User Guide to Thresholds and Classification - Environmental ...

332User Guide for Thresholds and Classificationschemical classification context therefore requires an evaluation of the intrinsic properties of the substance,as well as of the experimental conditions under which BCF has been determined. Based on the guide, adecision scheme for application of bioconcentration data or log KOW data for classification purposes hasbeen developed. The emphasis of the present section is organic substances and organo-metals.Bioaccumulation of metals is also discussed in section 19D.6.Data on bioconcentration properties of a substance may be available from standardised tests or may beestimated from the structure of the molecule. The interpretation of such bioconcentration data forclassification purposes often requires detailed evaluation of test data.See Appendix 19F for more detailed guidance.Interpretation of bioconcentration dataEnvironmental hazard classification of a chemical substance is normally based on existing data on itsenvironmental properties. Test data will only seldom be produced with the main purpose of facilitating aclassification. Often a diverse range of test data is available which does not necessarily match theclassification criteria. Consequently, guidance is needed on interpretation of existing test data in the contextof hazard classification.Bioconcentration of an organic substance can be experimentally determined in bioconcentrationexperiments, during which BCF is measured as the concentration in the organism relative to theconcentration in water under steady-state conditions and/or estimated from the uptake rate constant (k1) andthe elimination rate constant (k2) (OECD 305). In general, the potential of an organic substance tobioconcentrate is primarily related to the lipophilicity of the substance. A measure of lipophilicity is the n-octanol-water partition coefficient (K OW ) which, for lipophilic non-ionic organic substances, undergoingminimal metabolism or biotransformation within the organism, is correlated with the BCF. Therefore, K OW isoften used for estimating the bioconcentration of organic substances, based on the empirical relationshipbetween log BCF and log K OW . For most organic substances, estimation methods are available forcalculating the K OW . Data on the bioconcentration properties of a substance may thus be (i) experimentallydetermined, (ii) estimated from experimentally determined K OW , or (iii) estimated from KOW values derivedby use of Quantitative Structure Activity Relationships (QSARs). Guidance for interpretation of such data isgiven below together with guidance on assessment of chemical classes, which need special attention.Bioconcentration factorThe BCF is defined as the ratio on a weight basis between the concentration of the chemical in biota and theconcentration in the surrounding medium; here water, at steady state. The BCF can thus be experimentallyderived under steady-state conditions, on the basis of measured concentrations. However, the BCF can alsobe calculated as the ratio between the first-order uptake and elimination rate constants; a method whichdoes not require equilibrium conditions.Different test guidelines for the experimental determination of bioconcentration in fish have beendocumented and adopted; the most generally applied being the OECD test guideline (OECD 305, 1996).January 2012 EPA0109