Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
sample (i.e., responder bias). Since the population in such
non-probability sample studies is often made up of volunteers, there
is usually some factor present which distinguishes them from those who
do not choose to participate. This factor could influence the results;
in particular, those who participate may tend to consider themselves
strongly affected or not affected by the pollutant being studied and
may alter their responses or behaviours as a result. This phenomenon
is a special case of responder bias, often termed self-selection
bias. Also, a poorly designed study can fail to control for temporal
and spatial variability, as well as meteorological, site and source
bias. This bias is a result of a single, "random-day", or grab
sampling and single-location sampling, which decreases the potential
for generalization.
Controlled experiments are useful to examine a few factors and to
study their influence on the resulting exposure. The use of
randomization and control ensures that the effects are real and not
the result of confounding causes, incorrectly measured variables or
missing variables. Examples include chamber studies and other
situations where the investigator has control over most of the
environmental factors.
3.5 Exposure assessment approaches
As discussed in Chapter 1, strategies for assessing environmental
exposure can be categorized as one of two general approaches; direct
or indirect. Direct approaches include personal exposure monitoring
and biological markers of exposure. Indirect approaches include
environmental sampling, combined with exposure factor information,
modelling and questionnaires.
3.5.1 Direct approaches to exposure assessment
Direct measures of exposure include samples collected at the
interface between an exposure medium and the human body, e.g., at the
breathing zone in the case of air pollutant exposure, or samples of
biological tissue in which concentrations of target pollutants can be
quantitated. Measurements in food or drinking-water (duplicate
portions) which are ingested could also be viewed as a direct way of
assessing exposure through these media. Thus, direct approaches to
exposure assessment include personal exposure monitoring and
biological markers of exposure. Personal monitoring methods are
discussed below, and the subject of biomarkers of exposure is
presented in detail in Chapter 10.
Personal monitoring of exposure to environmental contaminants
refers to collection of samples at the interface between the exposure
medium and the human receptor (e.g., the breathing zone). Personal
monitoring approaches are summarized in Table 6. Personal monitors
make it possible to measure exposures for an identified subset of the
general population. Moreover, if study participants maintain records
of their activities, then locations where highest exposure
concentrations occur as well as the nature of emission sources can
often be inferred. Personal monitoring can be done for all potential
exposure media (e.g., air, water, soil, food) and pollutants of
interest. Although available, personal monitoring methods may not be
employed in a particular investigation due to study design, time or
expense considerations. The principal limitation on the use of
personal monitoring for exposure assessment is the availability of
sample collection methods that are sensitive, easy to operate, able to
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