Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p3
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 3/5)
Character Range: 2961573–2964581

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 that are not widely distributed in the environment. In these cases, 100% of the threshold TRV has been allocated to exposure from soil. This assumption should be considered further where site-specific conditions suggest otherwise.

In addition, it is also possible for background exposure to exceed the threshold TRV (for example, intakes of methyl mercury from fish), in which case, theoretically an HIL cannot be derived. A few approaches are available to address this problem. In the UK, when background exposure comprises greater than 50% of the threshold TRV, then the background exposure is taken to be 50% of the TRV (EA 2008). New Zealand guidance (MfE 2011b) has considered the proportion allocated to exposure from soil on a case-by-case basis. In the derivation of the HILs, a case-by-case approach has also been adopted.

4.5              Bioavailability and bioaccessibility
Bioavailability and bioaccessibility are discussed and defined in Schedule B4.

Bioavailability (absolute) is the fraction or percentage of a compound which is ingested, inhaled or applied to the skin that actually is absorbed and reaches systemic circulation.

Relative bioavailability refers to the comparative bioavailability of different forms of a chemical or to different exposure media containing the chemical and is expressed as a fractional relative absorption factor.

Bioaccessibility is the fraction of a contaminant in an exposure medium that is soluble in the relevant physiological milieu (usually the gastrointestinal tract) and available for absorption.

Not all texts make an equivalent distinction between bioavailability (absolute and relative) and bioaccessibility, but in the assessment of contaminated soils it is a useful concept because it provides clarity on the modelling approach adopted in the derivation of the HILs.

Oral and inhalation TRVs are generally derived from direct administration of the chemical to an animal or human and as such they often intrinsically account for 'bioavailability' as defined above. TRVs represent tolerable 'uptake' or absorbed dose, which is different from total 'intake'. Uptake is the dose actually absorbed by the body; that is, the amount of the administered dose (or intake) that is bioavailable.

In risk assessments, the dermal pathway has a well-established mechanism for considering absorption and relative bioavailability. The lack of dermal-specific TRVs means that a dermal dose is often compared to the ingestion TRV. However the dermal dose represents an absorbed dose rather than applied dose (as is commonly the case in establishing ingestion TRVs). Hence it may be necessary to modify the ingestion TRV. This is commonly done by applying a gastrointestinal absorption