Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 508

In the second step, the average daily dose over the exposure period is calculated
by incorporating estimates of the number of events per day, exposure frequency in
days per year, exposure duration in years, and average body surface area exposed.
For exposures that span childhood into adulthood, it may also be necessary to
prorate and combine the average daily doses during childhood with those during
adulthood. The calculation of the average daily dose over the exposure period is as
follows:

DAevent × EF × ED × EV × SA

DADsoil = (11.65)

AT × BW

Where:

DAD soil = Dermal absorbed dose (mg/kg-day)

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

=

EF Exposure frequency (days/year)

=

ED Exposure duration (years)

=

EV Event frequency (event/d)

Skin surface area available for contact (cm 2 ) SA =

AT = Averaging time (days)

BW = Body weight (kg).

11.7.4 Input Parameters

Critical input parameters for estimating dermal uptake from soil include dermal
absorption fractions, skin adherence factors, and skin surface area exposed.

11.7.4.1 Dermal Absorption Fractions

As described above, many factors affect the absorption of contaminants through the
skin. For studies of dermal absorption of contaminants in soil many aspects of study
design affect the relevance of the results in predicting behaviour of contaminants
in soils at contaminated sites. The principal problem with studies reported in the
literature is the failure of most studies to use weathered soils from sites. Most lab-
oratory studies have been conducted using soil to which a contaminant dissolved
in a solvent has been added. In some studies the solvent was not even evaporated
before the soil-contaminant mixture was applied to the skin. Contaminant mobility
and bioavailability in soil have been shown to decline over time (Alexander 2000 ),
so for that reason much empirical data on dermal absorption from soil is likely to
overestimate the contribution of dermal exposures to contaminated site exposures.
A related study design flaw is use of contaminant concentrations exceeding the soil
saturation limit. Such studies are not truly studies of absorption from soil (Spalt
et al. 2008 ).