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Toxicity to aquatic algae and cyanobacteria

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Description of key information

The estimated algal, 72-hr EL50 value for this substance is 25.12mg/L based on growth inhibition.The estimated algal, 72-hr NOEL value for this substance is 5.62mg/L based on growth inhibition. This is supported by read across within the category from n-pentane, which gives an EC50 based on growth rate of 10.7 mg/l and a NOEC of 2.04 mg/l.

Key value for chemical safety assessment

Additional information

The aquatic toxicity was estimated using the Petrotox computer model, which combines a partitioning model used to calculate the aqueous concentration of hydrocarbon components as a function of substance loading with the Target Lipid Model used to calculate acute and chronic toxicity of non-polar narcotic chemicals. Petrotox computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a hydrocarbon substance and membrane-water partition coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism. The estimated algal, 72-hr EL50 value for this substance is 25.12mg/L based on growth inhibition.The estimated algal, 72-hr NOEL value for this substance is 5.62mg/L based on growth inhibition.

The toxicity of 2 -methylbutane to algae has been read across within the category from n-pentane. The key study (ExxonMobil Chemical 1997) follows the standard guideline and is GLP compliant, with only minor deviations which are not believed to have effected the test performance, the effect concentrations for 2 -methylbutane are therefore considered to be: EC 50 growth rate = 10.7 mg/l, and NOEC growth rate = 2.04 mg/L.

This is supported by a QSAR calculation. The toxicity of 2 -methylbutane is estimated to be 96hr EC50: 5.2 mg/l. The ECOSAR model is a reliable and appropriate QSAR model to apply to cyclopentane as it is based on a related chemical dataset that calculates the toxicity of neutral organic hydrocarbons whose mode of action is non-polar narcosis. The endpoint calculated here is therefore a reasonable estimate of its baseline toxicity.

The hydrocarbon block method has been used for environmental risk assessment (see REACH guidance, R7, app.13-1). Aquatic PNECs for hydrocarbon blocks have been derived using the HC5 statistical extrapolation method and the target lipid model using representative structures. See Product Library tab in PETRORISK spreadsheet attached to IUCLID Section 13. Given the large database of freshwater organisms included in the target lipid model, an assessment factor of one has been applied to the HC5.