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Ecotoxicological information

Short-term toxicity to fish

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Description of key information

The estimated freshwater fish 96 hour LL50 value is 34.05mg/l based on mortality. This is supported by an LC50 (96 hour) of 4.26 mg/l based on mean measured concentrations for n-pentane.

Key value for chemical safety assessment

LC50 for freshwater fish:
4.26 mg/L

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 partitioning coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism. The estimated freshwater fish 96 hour LL50 value is 34.05mg/l based on mortality.

There was no measured data available for 2 -methylbutane so read across to n-pentane was applied.The supporting study (ExxonMobil Chemical 1997) was conducted on n-pentane according to OECD Guideline 203 (Fish, Acute Toxicity Test) with some minor deviations to minimise volatilisation of the substance.During testing a decrease of 80 % of n-pentane concentration was observed so geometric mean of the measured concentrations were used and a 95 % confidence limit was established. The 96 hour LC50was reported as 4.26 mg/l (95% C.I 3.60 - 5.04 mg/l).

This is supported by a QSAR calculation, which gives an estimated 96hr LC50 of 12.8 mg/l. The ECOSAR model is a reliable and appropriate QSAR model to apply to 2 -methylbutane 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.