User Guide to Thresholds and Classification - Environmental ...

333User Guide for Thresholds and ClassificationsExperimentally derived BCF values of high quality are ultimately preferred for classification purposes as suchdata override surrogate data, for example, K OW .High quality data are defined as data where the validity criteria for the test method applied are fulfilled anddescribed, for example, maintenance of constant exposure concentration; oxygen and temperaturevariations, and documentation that steady-state conditions have been reached, etc. The experiment will beregarded as a high-quality study, if a proper description is provided (for example, by Good LaboratoryPractice (GLP)) allowing verification that validity criteria are fulfilled. In addition, an appropriate analyticalmethod must be used to quantify the chemical and its toxic metabolites in the water and fish tissue.BCF values of low or uncertain quality may give a false and too low BCF value; for example, application ofmeasured concentrations of the test substance in fish and water, but measured after a too short exposureperiod in which steady-state conditions have not been reached (compare with OECD 306, 1996, regardingestimation of time to equilibrium). Therefore, such data should be carefully evaluated before use andconsideration should be given to using K OW instead.If there is no BCF value for fish species, high-quality data on the BCF value for other species may be used(for example, BCF determined on blue mussel, oyster, or scallop (ASTM E 1022-94)). Reported BCFs formicroalgae should be used with caution.For highly lipophilic substances, for example, with log K OW above 6, experimentally derived BCF values tendto decrease with increasing log K OW . Conceptual explanations of this non-linearity mainly refer to eitherreduced membrane permeation kinetics or reduced biotic lipid solubility for large molecules. A lowbioavailability and uptake of these substances in the organism will thus occur. Other factors compriseexperimental artefacts, such as equilibrium not being reached, reduced bioavailability due to sorption toorganic matter in the aqueous phase, and analytical errors. Special care should thus be taken whenevaluating experimental data on the BCF for highly lipophilic substances as these data will have a muchhigher level of uncertainty than BCF values determined for less lipophilic substances.Bioconcentration factor in different test speciesBCF values used for classification are based on whole body measurements. As stated previously, theoptimal data for classification are BCF values derived using the OECD 305 test method or internationallyequivalent methods, which uses small fish. Due to the higher gill surface to weight ratio for smallerorganisms than larger organisms, steady-state conditions will be reached sooner in smaller organisms thanin larger ones. The size of the organisms (fish) used in bioconcentration studies is thus of considerableimportance in relation to the time used in the uptake phase, when the reported BCF value is based solely onmeasured concentrations in fish and water at steady-state. Thus, if large fish, for example, adult salmon,have been used in bioconcentration studies, it should be evaluated whether the uptake period wassufficiently long for steady state to be reached or to allow for a kinetic uptake rate constant to be determinedprecisely.Furthermore, when using existing data for classification, it is possible that the BCF values could be derivedfrom several different fish or other aquatic species (for example, clams) and for different organs in the fish.January 2012 EPA0109