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 2/5)
Character Range: 2171720–2174636

on the basis of the following:
    * Intakes of cobalt from other sources were estimated with dietary intakes considered most significant for the general population. Intakes calculated were based on limited data where total background intakes were estimated to be 1 µg/kg/day, up to 20% of the adopted upper PTDI of 5 µg/kg/day.
    * A provisional tolerable daily intake (PTDI) range of 15 µg/kg/day was derived, based on a LOAEL of 0.5 mg/kg/day from human therapeutic studies and a combined safety factor of 100500.
    * Ingestion of both soil and dust has been considered assuming 50% is bioavailable.
    * Dermal absorption has been considered to be negligible.
    * Inhalation of cobalt in dust has been considered both outdoors and indoors.

6.3              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.

    6.3.1         Oral Bioavailability
According to ATSDR (2004), the oral bioavailability of cobalt varies from 1897% depending on dose, form of cobalt compound and nutritional status of the subjects.

While bioavailability has been considered in the previous HIL, insufficient data is available to adequately define the bioavailability of cobalt from soil. On this basis, a default approach of assuming 100% oral (and inhalation) bioavailability has been adopted in the derivation of an HIL. It is noted that a site-specific assessment of bioavailability can be undertaken where required.

    6.3.2         Dermal absorption
In humans, inhalation and dermal exposure have been observed to result in sensitisation to cobalt (WHO 2006), hence it is reasonable to consider that dermal absorption may be more than negligible. Limited data is available regarding the dermal absorption of cobalt 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), which is higher than the dermally absorbed fraction of 0.0004 cited by Paustenbach (2000) for cobalt chloride (0.04%) in aqueous solution.

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

    6.3.4         Plant Uptake
In the review of cobalt presented by Buckett & Di Marco (1998), it was noted that, based on data presented by IARC (1991), cobalt can be detected in plants. Whether cobalt is essential to plant growth has not been well established, however it appears that plant uptake may be somewhat significant and, as