Appendix D Food Codes for NHANES - OEHHA

Scientific Review Panel Draft February, 2012
gastrointestinal fluids that then becomes available for absorption. By definition,
bioaccessibility should exceed bioavailability.
Published data for some chemicals considered in this section contain only data
for neat application of the chemical to skin in solvent or aqueous vehicle.
Generally, there is a lack of absorption data for chemicals bound to soil. To
avoid potential overestimation of absorption in these instances, bioaccessibility
and soil leaching studies of soil-bound chemicals are considered for adjusting the
fractional absorption of the pure chemical applied to skin. These studies can be
used to determine the extractable, or bioaccessible, fraction of a soil pollutant
that can be deposited on the skin surface. Water added to soil is often used to
determine the bioaccessibility of a soil-bound chemical, although human sweat or
synthetic sweat has also been used to estimate the amount of a pollutant that
can be leached from contaminated soils (Horowitz and Finley, 1993; Filon et al.,
2006; Nico et al., 2006).
F.2.3 Soil - Chemical - Tissue Interaction.
Soil is a complex matrix with a highly variable composition and absorptive
capacity. Organic content, mineral composition, particle size, and pH are all
highly variable. Because the dermal absorption of a compound from soil is often
dependent on these characteristics, it follows that transfer of a chemical from soil
particles to the skin surface for absorption is likely to vary with soil type.
Transfer of a chemical from soil particles to the skin surface is limited by the
chemical’s diffusion rate (McKone, 1990). Diffusion through the soil phase,
through the air, and through soil moisture is all possible. Fugacity-based
interphase transport models were constructed to describe the rate of each of
these processes for chemicals in soil particles and to predict the dermal uptake
rates. It was shown that predicted dermal uptake of chemicals from soil depends
on the Henry’s constant (vapor pressure/solubility in water), the octanol/water
partition coefficient of a chemical, and the soil thickness on skin. If the Henry’s
constant is very high, chemicals will be lost from soil particles (or the skin
surface) quite rapidly, so net dermal uptake of chemicals added to soil will be
low. If the Henry’s constant is very low, diffusion through the soil particle layer
will be too slow to allow much dermal uptake unless the soil particles are very
small. A high octanol/water partition coefficient is associated with tight binding to
soil and low water solubility; these properties also limit the ability of a chemical to
diffuse through the mixed lipid/water phases of the stratum corneum.
Other mathematical models have been developed by Bunge and Parks (1997) to
describe dermal absorption of organic chemicals provided the chemical fits
certain assumptions, such as falling within a defined octanol/water partition
coefficient range (1.59 ≤ log10Kow ≤ 5.53), and that the molecular weight of the
organic chemical is ≤ 700. Soil constraints for the model include contaminated
soils with about 0.2% organic carbon or more, and with a clay fraction less than
F-6