McKay, Donald. "Front matter" Multimedia Environmental Models ...

determine priority. This is a subjective process, and it becomes difficult for two
major reasons.
First, chemicals that are subject to quite different patterns of use are difficult to
compare. For example, chemical X may be produced in very large quantities, emitted
into the environment, and found in substantial concentrations in the environment,
but it may not be believed to be particularly toxic. Examples are solvents such as
trichloroethylene or plasticizers such as the phthalate esters. On the other hand,
chemical Y may be produced in minuscule amounts but be very toxic, an example
being the “dioxins.” Which deserves the higher priority?
Second, it appears that the adverse effects suffered by aquatic organisms and
other animals, including humans, are the result of exposure to a large number of
chemicals, not just to one or two chemicals. Thus, assessing chemicals on a caseby-case
basis may obscure the cumulative effect of a large number of chemicals.
For example, if an organism is exposed to 150 chemicals, each at a concentration
that is only 1% of the level that will cause death, then death will very likely occur,
but it cannot be attributed to any one of these chemicals. It is the cumulative effect
that causes death. The obvious prudent approach is to reduce exposure to all chemicals
to the maximum extent possible. The issue is further complicated by the
possibility that some chemicals will act synergistically, i.e., they produce an effect
that is greater than additive; or they may act antagonistically, i.e., the combined
effect is less than additive. As a result, there will be cases in which we are unable
to prove that a specific chemical causes a toxic effect but, in reality, it does contribute
to an overall toxic effect. Indeed, some believe that this situation will be the rule
rather than the exception.
A compelling case can be made that the prudent course of action is for society
to cast a fairly wide net of suspicion (i.e., assemble a fairly large list of chemicals)
then work to elucidate sources, fate, and effects with the aim of reducing overall
exposure of humans, and our companion organisms, to a level at which there is
assurance that no significant toxic effects can exist from these chemicals. The risk
from these chemicals then becomes small as compared to other risks such as accidents,
disease, and exposure to natural toxic substances. This approach has been
extended and articulated as the “Precautionary Principle,” the “Substitution Principle,”
and the “Principle of Prudent Avoidance.”
One preferred approach is to undertake a risk assessment for each chemical.
Formal procedures for conducting such assessments have been published, notably
by the U.S. Environmental Protection Agency (EPA). The process involves identifying
the chemical, its sources, the environment in which it is present, and the
organisms that may be affected. The toxicity of the substance is evaluated and routes
of exposure quantified. Ultimately, the prevailing concentrations or doses are measured
or estimated and compared with levels that are known to cause effects, and
conclusions are drawn regarding the proximity to levels at which there is a risk of
effect. This necessarily involves consideration of the chemical’s behavior in an actual
environment. Risk is thus assessed only for that environment. Risk or toxic effects
are thus not inherent properties of a chemical; they depend on the extent to which
the chemical reaches the organism.
©2001 CRC Press LLC