Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
the water contacts the skin. Dermal exposure in this situation is
defined as the concentration of the contaminant in the water and the
duration of contact.
3.5.2 Indirect approaches to exposure assessment
Indirect measures of exposure include estimates derived from
environmental monitoring (i.e., measurements made in locations
frequented by the study participants), models and questionnaires.
3.5.2.1 Environmental monitoring
Indirect estimates of exposure may be made by combining
measurements of pollutant concentrations at fixed sites with
information on rates of contact with these media recorded in data logs
and diaries or time-activity surveys. Examples include air pollutant
concentrations in specific areas combined with time budget records
(see Chapter 5), food contaminant data combined with information on
dietary patterns (see Chapter 7.4 for details), and pollutant
concentrations on skin combined with data on frequency and duration of
hand-to-mouth contact. Although collection of environmental,
time-activity and questionnaire data needed for this exposure
assessment approach is simpler than for personal monitoring, it is
still invasive and laborious, and may lead to selection bias.
Microenvironmental monitoring is a special case of environmental
monitoring in which the location where measurements are made is
considered to be homogeneous with respect to concentrations of the
target pollutants over the averaging time of interest. The concept of
a microenvironment has been widely applied in air pollution exposure
assessments. Examples of potentially important micro-environments used
for air pollution exposure assessment are listed in Table 7. The
general form of the equation used to calculate time-weighted
integrated exposure from micro environmental monitoring data is
(3.1)
where E is the time-weighted integrated exposure (e.g., mg/m 3 ),
C is the concentration (e.g., mg/m 3 ), t is the unit time (e.g.,
minute), T is the total elapsed time (e.g., minutes). The subscripts
i, j and k denote the medium, the pathway and the microenvironment
respectively (Duan, 1982). The most important assumptions inherent in
this model are:
* The concentration C j in microenvironment j is assumed to be
constant during the time that person i is there.
* The concentration C j within microenvironment j and the time
that person i spends there are assumed to be independent events.
* The number of microenvironments necessary to characterize personal
exposure adequately is assumed to be small.
The concept of a time-weighted integrated exposure is illustrated
in Fig. 10. A unit width is indicated on the j axis for each of five
microenvironments: indoors at home, indoors at work, indoors in other
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