Page 159 - Materials Chemistry, Second Edition

L1644_C04.fm Page 131 Tuesday, October 21, 2003 3:13 PM

choosing a more complex model over a simple one will not necessarily ensure a
better solution in all situations. In fact, because a model is a mathematical repre-
sentation of a complex system, usually some degree of mathematical simplification
must be made about the system being modeled. Due to the complexity of natural
systems, it is usually not possible to obtain all the input parameters, so data limita-
tions must be weighted appropriately when choosing a model. Here ERA is con-
fronted with the same problem as that of the LCA described previously because
both are methods based on system analysis.
Ultimately, the type of model selected will be dependent on the overall goal for
the assessment, complexity of the problem, type of contaminants of concern, and
nature of the impacted and threatened media, as well as the type of corrective actions
considered in the investigation.

4.5.2 HUMAN HEALTH EXPOSURE ASSESSMENT

After estimation of the increasing concentrations of pollutants in air, water, soil, and
food in the affected regions (by means of a monitorization, multicompartmental or
single model), an exposure assessment must be carried out. The exposure assessment
phase of human health risk assessment is the estimation of the rates at which
chemicals are absorbed by potential receptors. Because most potential receptors can
be exposed to chemicals from a variety of sources and/or in different environmental
media, an evaluation of the relative contributions of each medium and/or source to
total pollutant intake could be critical in a multipathway exposure analysis. In fact,
the accuracy of exposure characterization could be a main determinant for the
validity of a risk assessment.
Humans may be exposed to substances in their workplaces (occupational expo-
sure), due to use of consumer products (consumer exposure) and indirectly via the
environment. Different types of individuals and ages may be required to characterize
the population at greatest risk. Also, frequent exposure to adults and children must
be considered separately because adulthood provides the longest period of exposure
and childhood accentuates some exposure routes (such as the incidental ingestion
of soil) and potential sensitivities due to higher ratios of intake to body weight.
In a first step of the exposure assessment, the probability of an exposure of the
population to the substances under consideration must be evaluated. Exposure levels
and concentrations for each exposed population need to be evaluated based on
available measured data and/or modeling.
A contaminant can enter the body using any of three pathways: ingestion,
inhalation or by contact with the skin (dermal or other exterior surfaces such as
eyes). Once in the body it can be absorbed and distributed to various organs and
systems. The toxic may then be stored, for example in fat, as in the case of DDT,
or it may be eliminated from the body by transformation into something else and/or
by excretion. The biotransformation process usually provides metabolites more
readily eliminated from the body than the original chemicals; however, metabolism
can also convert chemicals into more toxic forms.
On the other hand, environmental exposure to chemicals can be direct (as a
result of exposure to the media where the emission directly takes place) or

© 2004 CRC Press LLC