User Guide to Thresholds and Classification - Environmental ...

283User Guide for Thresholds and Classificationsbased on reduction in biomass is available, or it is not indicated which EC50 is reported, these values maybe used.Ideally, data on all three standard taxa will be available for classification purposes, with classification basedon the most sensitive test result.19.1.2. Chronic aquatic toxicityChronic effects usually include a range of sublethal endpoints and are generally expressed in terms of a noobservable effect concentration (NOEC) or an equivalent ECx. Endpoints typically include survival, growth,and/or reproduction. Exposure durations vary widely, depending on the endpoint being assessed and thetest species being used.Chronic toxicity data are generally less available than are acute data, so for classification purposes thepotential for chronic or long-term toxicity is often assessed through a combination of acute toxicity, lack ofrapid degradability, and potential or actual bioaccumulation. Where chronic data do exist, these are takeninto account in assigning a classification to a substance.Use of chronic data to reduce a classificationWhere a substance is acutely toxic, not rapidly degradable, and/or has potential to bioconcentrate, chronictest data > 1 mg/L can be used to ‗de-classify‘ or reduce the classification of a substance from 9.1B or 9.1Cto the less restrictive classification of 9.1D. Several aspects of the chronic data must be considered before aclassification can be reduced.The general approach is to demonstrate the chronic NOEC > 1 mg/L for the most sensitive species identifiedby the acute toxicity data. For example, if the classification has been applied on the basis of acute toxicity tofish, it would generally not be possible to reduce the classification based on a NOEC for an aquaticinvertebrate. If the classification has resulted from acute toxicity to more than one taxa, a NOEC > 1 mg/L foreach would be needed to reduce the classification.Tests with algae and Lemna cannot be used to reduce the classification of a substance because the:duration of the studies is not long term;acute to chronic ratio is generally narrow; andendpoints are more consistent with those for other organisms.Combined acute and chronic classificationsWhile the current HSNO Act classification system will continue to rely on the use of acute toxicity data incombination with a lack of rapid degradation and/or a potential to bioaccumulate as the basis for aquatichazard classification, actual chronic toxicity data form a better basis for classification where these data areavailable. The OECD is developing a chronic aquatic hazard classification system based on chronic aquatictest data. It is anticipated that under the United Nations Globally Harmonized System of Classification andLabelling of Chemicals (GHS) (United Nations, 2007) the available chronic toxicity data would be used toassign a chronic hazard classification in preference to that derived from acute toxicity in combination with aJanuary 2012 EPA0109