Appendix D Food Codes for NHANES - OEHHA

Scientific Review Panel Draft February, 2012
with data collected under similar conditions, the most comprehensive study was
selected. Or if the studies were of equal reliability, their absorption values would
be averaged for ABS determination. In some cases experimental data are not
even sufficient for a point estimate ABS and a default ABS is recommended (see
below).
F.1.2 Skin Morphology and Dermal Absorption Issues for ABS
Determination
The transepidermal uptake of chemicals across skin involves a complex process
of transport from the soil matrix to the external protective skin layer called the
epidermis, and then through the epidermis to the underlying dermis. The
outermost layer of the epidermis is called the stratum corneum, which is thought
to provide the major barrier to the absorption of most substances deposited onto
the skin surface. The stratum corneum in humans varies in thickness from about
5 µm to over 400 µm on the palms and soles of the feet (Poet and McDougal,
2002; Hostynek, 2003). Below lies the viable epidermis, about 50-100 µm thick,
containing keratinocytes that proliferate and differentiate while moving upwards
and replacing the stratum corneum cells as they wear away. Below the
epidermis lies the hydrous tissue of the dermis perfused by the blood and
lymphatic circulation.
Skin appendages, including hair follicles and sweat ducts, transit through all
these layers and may provide an alternate pathway for dermal diffusion of some
ions such as metal salts (Tregear, 1966; Flynn, 1990). However, skin
appendages occupy only a fraction of the surface area of the skin, which may
limit their potential as a major diffusion pathway into the systemic circulation.
During the transport through the viable-epidermal and dermal layers, metabolism
may also play a role in the absorption process (Kao and Carver, 1990).
Metabolism in the dermal layers could also activate a toxicant, resulting in skin as
a target organ or producing toxicity elsewhere following systemic absorption. As
noted above, specific dermal ABSs for soil-bound chemicals are difficult to obtain
due in part to the complex multiphasic nature of the system and lack of published
absorption data. Hawley (1985) suggested a default factor of 15 percent to
correct for the effect of the soil matrix on the dermal uptake of organic chemicals.
Experimental evidence, however, suggests absorption from soil will be chemical
dependent. Hence, it is important to determine dermal uptake point estimate
values for specific soil-bound chemicals where appropriate data are available, as
they will be more accurate than those derived on broad-based assumptions.
To obtain the ABS, a measured amount of chemical in a given amount of soil is
administered to the skin surface; this amount (wt chemical/area skin) is referred
to as the applied dose. The amount of chemical that crosses the skin barrier is
measured and the ABS is calculated by dividing the amount absorbed by the
amount applied. When measurements are made in excreta or specific organs,
F-3