Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
Models developed for assessing the behaviour of contaminants in
soil can be categorized in terms of the transport/transformation
processes being modelled. Partition models such as the fugacity models
of Mackay (1979) and Mackay & Paterson (1981, 1982) describe the
distribution of a contaminant among the liquid, solid and water phases
of soils. Jury et al. (1983) have developed an analytical screening
model that can be used to calculate the extent to which contaminants
buried in soil evaporate to the atmosphere. The multiple-media model
GEOTOX (McKone & Layton, 1986) has been used to determine the
inventory of chemical elements and organic compounds in soil layers
following various contamination events. This model addresses
volatilization to atmosphere, runoff to surface water, and leaching to
groundwater and first-order chemical transformation processes.
6.5 Multiple-media modelling
Human beings come directly into contact with certain media via
certain routes and are exposed to the chemicals therein as depicted in
Table 19. Efforts to assess human exposure from multiple media date
back to the 1950s when the need to assess human exposure to global
fallout led rapidly to a framework that included transport both
through and among air, soil, surface, water, vegetation and food
chains (Whicker & Kirchner, 1987). Efforts to apply such a framework
to non-radioactive organic and inorganic toxic chemicals have been
more recent and have not as yet achieved such a high level of
sophistication. In response to the need for multiple-media models in
exposure assessment, a number of transport and transformation models
have recently appeared. In an early book on multiple-media transport,
Thibodeaux (1996) proposed the term "chemodynamics" to describe a set
of integrated methods for assessing the cross-media transfers of
organic chemicals. The first widely used multiple-media compartment
models for organic chemicals were the fugacity models proposed by
Mackay (1979, 1991) and Mackay & Paterson (1981, 1982). Cohen and his
co-workers introduced the concept of the multiple-media compartment
model and more recently the spatial multiple-media compartment model,
which allows for non-uniformity in some compartments (Cohen & Ryan,
1985, Cohen et al., 1990). Another multiple-media screening model,
called GEOTOX (McKone & Layton, 1986; McKone et al., 1987), was one of
the earliest to explicitly address human exposure.
The preceding models deal with inter-media transfer of
contaminants on a relatively large scale, but other models are scaled
to the residence and exposures that may occur therein. Exposure to
chemicals in consumer products such as cleaning agents and paint are
the focus of a model called CONSEXPO (van Veen, 1996).
All multiple-media exposure models have at least two features in
common, regardless of the objective for which they were designed.
First, movement of contaminants from one medium to another is
characterized. Second, the rate and/or frequency of human contact with
environmental media is modelled. The former may be referred to as
inter-media transfer factors and the latter as exposure factors.
Table 19. Potential human exposure media and routes
Environmental medium Exposure routes
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