User Guide to Thresholds and Classification - Environmental ...

394User Guide for Thresholds and ClassificationsRationaleThe toxicity criteria for the soil hazard classification categories differ by a factor of 10 from one category toanother. Substances with a classification in a high toxicity band may, therefore, contribute to theclassification of a mixture in a lower band. The calculation of these classification categories, therefore, needsto consider the contribution of all substances that are classified for toxicity to the soil environment.When components are classified as 9.2A and their acute toxicity is well below the cut-off value (that is, 30 days.If the mixture is classified as 9.2C using the summation approach and the weighted sum of components witha soil half-life > 30 days is 30 days or with no data ondegradation. See Table 20.6 for a worked example.The steps to follow in applying the summation approach to soil hazard classification are set out below andsummarised in Table 20.4 and Figure 20.2.Mixtures with no highly toxic componentsThe steps to follow in applying the summation approach to soil hazard classification for mixtures with nohighly toxic components are set out below.Step 1: Consider all components classified as 9.2A.A mixture is classified as 9.2A if the sum of the components is ≥ 25%, and then the classification processis complete.Step 2: Where the mixture is not classified as 9.2A, consider classification of the mixture as 9.2B.A mixture is classified as 9.2B if:∑(9.2A)% x 10 + ∑ (9.2B)% ≥ 25%If so, the classification process is complete.Step 3: Where the mixture is not classified as 9.2A or 9.2B, consider classification of the mixture as 9.2C.January 2012 EPA0109