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 2/8)
Character Range: 1034967–1037995

a single mechanism to express risk.

Currently, the threshold and non-threshold approaches still dominate risk assessment practice, and the TRVs associated with each are described below.

    5.3.2          Threshold toxicity reference values
The US EPA's IRIS database describes the toxicity criteria for 'threshold' substances as follows:

'Oral reference doses and inhalation reference concentrations (RfDs and RfCs, respectively) for effects known or assumed to be produced through a nonlinear (possibly threshold) mode of action. In most instances, RfDs and RfCs are developed for the non-carcinogenic effects of substances.'

Where a carcinogen is assessed on the basis of a threshold doseresponse relationship, the threshold TRVs adopted need to consider both carcinogenic and non-carcinogenic end-points.

For threshold chemicals, TRVs reflect a measure of tolerable daily exposure. The threshold is typically selected from a point of departure in the dose-response curve below which adverse effects are not observed (NOAEL) and includes a range of safety (or uncertainty) factors. There are a number of terms that are used by different agencies to define a threshold TRV. Most commonly these include an ADI (acceptable daily intake), TDI (tolerable daily intake), TC (tolerable concentration in air), RfD (reference dose), RfC (reference concentration), MRL (minimal risk level) and REL (reference exposure level), as noted in Table 6.

In contaminated land health risk assessment, the estimated exposures from oral (ingestion), dermal and inhalation routes are compared to the corresponding TRV. This process is described in the risk characterisation section of this Schedule.

    5.3.3          Cancer toxicity reference values
The US EPA's IRIS database describes the TRVs for 'non-threshold' substances that it presents as follows:

'Descriptors that characterise the weight of evidence for human carcinogenicity, oral slope factors, and oral and inhalation unit risks for carcinogenic effects. Where a nonlinear mode of action is established, RfD and RfC values may be used.'

For non-threshold chemicals, TRVs reflect a cancer risk value commonly referred to as a cancer slope factor (CSF) or unit risk factor (URF).
Table 6. Threshold toxicity reference values
Toxicity criterion           Units      Meaning
Acceptable daily intake ADI  mg/kg-day  The daily intake of a substance that, during a lifetime, appears to be without appreciable risk, on the basis of all the facts known at the time (WHO 1994). The term ADI is generally used for chemicals such as pesticides that are deliberately used on food or crops. ADI is very similar conceptually to RfD; the different terminology arises from the measure having been defined by a different body.
Tolerable daily intake TDI   mg/kg-day  An estimate of the intake of a substance that can occur over a lifetime without appreciable health risk (WHO 1994). TDI is generally used when a chemical is a food or environmental contaminant. Like ADI, TDI is conceptually