ABSTRACT
In Australia, the most significant pathway for human exposure to arsenic (As) is via soil ingestion. Generally, soil ingestion results from incidental ingestion which is most significant for children under the age of five due to the prevalence of handto-mouth contact (Basta et al. 2001; USEPA 2007). However, following incidental ingestion, the dose of As that is able to exert an effect is dependent on its bioavailability in the soil (Ruby et al. 1996; Rodriguez et al. 1999; Juhasz et al. 2007). When defining bioavailability in terms of human receptors, the bioavailable fraction is the fraction of a compound that is absorbed into systemic circulation. Relative bioavailability may also be calculated which refers to comparative bioavailabilities of different forms of a chemical or for different exposure media containing the chemical (NRC 2003). Bioaccessibility is another related term which refers to the fraction of a compound that is soluble in the gastrointestinal tract and is therefore available for absorption (Ruby et al. 1996). Bioaccessibility is specifically referred to when in vitro assessment models are used. The need to evaluate contaminant bioavailability arises from the fact that As species vary in their solubility and capacity to sorb to soil constituents, which may influence their uptake by receptor organisms. As a result, the assumption that all soil-borne As is bioavailable may overes-
2 TECHNIQUES FOR ASSESSING/ PREDICTING ARSENIC RELATIVE BIOAVAILABILITY
2.1 In vivo assays
Young swine are considered to be a good physiological model for the gastrointestinal absorption of contaminants in children. An outline of advantages of utilizing swine for contaminant bioavailability assessment is provided in Weis and LaVelle (1991). Following soil dosing, As relative bioavailability may be quantified by the determination of As urinary excretion or changes in As concentration in the blood (area under the curve dose response) compared to sodium arsenate reference doses (Rodriguez et al. 1999; Rees et al. 2009). Monitoring urinary excretion is a convenient endpoint for the assessment of As relative bioavailability, however, it may underestimate absorption as some absorbed As may be excreted in the faeces (via the bile) or enter tissue compartments (e.g. skin, hair) from which it is cleared slowly or not at all. Blood analysis overcomes these issues, however, the procedure is more invasive for the animal (i.e. a minor surgical procedure is required for the insertion of jugular catheters; see Rees et al. 2009) and analytical costs are higher due to the number data points required to generate area under the curve dose responses (e.g. Fig. 1).
