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and food. Although the organic compounds are not naturally occurring, they are
inadvertently produced as an impurity in the manufacture of many chemicals or as
a byproduct of many combustion processes and thus considered to be ubiquitous in
the environment.

4.5.1.2 Fate and Exposure Models

Examination of the total exposure is a comprehensive evaluation that necessitates
modeling. The models or the mathematical calculations for the exposure scenarios
provide a means of calculating exposure levels. A chemical’s final distribution in
the media and its respective concentrations are the result of numerous highly complex
and interacting processes that are not easy to estimate. Fate and transport models
have been developed to estimate pollutant transport among and transportation within
multiple environmental media.
Once a chemical has been emitted to a medium (air, water or soil), it is distributed
in the environment. The distribution is not normally restricted to one environmental
compartment, but is partitioned among different compartments. Therefore, it may
cause one or more types of environmental impacts. Also, it can enter the food chain
and become a risk to human beings. This ability depends on the specific physical,
chemical or biological (toxicological) properties of the compound and of the prop-
erties and characteristics of the medium to which the emission is released. On the
other hand, chemicals can suffer different processes (i.e., degradation, biodegrada-
tion, metabolization, transformation, dissociation, hydrolytic process, etc.) in the
environment. In this way, fate and transport models can help resolve how a chemical
will be distributed in the different media and which transformation may suffer.
Two different modeling approaches exist: 1) multimedia fate and exposure modeling
and 2) specific single-medium models. Integrated multimedia fate and exposure models
represent the distribution of a chemical among different compartments and the transfer
of chemicals through various exposure routes to a species of interest. For human toxicity,
the models calculate a potential dose, which is indicative of the level of impact expected.
For ecological toxicity, the models calculate environmental compartment concentrations
or potential doses for animals at different levels of the food chain.
The examination of the total exposure is a comprehensive evaluation most
accurately carried out for micropollutants (organics and heavy metals) by the use
of multimedia modeling. For macropollutants (SO , NO and particles) single-
x
2
medium models are generally applied. Table 4.1 consists of selected models that
may be applied to some aspects of risk assessment and environmental management
problems. The choice of one particular model over another will generally be specific
to the problem.
Environmental fate models determine the concentration in different compart-
ments (air, surface water, sediments) through the solution of mass balance equations
describing the release, transformation, and intercompartmental distribution of a
pollutant. Exposure pathway models calculate the exposure of an organism via a
stated pathway resulting from a given environmental concentration. These factors
take into account transfer factors, uptake rates such as the rate of inhalation, parti-
tioning and bioconcentration factors, and environmental concentrations.

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