Appendix D Food Codes for NHANES - OEHHA

Scientific Review Panel Draft February, 2012
extensive percutaneous metabolism when absorbed, and reducing the metabolic
capabilities of skin samples will reduce dermal penetration of absorbed PAHs
(Kao et al., 1985; Ng et al., 1992; Moody et al., 2009a).
For metal salts, it has been postulated that low diffusion values through the
stratum corneum in vitro are a result of skin shunts (e.g., hair follicles and sweat
ducts) swelling shut upon hydration of skin samples (Tregear, 1966; Hostynek,
2003). Skin shunts that bypass the stratum corneum are thought by some to be
a significant absorption route for charged metals. For example, dermal
absorption of nickel salts shows there is a surge in diffusion at the earliest stage,
which then rapidly decreases towards steady state (Tanojo et al., 2001). The
decrease in diffusion rate has been proposed to be a result of the skin tissue
becoming hydrated, shutting down the skin shunts.
A further potential limitation under in vitro conditions is that diffusing compounds
must traverse the epidermis and the entire dermis in order to reach the receptor
fluid. In vivo, the majority of the absorption into the cutaneous microcirculation is
thought to occur in the upper dermis and the penetrant compounds may not have
to diffuse across the entire thickness of the dermis. However, the bulk of the
connective tissue in the dermis is often eliminated from the skin preparation by
cutting the skin parallel to the skin surface with a dermatome (Poet and
McDougal, 2002).
In vivo studies are not without limitations. Dermally applied chemicals are often
radiolabeled to facilitate quantification of the usually low absolute amounts of
chemical dermally absorbed. In small mammals, a total accounting of all
dermally absorbed radioactivity can be estimated from excreta, carcass, and site
of skin absorption. However, in larger mammals measurements of radiotracer
are quantified in excreta and measurements from intravenous, intramuscular, or
oral dosing are applied as a correction for tissue absorbed chemical. The validity
of this method depends on the underlying assumption that metabolism and
disposition of the applied compound is route independent, and that the
pharmacokinetic behavior of the intravenous and topical doses is similar (Kao,
1990).
F.2.8 Inter- and Intra-Species Specificity
The variability in dermal absorption of chemicals among mammalian species has
been investigated in vivo and in vitro. Bartek et al. (1972) suggest that the extent
of in vivo uptake among animals follows the rank: rabbit > rat > pig ≈ monkey ≈
humans, based on dermal absorption of benzoic acid, hydrocortisone,
testosterone, caffeine, N-acetylcysteine, and butter yellow. However, the species
ranking did not strictly hold for all chemicals, indicating not only species-specific
differences but also chemical-specific differences.
F-12