Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:5:p2
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 5 (pt 2/18)
Character Range: 2971086–2973949

young children, the average body weight for a child aged 23 years of 15 kg from enHealth (2012a) has been adopted.
Adults have been assessed on the basis of the average lifetime adult body weight of 70 kg from enHealth (2012a). This body weight is lower than the average adult body weight of 78 kg from enHealth (2012a) and has been selected for the purpose of deriving HILs as it better reflects the larger age range considered for adults in the derivation of HILs (all ages older than a young child aged 05 years) and is consistent with the body weight adopted in the derivation of the Australian Drinking Water Guidelines (NHMRC 2011) and other screening level guidelines developed in New Zealand, the USA, Canada and the Netherlands.

The World Health Organization drinking water guidelines are based on an average adult body weight of 60 kg (WHO 2011), but they are designed to be applicable worldwide and to cater for countries where average body weight would be much lower than that in Australia.

    5.2.2         Exposure duration and frequency
Child exposure duration has been set at 6 years for all land use scenarios, based on the critical child receptor characterised on the basis of exposure parameters for the more sensitive ages of 23 years, but expected to be representative of exposures over all ages from 05 years. Adult residential and recreational exposure duration has been set at 29 years, reflecting total residential exposure duration (child plus adult) of 35 years based on the 95th percentile from enHealth (2012a). An exposure duration of 30 years has been applied for adult commercial receptors as per enHealth (2004).

The exposure frequency applied in the residential and open space scenarios is 365 days/year (enHealth 2012a). This reflects the assumption that exposed populations are potentially using the contaminated site daily; this is a necessary assumption for residential scenarios, but is a worst-case assumption for the recreational scenario. The exposure frequency applied in the commercial/industrial land use scenario is 240 days/year; this value assumes a 5-day working week for 48 weeks/year.

    5.2.3         Averaging time
The averaging time selected depends on the type of toxic effect being assessed. The distinction between the approach for threshold and non-threshold compounds relates to the currently held scientific opinion that the mechanism of action differs for these groups (US EPA 1989).

When evaluating chronic exposures (as is the case in the derivation of HILs) to threshold toxicants, intakes are typically calculated by averaging intakes over the period of exposure (essentially the exposure duration multiplied by 365 days in a year). It is noted that the exposure duration cancels out in the exposure equations for threshold compounds.

For non-threshold toxicants, intakes are calculated