Appendix D Food Codes for NHANES - OEHHA

Scientific Review Panel Draft February, 2012
and act as a reservoir for the slow systemic absorption of chemicals, the
chemical remaining in skin at the end of dermal absorption experiments is
considered available for systemic absorption unless data are available that
shows otherwise.
Some fraction of dermally-absorbed chemicals may be only superficially diffused
into skin and deposit in the stratum corneum where they are subject to countercurrent
forces of skin shedding, or desquamation, and ultimately removed from
the body before becoming systemically absorbed. Continuous desquamation
with total stratum corneum turnover has been estimated to take 2-3 weeks
(Hostynek, 2003). Modeling calculations by Reddy et al. (2000) indicate that
epidermal turnover can significantly reduce subsequent chemical absorption into
the systemic circulation for highly lipophilic (log Kow > about 4) or high molecular
weight chemicals (MW > about 350-400 Da). However, some highly lipophilic
chemicals retained in skin at the end of dermal absorption studies have been
shown to be predominantly available for eventual absorption into the systemic
circulation. Chemicals of concern that fall into this category include the PAHs
and DEHP (Chu et al., 1996).
Loss of absorbed chemical through skin shedding appears to occur more readily
with some hydrophilic metal salts in which a portion of the metal becomes
irreversibly bound in the epidermis and subject to eventual shedding with skin.
Some metal salts have such a slow diffusion (i.e., long lag time) through skin that
the stratum corneum turnover rate exceeds the chemical diffusion rate
(Hostynek, 2003).
Tape stripping methods to remove thin layers of stratum corneum have been
used in several studies discussed below to estimate the fraction of chemical in
the stratum corneum that may be lost through desquamation. A more definitive
approach used in a few cases is to extend the dermal uptake study for an
additional few days (after chemical is removed from skin) to determine if more of
the chemical retained in the skin becomes available for systemic absorption.
Other studies that help determine the fate of chemicals retained in skin include
skin localization techniques and skin binding studies (Miselnicky et al., 1988;
Yourick et al., 2004). But in many instances the dermal uptake studies for
individual chemicals did not provide enough data to determine the fate or location
of the chemical retained in skin. Thus, as discussed above, the ABS will then
represent that fraction of chemical still retained in skin, plus the fraction that has
already passed through the skin.
F.2.2 Dermal Bioavailability of Chemicals in Soil
The term dermal bioavailability as it applies in this section refers to the fraction of
chemical in soil that is actually dermally absorbed. Dermal bioaccessibility is
another term used in reference to chemical-laden soils and represents that
fraction of chemical solubilized from soil, usually into water, sweat, or
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