Building on an understanding of how individual substances are classified, it is now possible to consider how one should classify a mixture of substances according to the new criteria for Environmentally Hazardous Substances (EHS).  Criteria for classification of mixtures are described in 2.9.3.4 in Amendment 34-08 of the IMDG Code. 

An important rule to keep in mind when classifying mixtures according to the EHS criteria is that components present in concentrations of less than 1% may be disregarded for classification purposes unless there is reason to believe that such a component could significantly affect the mixture classification.  For example, this could be the case if such a component was highly toxic. If a low concentration component was present at just below 1% and was toxic to fish, daphnia or algae at 0.01 mg/1, that component by itself would cause the overall toxicity of the mixture to be on the order of 1 mg/l even if all other components were nontoxic.

The new provisions specify a tiered approach to mixture classification.  To properly classify a mixture one should work through the various tiers according to data available on the mixture and its components.  The tiers are:

classification according to data available for the mixture itself;
classification based on similar mixtures using bridging principles;
classification by calculation when data is available for all ingredients; and
classification using available data of known ingredients.

The use of chronic toxicity, particularly the use of surrogate data supporting chronic toxicity classification, complicates mixture classification considerably.  In some cases it is necessary to classify a mixture using approaches in different tiers.

Classification According to Mixture Data
As is the case in classifying mixtures for other hazards (e.g., toxicity), mixture classification should first be attempted on the basis of actual data on the mixture. Classifying on the basis of acute toxicity data is straightforward.  If the mixture has an acute toxicity of 1 mg/l or less it is classified as Acute I.  As is the case in classifying substances, once it is known that the mixture is classified as an EHS on the basis of acute toxicity there is no need to consider the chronic toxicity of the mixture from a classification perspective.  Whether the mixture does or does not meet the chronic toxicity criteria will not affect the classification of the mixture already known to meet Acute I criteria.

When actual chronic toxicity data on the mixture is unavailable and chronic toxicity can only be assessed on the basis of surrogate data (i.e., bioaccumulation and biodegradation), data on the mixture itself cannot be used for the chronic classification.  It is the mixture components that biodegrade or bioaccumulate, not the mixture as a whole (see 2.9.3.4.3.1).


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