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312User Guide for Thresholds and ClassificationsAquatic toxicity conceptsThis section addresses the use of acute and chronic toxicity data in classification, and special considerationsfor exposure regimes, algal toxicity testing, and use of Quantitative Structure Activity Relationships (QSARs).For a more detailed discussion of aquatic toxicity concepts, see Rand (1995).Acute toxicityAcute toxicity for purposes of classification refers to the intrinsic property of a substance to be injurious to anorganism in a short-term exposure to that substance. Acute toxicity is generally expressed in terms of aconcentration which is lethal to 50% of the test organisms (LC50), causes a measurable adverse effect to50% of the test organisms (EC 50 , for example, immobilisation of daphnids), or leads to a 50% reduction intest (treated) organism responses from control (untreated) organism responses (EC 50 , for example, growthrate in algae).Chronic toxicityChronic toxicity, for purposes of classification, refers to the potential or actual properties of a substance tocause adverse effects to aquatic organisms during exposures that are determined in relation to the life-cycleof the organism. Such chronic effects usually include a range of sublethal endpoints and are generallyexpressed in terms of a no observable effect concentration (NOEC), or an equivalent EC x . Observableendpoints typically include survival, growth and/or reproduction. Chronic toxicity exposure durations can varywidely depending on test endpoint measured and test species used.Since chronic toxicity data are less common in certain sectors than acute data, for classification schemes,the potential for chronic toxicity is identified by appropriate combinations of acute toxicity, lack ofdegradability, and/or the potential or actual bioaccumulation. Where such data exist and show long-termNOEC > 1 mg/L, this can be taken into account when deciding whether the classification based on the acutedata should be applied. In this context, the following general approach should be used. In order to remove a9.1B or 9.1C classification, it must be demonstrated that the NOEC used would be suitable in removing theconcern for all taxa that resulted in classification. This can often be achieved by showing a long-term NOEC> 1 mg/L for the most sensitive species identified by the acute toxicity. Thus, if a classification has beenapplied based on a fish acute LC50, it would generally not be possible to remove this classification using along-term NOEC from an invertebrate toxicity test. In this case, the NOEC would normally need to be derivedfrom a long-term fish test of the same species or one of equivalent or greater sensitivity. Equally, ifclassification has resulted from the acute toxicity to more than one taxa, it is likely that NOECs > 1 mg/L fromeach taxa will need to be demonstrated. In case of classification of a poorly soluble substance as 9.1D, it issufficient to demonstrate that NOECs are greater than the water solubility of the substances underconsideration.Testing with algae/Lemna cannot be used for de-classifying chemicals because (1) the algae and Lemnatests are not long-term studies, (2) the acute to chronic ratio is generally narrow and (3) the endpoints aremore consistent with the endpoints for other organisms. However, where classification is applied solely dueto the acute toxicity (L(E)C 50 ) observed in single algae/aquatic plant tests, but there is evidence from a rangeJanuary 2012 EPA0109
313User Guide for Thresholds and Classificationsof other algae tests that the chronic toxicity (NOECs) for this taxonomic group is above 1 mg/L, this evidencecould be used to consider declassification. At present, this approach cannot be applied to aquatic plantssince no standardised chronic toxicity tests have been developed.The GHS is intended to contain a specific value of chronic toxicity below which substances would beclassified as chronically toxic, but the criteria are not yet set.Exposure regimesFour types of exposure conditions are employed in both acute and chronic tests and in both freshwater andsaltwater media: static, static-renewal (semi-static), recirculation, and flow-through. The choice for which testtype to use usually depends on test substance characteristics, test duration, test species, and regulatoryrequirements.Test media for algaeAlgal tests are performed in nutrient-enriched media and use of one common constituent, EDTA, or otherchelators, should be considered carefully. When testing the toxicity of organic chemicals, trace amounts of achelator like EDTA are needed to complex micronutrients in the culture medium; if omitted, algal growth canbe significantly reduced and compromise test utility. However, chelators can reduce the observed toxicity ofmetal test substances. Therefore, for metal compounds, it is desirable that data from tests with highconcentration of chelators and/or tests with stoichiometrical excess of chelator relative to iron be criticallyevaluated. Free chelator may mask heavy metal toxicity considerably, in particular with strong chelators likeEDTA. However, in the absence of available iron in the medium, the growth of algae can become ironlimited, and consequently data from tests with no or with reduced iron and EDTA should be treated withcaution.Use of Quantitative Structure Activity RelationshipsFor purpose of classification, and in the absence of experimental data, QSARs can be relied upon to providepredictions of acute toxicity for fish, Daphnia, and algae for non-electrolyte, non-electrophilic, and otherwisenon-reactive substances (See section 19D.5 on the use of QSARs.) Problems remain for substances suchas organophosphates that operate by means of special mechanisms such as functional groups whichinteract with biological receptors, or which can form sulfhydryl bonds with cellular proteins. Reliable QSARshave been derived for chemicals acting by a basic narcosis mechanism. These chemicals arenonelectrolytes of low reactivity such as hydrocarbons, alcohols, ketones and certain aliphatic chlorinatedhydrocarbons that produce their biological effects as a function of their partition coefficients. Every organicchemical can produce narcosis. However, if the chemical is an electrolyte or contains specific functionalgroups leading to non-narcotic mechanisms as well, any calculations of toxicity based on partition coefficientalone would severely underestimate the toxicity. QSARs for acute aquatic toxicity of parent compoundscannot be used to predict the effects of toxic metabolites or degradates, when these arise after a longerperiod than the duration of acute tests.January 2012 EPA0109
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