Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:3:p2
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 3 (pt 2/5)
Character Range: 2113724–2116656

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.

    3.3.1         Oral Bioavailability
While bioavailability (inhalation only) has been considered in the previous HIL, insufficient data is available to adequately define the bioavailability of boron. 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.

    3.3.2         Dermal absorption
Dermal absorption of boron is considered to be low. While limited data is available, reviews by WHO (1998), ATSDR (2010) and US EPA (2004b) suggest the boron is not absorbed across intact skin. Review by MfE (2011) has also noted that dermal absorption is considered negligible and has not considered this pathway in the derivation of a soil guideline. It is noted that review of the derived HIL (based on 1% dermal absorption) contributes less than 5% of the HIL, and hence it is considered appropriate that, based on the available data, dermal absorption is not considered a significant pathway in the derivation of a soil HIL. This pathway has not been considered further.

    3.3.3         Inhalation of Dust
Boron 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.

    3.3.4         Plant Uptake
Review of plant uptake of boron by MfE (2011) has indicated that 'it has not been possible to develop bioconcentration factors for boron. Reviewing the literature shows that boron uptake into plants is highly variable between species with no relationship with soil concentration or other soil parameters. Boron is an essential element for plant growth, but what may be optimal boron for one species may be toxic or insufficient for other species'. 'Determining the significance of plant uptake of boron to human exposure is difficult, given the wide-ranging and overlapping concentrations that determine boron essentiality and toxicity in various species. Nonetheless, it appears that 300 mg/kg is a reasonable upper limit of non-toxic plant boron concentrations and thus can be used as the reasonable maximum amount of boron likely to be taken up in home-grown vegetables. Beyond that point vegetables are unlikely to be harvestable.'

The approach adopted by MfE (2011) in the derivation of a soil guideline for boron was to consider potential intakes associated with consumption of home-grown produce in soil concentrations that are not phytotoxic (300 mg/kg), as part of the overall intake from other sources. To obtain the additional background intake, a child's produce consumption (0.048