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Measured ecotoxicity data are not available for the aquatic toxicity endpoints. In accordance with column 2 of REACH Annex VII and VIII, the acute toxicity tests do not need to be conducted if there are mitigating factors indicating that aquatic toxicity is unlikely to occur. Butenes are gases at standard temperature and pressure and are expected to partition primarily to air; therefore aquatic toxicity tests may not be relevant. In addition, it is technically difficult to maintain aqueous concentrations of gases for toxicity testing. Therefore, in the absence of measured data, the toxicity of butenes has been estimated using a suitable QSAR (Nabholz and Mayo-Bean 2009). The equation used is suitable for non-polar narcotic chemicals. The endpoints calculated here are therefore reasonable estimates of their baseline toxicity.