User Guide to Thresholds and Classification - Environmental ...

291User Guide for Thresholds and ClassificationsNote: L(E)C 50 = median lethal concentration or median effective dose19.3. Classification of mixturesTo make use of all available data for classifying the aquatic environmental hazards of the mixture, thefollowing assumption has been made and should be applied where appropriate.The ‗relevant components‘ of a mixture are those that are present in a concentration of 1% (weight/weight) orgreater, unless there is a presumption (for example, in the case of highly toxic components) that acomponent present at less than 1% can still be relevant for classifying the mixture for aquatic environmentalhazards.The approach for classifying aquatic hazards is tiered, and depends on the type of information available forthe mixture itself and for its components. Elements of the tiered approach include classification based on:tested mixtures (see section 19.3.1);bridging principles (see section 19.3.2); andthe summation approach, using the classifications of components (see section 19.3.3).19.3.1. Tested mixturesFor aquatic hazard classification, the test data on the mixture can be used directly to assign a classificationto a substance on the basis of acute toxicity, as indicated in the examples in Table 19.2, with additionalconsideration given to whether the components of the mixture are not rapidly degradable and/or arepotentially bioaccumulative.Where components of the mixture are acutely toxic and either are not rapidly degradable or arebioaccumulative, or in the absence of data on these properties, the concentrations of components with theseproperties are weighted and summed to determine the classification of the mixture. Where the weighted sumof these components is ≥ 25% the more conservative classification applies.To calculate the weighted sum of the components which are not rapidly degradable and/or arebioaccumulative, use the summation approach set out in section 19.3.3 and see the worked example inTable 19.5. and accompanying text.Table 19.2: Aquatic classification of a tested mixtureAcute L(E)C 50 of thetested mixtureChronic NOEC oftested mixture1 mg/L 1 mg/L or unknown1 mg/L > 1 mg/LComponents inmixture are notrapidly degradableand/or arebioaccumulativeNot considered for 9.1AclassificationClassificationcategory9.1A> 1 and 10 mg/L 1 mg/L or unknown Yes or unknown 9.1B*January 2012 EPA0109