Frameworks for the application of toxicity data

Maria Consuelo Diaz-Baez and Bernard J. Dutka 

Because industrial effluents, emissions and toxicants such as pesticides, car exhausts, landfill site leachates, etc. affect life forms at different levels and in many ways, it has become accepted practice that single or multitrophic toxicity 

Toxic action is concentration dependent. For example, phenol can be metabolized at low concentrations but becomes toxic at higher concentrations. These types of actions often lead to the occurrence of a hormesis effect, i.e. the stimulating effect of small doses of a toxicant that is known to be inhibitory at larger concentrations. Toxicant action also depends on the presence of other chemicals in solution (Dutka and Kwan, 1982). 

However, it is believed that one of the most important effects of the toxic action of chemicals is on enzyme activity (Iverson and Brinckman, 1978). Also, in any toxicity study one must take into account the physicochemical factors (presence of other cations, pH, oxidation-reduction potential, temperature, organic matter, clay particles, etc.) that control the toxic action towards organisms (Sprague, 1985 Brezonik et al., 1991). 

From this brief overview of toxicity tests, their uses and methods of assessing their responses to toxic substances, it can be seen that this is a very complex topic with many unknowns and no single best way of addressing the problem of bioavailable toxicant estimation. In this chapter, an attempt will be made to describe some applications of toxicological data, how they could be analysed and how toxicity results could be integrated in ecological risk assessment frameworks. 

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A wide-ranging compilation of techniques, Extrapolation Practice for Ecotoxicological Effect Characterization of Chemicals describes methods of extrapolation in the framework of ecological risk assessment. The book, informally known as EXPECT, identifies data needs and situations where these extrapolations can be most usefully applied, making it a practical guide to the application of extrapolation procedures. It focuses on the extrapolation of chemical effects and covers the extrapolation of exposures in the context of interactions between toxicants and the matrix. 

For sublethal responses, the level of resource allocation is essential. The DEB approach offers great promise as a TD model in ecotoxicology. It has been applied to the combination of a toxicant with another stressor (food limitation), but its application to mixtures of toxicants requires further work, and a comparison to dedicated experimental data. The EU sixth framework project NoMiracle has delivered such data, which in time will help develop the DEB mixtures approach to also cover sub-lethal endpoints. 

Application of the HRF increases the transparency of delineation of the relative degrees of nncertainty associated with various options for considraation in assessment of risk for impacted populations. HRFs are also instrumental in acquiring transparency on critical data gaps that will further reduce uncertainty. They force distinction of choices made on the basis of science policy versns those that are science jndgment related, inclnding reliance on default, based on the erroneous premise that it is always health-protective (Meek and Doull 2009). They focus on early events in a toxicity pathway through relation of early perturbations to apical endpoints in frameworks designed to systematically address (a) consideration of key events in modes of action and (b) their snbseqnent implications for dose-response in risk assessment [see, for example. Meek (2008)]. 

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