Appendix D Food Codes for NHANES - OEHHA

Scientific Review Panel Draft February, 2012
to soil ageing. Leaching and weathering effects are assumed to be very long
(i.e., 10 8 days), unless site-specific information shows otherwise. Only a few
studies have investigated the decrease in dermal absorption for specific
inorganic metals and metalloids aged in soils, including arsenic, nickel and
mercury. The soil aging results from these studies are considered in the
development of the ABS, although the volume of literature available is sparse.
Therefore, dermal fractional absorption still relies primarily on data for freshly
applied metals to soil to avoid underestimation of the ABS.
For organic chemicals, the soil half-life variable in Eq. 6.2 will account to some
degree for the effects of soil aging, depending on the rigor of the extraction
process used (Abdel-Rahman et al., 2002). Use of a strong acid extraction
method may solubilize some of the desorption-resistant chemical from soil and
overestimate the dermal bioaccessibility of a soil-aged organic chemical. That is
why milder extraction methods have been recommended, such as soil extraction
in synthetic sweat, to obtain a more applicable estimate of soil half-life.
F.2.6 Dermal Soil Loading and Adherence Characteristics
The ABS from soil depends on the amount of soil in contact with the skin.
Maximal fractional absorption of a soil-bound chemical occurs when a monolayer
of soil covers the skin (monolayer threshold). A monolayer can be defined, in
this case, as layer of soil on the skin equal in thickness to the average soil
particle diameter. Theoretical calculations and experimental data show that
increased soil loading (mg soil/cm 2 skin) beyond monolayer coverage usually
leads to decreased fractional absorption as a result of some of the soil not being
in direct contact with skin (McKone, 1990; Duff and Kissel, 1996; Bunge and
Parks, 1997). Soil loading at which the monolayer exists depends on the soil
particle size (Duff and Kissel, 1996). For example, sand with an average particle
diameter of 0.044 cm reaches monolayer coverage at 61 mg/cm 2 , whereas
monolayer coverage with clay at a particle diameter of 0.0092 cm is 13 mg/cm 2
(USEPA, 2004).
Early soil loading experiments were carried out under conditions of high loading,
e.g. 20-40 mg/cm 2 (Shu et al., 1988; Wester et al., 1990a; Wester et al., 1992) ,
without estimating monolayer coverage or providing average soil particle
diameter to estimate monolayer coverage. High soil loadings that are greater
than monolayer coverage may underestimate the fraction of chemical absorbed
from soil. Coarse grain size (180 to 300 µm) used under the high loading
conditions of 20-40 mg/cm 2 was at, or only, slightly more than monolayer
coverage (Duff and Kissel, 1996). However, using such soil loadings with soils
sieved to