Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:5:p14
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 5 (pt 14/18)
Character Range: 3008187–3011106

have been adopted.
    * The uptake component models the process by which the lead intake is transferred to blood plasma. The amount of lead that is taken up is controlled by the bioavailability of the lead, which can be specified separately for soil, water and food.
    * The biokinetic component models the balance of lead in the body between uptake and excretion. A central estimate of blood lead concentration is output from this component.
    * The variability component applies a log-normal distribution to the output of the biokinetic component using a geometric standard deviation of 1.6. This value is based on empirical studies where blood lead concentrations of young children and environmental lead concentrations were measured. It models the predicted variability likely to apply to the population.
The model contains 100 variables, of which 46 can be modified by the user. Those which cannot be modified are based on considerable research, and are detailed in the model user guide (US EPA 2007b). In calculating the HILs for lead, input variables consistent with those used for the other HILs have been applied. A full list of variables input to the model is provided in Appendix D, including important variables where the model defaults were retained.

    5.4.3         Bioavailability and bioaccessibility of lead
Lead is the only substance for which adequate data is available to support an estimate of bioavailability. Because the toxicity criterion in lead modelling is a blood lead value, it represents an absorbed quantity and estimation of both bioavailability and bioaccessibility is appropriate. A single factor, labelled 'bioavailability' is used to represent both concepts.

US EPA (2007a) recommends use of 30% oral bioavailability of lead in soil for children, and 12% bioavailability of lead in soil for adults. There is also data available from Australian sites, which indicate that an oral bioavailability of at least 45% is likely (David Simon, South Australia Health, pers comm.).

Following review of the available data, an oral bioavailability of 50% (based on a review of data presented by IARC (2006)) was used in the models used for the derivation of an HIL associated with exposures to lead.

5.5              Vapour assessment

    5.5.1         Introduction
The inhalation of vapours in the indoor and outdoor environment is an important exposure pathway for volatile and semi-volatile soil contaminants. The approach adopted for the derivation of HILs has used an empirical approach using an attenuation factor, rather than a model, for estimating concentrations indoors and outdoors from soil vapours. There are a number of limitations and uncertainties associated with the use of any model in the estimation of exposure concentrations. In particular, the methodology and uncertainties associated with vapour modelling from a soil source are not fully resolved.

Hence, at