Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 506

11.7 Exposure Through Dermal Uptake
11.7.1 Significance
At contaminated sites contaminants may come in contact with the skin either in
surface water and groundwater or in soils and sediment.
Dermal uptake has often been omitted from Exposure Assessments based on the
assumption that dermal uptake is a minor contributor to total exposures. In many
cases that assumption is correct. The relative contribution of dermal uptake to total
exposures to contaminants in soil varies depending on the exposure scenarios being
considered and the nature of the contaminants being assessed. Volatile contaminants
are typically assumed to have minimal potential for dermal uptake due to the like-
lihood that they will volatilize faster than dermal uptake could occur. Most metals
are also expected to have limited dermal uptake due to a low tendency to partition
into skin lipids. This leaves semi-volatile organic contaminants as the focus of most
dermal uptake assessments.
Typical residential exposures for young children are likely to include the poten-
tial for substantial dermal contact with soil through routine play activities, whereas
adults may only have substantial soil contact with skin while engaged in selected
activities such as gardening. Workers may also have highly variable soil contact,
with construction workers or utility workers digging trenches having the greatest
potential exposure via the dermal route. As intake assumptions are varied within
any one scenario, the relative contributions from different exposure routes may also
change. For example, if soil ingestion rates in young children are reduced, the rel-
ative contribution of dermal uptake to total exposures would increase. Similarly, if
a larger area of skin is assumed to be in contact with soil, the importance of dermal
uptake will increase.
11.7.2 Conceptual Model
Systemic absorption of contaminants from the skin surface requires a series of trans-
fers; contaminants must cross into the stratum corneum (outer skin layer) followed
by transfer into the epidermis (live skin cell layer), and then into the blood. The rate
and extent of these transfers is a function of many factors, including the condition
and thickness of the skin and the characteristics of the contaminant itself. Skin thick-
ness varies over different areas of the body, so the nature of the exposed body parts
should be specified when estimating dermal uptake. Lipophilic contaminants such
as PCBs, some PAHs, dioxins, and some pesticides typically have greater poten-
tial for dermal uptake compared with ionic or hydrophilic contaminants. In the case
of soil and sediment, the rate and extent of dermal uptake will also be controlled
by the nature of the soil particles that adhere to skin and by the interactions of the
contaminant with the soil particles.
The model typically used to estimate dermal uptake of contaminants from soil
and sediment is derived from the idea that a finite dose of contaminant comes