Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
partition coefficients such as these (e.g., MacIntosh et al., 1994).
6.5.2 Exposure factors
In constructing exposure models one needs to define the
characteristics of individuals in various age and sex categories and
the characteristics of the microenvironments in which they live or
from which they obtain water and food. The types of data needed to
carry out the exposure assessment include exposure duration and
averaging time, time-activity patterns of individuals, food
consumption patterns, household parameters, human factors such as body
weight, surface area, soil ingestion and breast milk intake, and
parameters associated with food crops and food-producing animals.
Time-activity patterns provide information on how individuals
distribute their time among a number of potential exposure media.
Time-activity pattern data describe such things as the average number
of hours spent indoors at home and in what rooms and the nature of
activity. Time-activity data also includes information on time spent
outdoors at home or spent in microenvironments, such as bathrooms
(including shower and bathing time). Exposure times are activity data
that involve the number of days per year and hours per day spent in
contact with soil during recreation and home gardening and in contact
with surface water during swimming or other water recreation.
Household factors relate to drinking-water supply and use,
room-ventilation rates, and soil and dust concentrations within homes.
Soil ingestion rates and soil contact on skin are also needed. Methods
for measuring time-activity patterns and related considerations are
discussed in detail in Chapter 5.
Input data of these types may be measured in the population under
investigation, i.e., site specific, or may be drawn from standard
references such as AIHC (1994), Finley et al. (1994b) and US EPA
(1996a). Site-specific data are preferred, in case the population of
interest may exhibit unique characteristics expected to influence
exposure. If site-specific data are not available, values observed in
other populations or estimates may be applied. Some model applications
may rely solely on estimated inputs. For example, screening models are
often used to assess exposure and health risks associated with new
products such as pesticides designed for agricultural and residential
use. In this case, model inputs may be determined in a manner such
that the model result is unlikely to underestimate the true level of
exposure experienced by the population of interest. Models such as
these are often referred to as "worst-case" models. An exposure
modelling system recently developed by the European Union contains a
suite of screening models (EC, 1996).
6.5.3 Multiple-media/multiple-pathway models
Multiple-media or so-called "total" exposure models provide
methods for integrating multiple exposure pathways from multiple
environmental media into a model system that relates concentrations of
toxic chemicals to potential total human dose at toxic substances
release sites. This type of simulation matrix is used to generate the
hypothetical histogram shown in Fig. 22. The scenarios used to develop
this particular histogram are for a representative VOC incorporated in
the top several metres of soil. Here we can see that, based on a
multiple-media and multiple-pathway assessment, we get indications of
where it is most valuable to focus our resources to more fully
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