Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
include exposure concentration and the duration of contact. If the
exposure concentration is integrated over the duration of contact
(Table 2), the area under the resulting curve is the magnitude of the
exposure in units of concentration multiplied by time (e.g.,
mg/litreÊday, mg/kgÊday, µg/m 3 Êh). This is the method of choice to
describe and estimate short-term doses, where integration times are of
the order of minutes, hours or days.
Over periods of months, years or decades, exposures to most
environmental agents occur intermittently rather than continuously.
Yet long-term health effects, such as cancer, are customarily
evaluated based on an average dose over the period of interest
(typically years), rather than as a series of intermittent exposures.
Consequently, long-term doses are usually estimated by summing doses
across discrete exposure episodes and then calculating an average dose
for the period of interest (e.g., year, lifetime). Although the
integration approach can also be used to estimate long-term exposures
or doses, its application to time periods longer than about a week is
usually difficult and inconvenient.
1.2.3 Exposure measurements and models
Direct measurements are the only way to establish unequivocally
whether and to what extent individuals are exposed to specific
environmental agents. But it is neither affordable nor technically
feasible to measure exposures for everyone in all populations of
interest. Models, which are mathematical abstractions of physical
reality, may obviate the need for such extensive monitoring programmes
by providing estimates of population exposures (and doses) that are
based on a smaller number of representative measurements (Fig. 1). The
challenge is to develop appropriate and robust models that allow for
extrapolation from relatively few measurements to estimates of
exposures and doses for a much larger population (US NRC, 1991b).
For relatively small groups, measurements or estimates can be
made for some or all of the individuals separately, and then combined
as necessary to estimate the exposure (or dose) distribution. For
larger groups, exposure models and statistics can sometimes be used to
derive an estimate of the distribution of population exposures,
depending on the quantity and quality of existing data. Monte Carlo
and other statistical techniques are increasingly being used to
generate and analyse exposure distributions for large groups (US EPA,
1992a).
1.2.4 Exposure in the context of an environmental health paradigm
The presence of hazardous substances in our environment does not
necessarily imply a risk to human health or to the ecosystem. Exposure
is an integral and necessary component in a sequence of events having
potential health consequences. An expanded and more detailed version
http://www.inchem.org/documents/ehc/ehc/ehc214.htm
Page 23 of 284
6/1/2007