Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:6:p2
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 6 (pt 2/4)
Character Range: 3022006–3025087

Uncertainty has the potential to result in a cumulative overestimation or underestimation of potential health risks during an assessment.

The three broad types of uncertainty inherent in any risk assessment are:
    * Scenario uncertainty — uncertainty arising from missing or incomplete information such as descriptive errors, aggregation errors, errors in professional judgement and incomplete analysis
    * Parameter uncertainty — uncertainty affecting a particular parameter such as measurement errors, sampling errors, variability, and use of generic or surrogate data
    * Model uncertainty — uncertainties in scientific theory affecting the ability of a model to make predictions.
Scenario uncertainty in the HIL assessment is largely not applicable, since the circumstances of the assessment are hypothetical.

There is considerable parameter uncertainty in the HIL assessment. Parameter uncertainty is usually dealt with by sensitivity analysis (see below); however, because of the generic purpose of the HILs, many of the parameters for tier 1 assessment are set, for example by enHealth (2012a, 2012b), and sensitivity analysis for these was not considered appropriate. The approach used to address parameter uncertainty during the derivation of the HIL values was the use of conservative or reasonable high-end exposure assumptions, allowing them to be applied across the majority of Australian sites.

Assumptions applied during the development of the HILs include:
    * the use of human physical and behavioural characteristics outlined by enHealth (2012a, 2012b) as estimates for the Australian population
    * the use of vapour attenuation factors that are not site-specific and do not account for potential degradation of either the contaminant source or migrating vapours
    * the use of vegetable uptake models identified by EA (2009e) as being likely to overestimate potential chemical uptake by vegetable and fruit crops
    * the use of toxicity criteria that are established by authoritative Australian and international public health bodies, and which are intended to be used for derivation of health protective guidelines.
The models used to estimate exposure are inherently uncertain, and are not necessarily able to accurately predict actual exposure. The soil and dust ingestion, vapour inhalation, and dermal pathway approaches/models are well established and have been in use in international risk assessment for many years. Although quantitatively they may not be very good absolute predictors of exposure, they are very simple and primarily dependent on the exposure settings and toxicity criteria.

The pathways most subject to model uncertainty were the concentration of contaminants in vegetables and fruit, and the prediction of airborne dust concentrations. Uncertainty analysis was carried out by evaluating the pathways driving the HIL values (that is, percentage contributed by each) and assessing the likely reality of the proportions of exposure from each pathway. In the case of both the vegetable uptake and airborne dust pathway a