer products all contribute to the quantity of the chemical that a person might be exposed to on a daily basis. The exposure from non-site sources is referred to throughout this document as 'background exposure'.

The threshold TRV is associated with a tolerable total intake from all sources, which includes food, air, water, consumer products and contamination sources. If it is known that a significant background exposure is likely to exist, then a proportion of the threshold TRV should be allocated to the background before comparing exposures derived from contamination in soil to the TRV.

This is only applied to threshold substances, because intakes of non-threshold contaminants are considered on the basis of an increase in risk, which is irrespective of background exposure (Health Canada 2004).

In the derivation of the HILs, this has been done on a chemical-specific basis by applying a factor to the threshold TRV, as outlined in the equations in Appendix B. Essentially this is calculated as follows for threshold contaminants:
TRV(adjusted) = (1 – Background) x TRV

The background concentration has been considered for each threshold chemical (refer to Appendix A) based on available data from Australia and, where limited data is available, from other countries. Where no data is available, an evaluation is undertaken on a chemical-specific basis with a default value for background exposure assumed where relevant.

It is possible for background exposure to be essentially negligible (contributing less than 5% of the threshold TRV) for chemicals