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346User Guide for Thresholds and ClassificationsProlonged toxicity for fish and DaphniaCalculated values for chronic toxicity to fish and Daphnia should not be used to overrule classification basedon experimental acute toxicity data. Only a few validated models are available for calculating prolongedtoxicity for fish and Daphnia. These models are based solely on log KOW correlations and are limited in theirapplication to non-reactive, non-electrolyte organic compounds, and are not suitable for chemicals withspecific modes of action under prolonged exposure conditions. The reliable estimation of chronic toxicityvalues depends on the correct discrimination between non-specific and specific chronic toxicity mechanisms;otherwise, the predicted toxicity can be wrong by orders of magnitude. It should be noted that although formany compounds, excess toxicity in a chronic test correlates with excess toxicity in an acute test, this is notalways the case; where:Excess toxicity = (Predicted baseline toxicity)/Observed toxicity19D.6 Classification of metals and metal compoundsIntroductionThe harmonised system for classifying chemical substances is a hazard-based system, and the basis of theidentification of hazard is the aquatic toxicity of the substances, and information on the degradation andbioaccumulation behaviour (OECD, 1998). Since this document deals only with the hazards associated witha given substance when the substance is dissolved in the water column, exposure from this source is limitedby the solubility of the substance in water and bioavailability of the substance in species in the aquaticenvironment. Thus, the hazard classification schemes for metals and metal compounds are limited to thehazards posed by metals and metal compounds when they are available (that is, exist as dissolved metalions, for example, as M+ when present as M-NO3), and do not take into account exposures to metals andmetal compounds that are not dissolved in the water column but may still be bioavailable, such as metals infoods. This section does not take into account the non-metallic ion (for example, CN-) of metal compoundswhich may be toxic or which may be organic and may pose bioaccumulation or persistence hazards. Forsuch metal compounds the hazards of the non-metallic ions must also be considered.The level of the metal ion that may be present in solution following the addition of the metal and/or itscompounds, will largely be determined by two processes: the extent to which it can be dissolved, that is, itswater solubility, and the extent to which it can react with the media to transform to water soluble forms. Therate and extent at which this latter process, known as ‗transformation‘ for the purposes of this guidance,takes place can vary extensively between different compounds and the metal itself, and is an importantfactor in determining the appropriate hazard class. Where data on transformation are available, they shouldbe taken into account in determining the classification. The protocol for determining this rate is in Appendix19G.Generally speaking, the rate at which a substance dissolves is not considered relevant to the determinationof its intrinsic toxicity. However, for metals and many poorly soluble inorganic metal compounds, theJanuary 2012 EPA0109
347User Guide for Thresholds and Classificationsdifficulties in achieving dissolution through normal solubilisation techniques is so severe that the twoprocesses of solubilisation and transformation become indistinguishable. Thus, where the compound issufficiently poorly soluble that the levels dissolved following normal attempts at solubilisation do not exceedthe available L(E)C50, it is the rate and extent of transformation, which must be considered. Thetransformation will be affected by a number of factors, not least of which will be the properties of the mediawith respect to pH, water hardness, temperature, etc. In addition to these properties, other factors such asthe size and specific surface area of the particles which have been tested, the length of time over whichexposure to the media takes place and, of course the mass or surface area loading of the substance in themedia will all play a part in determining the level of dissolved metal ions in the water. Transformation datacan generally, therefore, only be considered as reliable for the purposes of classification if conductedaccording to the standard protocol in Appendix 19G.This protocol aims at standardising the principal variables such that the level of dissolved ion can be directlyrelated to the loading of the substance added. It is this loading level that yields the level of metal ionequivalent to the available L(E)C50 that can then be used to determine the hazard category appropriate forclassification. The strategy to be adopted in using the data from the testing protocol, and the datarequirements needed to make that strategy work, will be described.In considering the classification of metals and metal compounds, both readily and poorly soluble, recognitionhas to be paid to a number of factors. As defined in section 19D.3, the term ‗degradation‘ refers to thedecomposition of organic molecules. For inorganic compounds and metals, clearly the concept ofdegradability, as it has been considered and used for organic substances, has limited or no meaning.Rather, the substance may be transformed by normal environmental processes to either increase ordecrease the bioavailability of the toxic species. Equally, the log KOW cannot be considered as a measure ofthe potential to accumulate. Nevertheless, the concepts that a substance, or a toxic metabolite/reactionproduct may not be rapidly lost from the environment and/or may bioaccumulate are as applicable to metalsand metal compounds as they are to organic substances.Speciation of the soluble form can be affected by pH, water hardness, and other variables, and may yieldparticular forms of the metal ion that are more or less toxic. In addition, metal ions could be made nonavailablefrom the water column by a number of processes (for example, mineralisation and partitioning).Sometimes these processes can be sufficiently rapid to be analogous to degradation in assessing chronicclassification. However, partitioning of the metal ion from the water column to other environmental mediadoes not necessarily mean that it is no longer bioavailable, nor does it mean that the metal has been madepermanently unavailable.Information pertaining to the extent of the partitioning of a metal ion from the water column, or the extent towhich a metal has been or can be converted to a form that is less toxic or non-toxic is frequently notavailable over a sufficiently wide range of environmentally relevant conditions, and thus, a number ofassumptions will need to be made as an aid in classification. These assumptions may be modified ifavailable data show otherwise. In the first instance it should be assumed that the metal ions, once in thewater, are not rapidly partitioned from the water column and thus these compounds do not meet the criteria.January 2012 EPA0109
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