Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:13:p2
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 13 (pt 2/9)
Character Range: 2312728–2315635

Imray & Neville (1996). In summary, the HIL was derived on the basis of the following:
    * Based on data from the US, background intakes for adults were estimated to be 0.23 mg/kg/day, 77% of the adopted threshold TDI of 0.3 mg/kg/day.
    * A tolerable daily intake (TDI) of 0.3 mg/kg/day, referenced from ATSDR, relevant for soluble zinc salts was adopted. It was noted that insoluble salts and metallic zinc may have a lower bioavaiability.
    * Dermal absorption was considered to be negligible.
    * An HIL was derived on the basis of soil ingestion by a 2-year-old child, assuming 100% bioavailability.

13.3          Significance of Exposure Pathways
Ingestion of soil and dust is considered the most significant pathway of exposure for inorganics in soil. The consideration of bioavailability and inclusion of other exposure pathways in the derivation of a soil HIL has been further reviewed, as noted below.

    13.3.1     Oral Bioavailability
Insufficient data isavailable to adequately define the bioavailability of zinc from soil as it will be dependent on the form present. On this basis a default approach of assuming 100% oral (and inhalation) bioavailability has been adopted in the derivation of an HIL.

The toxicokinetic properties of ingested zinc have been extensively studied. The bioavailability of zinc from specific foods ranges from 1040%. Absorption from the gastrointestinal tract is homeostatically controlled. Under normal physiological conditions 2030% of ingested zinc is absorbed (Imray & Neville 1996). It is expected that bioavailability from soil will depend on the form present and may be considered further in a site-specific assessment.

    13.3.2     Dermal absorption
Imray & Neville (1996) note that dermal absorption of zinc occurs, but the mechanism is undefined and studies are limited. Certain zinc compounds (such as acetate and chloride) are skin irritants, though zinc oxide is a common constituent in many topical skin creams, such as sunscreen/block. Data presented by WHO (2001) shows some (low) dermal absorption of zinc in animal studies. Based on the limited data available, it is reasonable to consider that dermal absorption may be more than negligible. Limited data is available regarding the dermal absorption of zinc from soil and hence a default value of 0.1% has been considered. The default value of 0.1% is the lower end of the range considered relevant for metals, as presented by US EPA (1995).

    13.3.3     Inhalation of Dust
Zinc is not volatile and inhalation exposures associated with particulates outdoors and indoors are expected to be of less significance than ingestion of soil. While likely to be negligible, potential inhalation exposures associated with dust have been considered in the HIL derived.

    13.3.4     Plant Uptake
An important aspect of the potential for plant uptake of zinc is the potential for zinc to