Chemistry Reference
In-Depth Information
13.2 meaSurIng tHe toxIcIty of mIxtureS
As explained earlier, toxicity testing of pesticides and industrial chemicals for the
purposes of statutory environmental risk assessment is very largely done on single
compounds (Chapter 2, Section 2.5). For reasons of practicality and cost, only a
minute proportion of the combinations of pollutants that occur in the natural envi-
ronment can be tested for their toxicity. This dilemma will be discussed further in
Section 13.3. A different and more complex situation exists, however, in the real
world; mixtures of pollutants are found in contaminated ecosystems, in effluents
discharged into surface waters, for example, sewage and industrial effluents, and in
waste waters from pulp mills. The tests or bioassays employed here usually measure
the toxicity expressed by mixtures, and investigators are presented with the prob-
lem of identifying the contributions of individual components of a mixture to this
toxicity. Simple toxicity tests/bioassays often establish the presence of toxic chemi-
cals without identifying the mechanisms by which toxicity is expressed. The issue
is further complicated by the possibility that naturally occurring xenobiotics, such
as phytoestrogens taken up by fish, may contribute significantly to the toxicity that
is measured.
In the simplest situation, chemicals in a mixture will show additive toxicity. If envi-
ronmental samples are submitted for both toxicity testing and chemical analysis, the
toxicity of the mixture may be estimated from the chemical data, to be compared
with the actual measured toxicity. As explained earlier for the estimation of dioxin
equivalents (Chapter 7, Section 7.2.4), the toxicity of each component of a mixture
may be expressed relative to that of the most toxic component (toxic equivalency fac-
tor or TEF). Using TEFs as conversion factors, the concentration of each component
can then be converted into toxicity units (toxic equivalents or TEQs) the summation of
which gives the predicted toxicity for the whole mixture. Often, the estimated toxicity
of mixtures of chemicals in environmental samples falls short of the measured toxicity.
Two major factors contribute to this underestimation of toxicity: first, failure to detect
certain toxic molecules (including natural xenobiotics), and second, the determination
by analysis of chemicals that are of only limited availability to free-living organisms,
as when there is strong adsorption in soils or sediments. In the latter case, analysis
overestimates the quantity of a chemical that is actually available to an organism.
Potentiation (synergism) between pollutants can also contribute to the underestimation
of toxicity when making calculations based on chemical analysis (see Doi, Chapter 12
in Volume 2 of Calow 1994). Sometimes, during the course of analysis, mixtures of
pollutants present in environmental samples are subjected to a fractionation procedure
in an attempt to identify the main toxic components. By a process of elimination, tox-
icity can then be tracked down to particular fractions and compounds.
The advantages of combining toxicity testing with chemical analysis when deal-
ing with complex mixtures of environmental chemicals are clearly evident. More
useful information can be obtained than would be possible if one or the other were
to be used alone. However, chemical analysis can be very expensive, which places a
limitation on the extent to which it can be used. There has been a growing interest
in the development of new, cost-effective biomarker assays for assessing the toxic-
ity of mixtures. Of particular interest are bioassays that incorporate mechanistic
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