Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 507

in contact with the skin during each exposure event, and that some fraction of
that applied dose is subsequently absorbed (US EPA 2004b ). This model differs
from the model used to estimate uptake from water, which is based on use of
contaminant-specific permeability constants to estimate rates of absorption over a
specified exposure period. Practical application of the soil absorption model and the
most critical input parameters are described in the following subsections.
Due to the variable contribution of dermal uptake to exposures, initial screening
evaluations may be useful to determine if there is a need to more fully characterize
dermal uptake. Where dermal exposures are predicted to contribute 5% to 10 % or
less to total exposures, it may not be necessary to include dermal uptake calculation
in Risk Assessments.
Assessing human health risks from dermal uptake differs from assessments for
other exposure routes, because there is little data from studies that directly measure
systemic toxicity of environmental contaminants via dermal exposure. Typically,
it is necessary to rely on oral Toxicological Reference Values to assess risks from
dermal exposures. Most oral Toxicological Reference Values are stated in terms of
intake (“external exposure”) rather than uptake (“internal exposure”). Consequently,
it is necessary to convert oral toxicological reference to an absorbed dose or uptake
before using those values to assess dermal exposures.
For non-cancer effects, this conversion is accomplished by multiplying the oral
Toxicological Reference Value by the fraction of the oral dose that is absorbed
into the systemic circulation. For example, if the tolerable daily intake (TDI) is
100 mg/kg body weight /d, and 5% of the dose is absorbed, then the tolerable absorbed
dose will be 5 mg/kg body weight /d (i.e., 0.05 times 100). Conversion of a cancer slope
factor to an absorbed dose basis requires dividing the oral slope factor by the frac-
tion of dose absorbed. US EPA ( 2004b ) provides a more detailed explanation of this
process and recommended assumptions for the oral absorption of a limited number
of contaminants.

11.7.3 Mathematical Equations

Dermal uptake for chronic exposure is typically calculated in a two step procedure.
In the first step, the absorbed dose for a single exposure event is calculated from the
soil concentration, dermal absorption fraction and assumed amount of soil adhering
to the skin. The calculation of the absorbed dose per event is as follows:

DA event = C soil × CF × AF × ABSd (11.64)

Where:

Absorbed dose per event (mg/cm 2 -event)

DA event =

C soil = Concentration of contaminant in soil (mg/kg dw )

Unit conversion factor (1 x 10 − 6 kg/mg) CF =

Adherence factor (mg/cm 2 -event)

AF =

ABSd = Dermal absorption fraction, contaminant-specific (unitless).