Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
error in environmental epidemiology (Schulte, 1987; Hulka & Wilcosky,
1988; Hatch & Thomas, 1993). Since they represent internal dose, they
are anticipated to be more predictive of health effects than external
measures of exposure (US NRC, 1987; Hulka, 1991) and they can be used
to validate population exposure models (Georgopoulos & Lioy, 1994).
For risk assessment purposes, biological markers can be useful tools
in evaluation of intermediate end-points and improving the transition
from environmental exposure assessment and animal dose-response models
to actual human health outcomes (Hattis, 1986; IPCS, 1993; Mercier &
Robinson, 1993).
10.4.3 Internal exposure sources
Contaminants can be stored long-term in the body and may be
produced endogenously. These sources of exposure cannot be
characterized without biological markers. Breast milk, bone mineral
and adipose tissue as well as blood represent biological media
available to assess body burden of contaminants, especially those that
concentrate in biological tissue. Mobilization of contaminants from
internal storage can be assessed using biological markers; for
example, pesticide mobilization from adipose tissue can be measured in
blood following fasting.
10.5 Limitations of biological markers for exposure assessment
Although the use of biological markers may result in improved
exposure assessment, their use is not without limitations, since few
validated markers are currently available. Challenges associated with
the use of biological markers include source identification,
pharmacokinetics, timing of exposure, biological variability, altered
response as a result of exposure, potentially invasive sampling
procedures and ethical concerns.
10.5.1 Source identification
Although the ability to integrate over all exposure sources is an
advantage for total human exposure assessment, it limits the ability
to identify the sources that contribute to exposure. Considerations
for source identification include multiple exposure sources, multiple
exposure regimes, non-specific biological response to exposure and
endogenous production. Multiple sources of exposure can result in
multimodal excretion rates because some exposures may be constant
whereas additional exposures may be intermittent. Therefore,
performing cross-sectional studies may miss multiple exposures (Que
Hee, 1993) or changes in exposure patterns and/or concentrations. Some
organic chemicals can be produced endogenously; this may result in an
overestimate of the impact of external sources of exposure.
10.5.2 Biological variability and altered exposure response
Variability of biological response is an inherent characteristic
of biological markers. Although variability may be associated with
exposure levels, genetic differences in toxicokinetics and lifestyle
factors such as diet, smoking and alcohol consumption may also result
in biological variability. The impact of the inherent biological
variability can be reduced by stratified sampling and the use of large
sample sizes. Quantification of biological variability is difficult,
and without adequate information on sources of variation the
interpretation of results for exposure assessment purposes may be
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