User Guide to Thresholds and Classification - Environmental ...

322User Guide for Thresholds and Classificationstransformed by normal environmental processes to either increase or decrease the bioavailability of the toxicspecies. Therefore, the present section deals only with organic substances and organo-metals. Section19D.6 provides detailed guidance on assessing the transformation of metals in the aquatic environment.Data on degradation properties of a substance may be available from standardised tests or from other typesof investigations, or they may be estimated from the structure of the molecules. The interpretation of suchdegradation data for classification purposes often requires detailed evaluation of the test data. Guidance isgiven in the present section and more details can be found in Appendix 19E.Interpretation of degradability dataRapid degradabilityAquatic hazard classification of chemical substances is normally based on existing data on theirenvironmental properties. Only seldom will test data be produced with the main purpose of facilitating aclassification. Often a diverse range of test data is available that does not necessarily fit directly with theclassification criteria. Consequently, guidance is needed on interpretation of existing test data in the contextof the aquatic hazard classification. Guidance for interpretation of degradation data is set out below for thethree types of data indicated in the HSNO Act definition of ‗rapid degradation‘ in the aquatic environment.Ready biodegradabilityReady biodegradability is defined in the OECD Test Guideline 301). All organic substances that degrade to alevel higher than the pass level in a standard OECD ready biodegradability test or in a similar test should beconsidered readily biodegradable and consequently also rapidly degradable. Many literature test data,however, do not specify all of the conditions that should be evaluated to demonstrate whether or not the testfulfils the requirements of a ready biodegradability test. Expert judgement is therefore needed as regards thevalidity of the data before use for classification purposes. Before concluding on the ready biodegradability ofa test substance, however, at least the following parameters should be considered.Concentration of test substanceRelatively high concentrations of test substance are used in the OECD ready biodegradability tests (2–100mg/L). Many substances may, however, be toxic to the inocula at such high concentrations causing a lowdegradation in the tests although the substances might be rapidly degradable at lower non-toxicconcentrations. A toxicity test with micro-organisms (as, for example, the OECD Test Guideline 209‗Activated Sludge, Respiration Inhibition Test‘, the International Organization for Standardization (ISO) 9509nitrification inhibition test, or the ISO 11348 luminescent bacteria inhibition test) may demonstrate the toxicityof the test substance. When it is likely that inhibition is the reason for a substance being not readilydegradable, results from a test employing lower non-toxic concentrations of the test substance should beused when available. Such test results could on a case by case basis be considered in relation to theclassification criteria for rapid degradation, even though surface water degradation test data withenvironmentally realistic microbial biomass and non toxic realistic low concentration of the test substance ingeneral are preferred, if available.January 2012 EPA0109