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The registered substance MEA-LAS easily dissociates into MEA (cation) and LAS (anion) ions in aqueous conditions. The toxicity of MEA-LAS can be explained by the toxicity of the main contributor LAS (the driver of toxicity) combined with the effects of the dissociated MEA. The MEA-LAS registration makes use of data on MEA-LAS (target material), as well as data on the dissociated subunits MEA and LAS (source substances), as summarised in Sections 7.1.1 to 7.1.4.

 LAS is a highly studied compound, with a well-defined narcotic toxic mode of action. In fact, LAS has been used as an example compound to illustrate the narcosis Adverse Outcome Pathway (Volz et al., 2011). It is well documented that the toxicity of LAS increases with increasing alkyl chain length. Chain length is also a surrogate for hydrophobicity, much like log P is a surrogate for hydrophobicity. Predictably, log P increases with increasing carbon chain length. Following the narcosis AOP, increased chain length/hydrophobicity leads to an increase in binding and disruption of cellular membranes, ultimately leading to mortality.

Numerous studies have shown that the contribution of each carbon chain length is proportional to its intrinsic toxicity and as the fraction present in the mixture. Thus, a mixture containing a broad range of homologues can be readily assessed. The average chain length well predicts the overall toxicity of the mixture. The available chronic toxicity data on LAS spans a variety of mixtures which vary in their composition of alkyl chains. This data can be normalized to a single representative chain length using LAS-specific (Q)SARs. 

The following paragraphs describe how the data for LAS was normalized to C11.6 LAS. Relevant chronic endpoint results were then used for a species sensitivity distribution analysis (SSD) to calculate an HC5 and determine whether the data is ordered and adheres to statistical assumptions.

The normalisation process and the importance of the SSD results to support the establishment of an overall assessment factor for aquatic PNEC calculation are detailed inSection 7.6.1. 

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