Toxicants ecological effects

PCDDs and PCDEs, together with coplanar PCBs, can express Ah-receptor-mediated toxicity. TCDD (dioxin) is used as a reference compound in the determination of TEFs, which can be used to estimate TEQs (toxic equivalents) for residues of PHAHs found in wildlife samples. Biomarker assays for Ah-receptor-mediated toxicity have been based on the induction of P450 lAl. TEQs measured in field samples have sometimes been related to toxic effects upon individuals and associated ecological effects (e.g., reproductive success). 

EPA Ecological Effects Test Guidelines, OPPTS 850.1400 Fish Early-Life Stage Toxicity Test, 1996. 

These relationships allow for screening and ranking of toxicity so that the least toxic option may be used if deemed appropriate. They are applied in many jurisdictions for regulatory use in the prediction of ecological effects (and fate) of chemicals when there are no actual toxicity data and decisions need to be made about their use [99]. QSARs have been developed, for example, to predict which chemicals may exhibit persistence, bioaccumulation, and toxicity (PBT) properties, or be very persistent and very bioaccumulative (vPvB) [99]. These methods have been applied to the prediction of chemicals that fall under the European REACH initiative and also high production volume (HPV) chemicals [99]. Currently available QSARs for predicting a compound s fall into two general classes those that have been developed for a nonspecific mode of action, and those that have been developed for specific types or classes of chemicals [99]. 

U.S. EPA (U. S. Environmental Protection Agency) (1996b) Ecological Effects Test Guidelines. OPPTS 850.1055. Bivalve Acute Toxicity Text (Embiyo-Larval). U.S. EPA, Prevention, Pesticides, and Toxic Substances. EPA 712-C-96-160. 

Metal speciation procedures, which have been verified under controlled laboratory conditions and evaluated by means of bioassays, will require further verification in order to determine their ecological effects. For example, how does the response of the bioassay test species to a toxic metal fraction relate to the toxicity to larger organisms such as fish in the natural environment Bioaccumulation of metals in populations has been very difficult to relate to metal speciation measurements. There is a challenge for analytical chemists to develop metal speciation procedures that are relevant to ecotoxicology (Morrison and Wei, 1991). 

The EPA uses QSARs to predict a large number of ecological effects, as well as for environmental fate within the PMN process. The EPA s website (www.epa.gov) provides a valuable source of further information on all these predictive methods, as well as a database and aquatic toxicity values and detailed information on how the models have been validated. Many of the predictive models have been brought together into the EPISUITE software (see Table 19.2 for a listing of the models available). This includes the OPPT s models used for the prediction of physical and chemical properties for new chemical substances. The EPISUITE software is downloadable free of charge (www.epa.gov/oppt/exposure/docs/episuitedl.htm). This provides not only an excellent resource for the development of QSARs, but also a transparent mechanism for the assessment of PMNs. 

Common Whole Mixtures There are few systematic studies of mixtures that are strictly based on the approach of the mixture of concern or similar mixtures as defined under human risk assessment of mixtures. Most ecological effect studies have more characteristics in common with a component-based or unique whole mixture approach than with the common mixture approach. A rare example of the common whole mixture approach in ecological risk assessment is the hydrocarbon block method. In this case, mixture effects are predicted on the basis of partial characterization of hydrocarbon mixtures. The hydrocarbon block method is used to determine the risks of a total hydrocarbon mixture on the basis of discriminating different chain length fractions of hydrocarbons, for each of which toxicities are known (King et al. 1996). 

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