Page 778 - The Toxicology of Fishes

758 The Toxicology of Fishes

that there is little chance that we would be exposed to that gasoline in a way and quantity that would
cause us serious harm. It is this weighing of exposure and potential effect that is, in essence, a form of
risk assessment.
Although we each evaluate risks many times each day, the actual process we use to judge risk is
probably something we would struggle to explain. Moreover, different people assess risks differently
and choose different behaviors as a result, but we would also struggle to articulate exactly how their
risk assessment process differs from our own. When it comes to environmental decision making, we
intuitively expect something more transparent and objective from regulatory authorities and other deci-
sion makers. The need for an objective and consistent framework for judging environmental risks has
led to the process we know today as ecological risk assessment.
Through the years, the term ecological risk assessment has been coined to refer to risk assessment
applied to nonhuman ecological receptors (e.g., fish, terrestrial wildlife, plants) or more generally to the
natural environment; the term human health risk assessment is used in reference to assessments focusing
on effects to humans. The remainder of this chapter focuses on ecological risk assessment in its broadest
definition, reviewing the evolution of the concepts into a formalized scientific process and describing
how the science of toxicology is woven into the risk assessment process. The primary context is ecological
risk assessment for chemical pollutants, although the principles are also generally applicable to other
stressors.

The Evolution of Environmental Toxicology and Ecological Risk Assessment
Regulatory decision making has been a major impetus in the development of the process of ecological
risk assessment, presumably because it is in this context that our basic understanding of the effects of
chemicals on organisms must be organized into a framework for making quantitative decisions about
the ecological acceptability of chemical exposures in the environment. Looking retrospectively at major
developments in environmental toxicology and regulation within the United States, one can see the
emergence of many concepts important to ecological risk assessment today.
In the 1960s, Mount and Stephan (1967) proposed the maximum acceptable toxicant concentration
(MATC), which is one of the earliest examples of a standardized process for describing toxicity in the
context of risk. The MATC was developed to describe the results of aquatic life-cycle toxicity tests and
was defined initially as the range between the highest toxicant concentration that did not cause an adverse
effect on any tested endpoint (i.e., survival, growth, or reproduction) and the lowest test concentration
that did cause an adverse effect. Later, this range was abbreviated to a single concentration, the geometric
mean of these values (often called the chronic value today). Although the concept itself is still in use,
the very term MATC implies more than an expression of risk (how much effect is induced by an exposure),
but also an expression of the acceptability of that risk (how much toxicant is allowable). As is described
later in this chapter, current risk assessment practices segregate these elements. Nonetheless, the MATC
concept marked a significant move forward in establishing a consistent means to relate chemical expo-
sures to expected effects. It is interesting to note that the same chronic effect endpoints used to derive
the original MATC are still in widespread use but have been renamed the no-observed-effect concentra-
tion (NOEC) or no-observed-effect level (NOEL) and the lowest-observed-effect concentration (LOEC)
or lowest observed effect level (LOEL). This newer terminology does, in fact, focus on an expression
of risk (projected biological effects) rather than environmental acceptability.
In 1977, a workshop was held in Pellston, Michigan, for the express purpose of gathering experts in
aquatic toxicology and environmental fate for a discussion of how to structure an assessment framework
for evaluating risk (then called hazard) to aquatic life posed by chemical substances (Cairns et al., 1978).
The proposed structure is very similar to the structures used for risk assessment today and provides a
succinct conceptual representation of the process. As illustrated in Figure 18.1, chemical exposure is
represented by an expected environmental concentration (EEC) and is compared to a threshold for
unacceptable biological effects, which can be thought of as a safe environmental concentration (SEC).
Each of these values has a degree of uncertainty about it, which is inversely proportional to the amount
of available data. When few data are available, uncertainties around one or both of these values may be