2-methylbutane

Short-term toxicity to aquatic invertebrates

Description of key information

The estimated freshwater invertebrate 48 hour EL50 value is 59.44mg/l based on mobility. This is supported by a measured value: 48hr EC50 for Daphnia magna of 2.3mg/l

Key value for chemical safety assessment

Fresh water invertebrates

Fresh water invertebrates

Effect concentration:

2.3 mg/L

Additional information

The aquatic toxicity was also 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 (K_MW) that describe the partitioning of the hydrocarbons between the water and organism. The estimated freshwater invertebrate 48 hour EL50 value is 59.44mg/l based on mobility.

The study for 2-methylbutane (Adema and Bakker 1986) is equivalent to OECD 202, using a static, closed system with low headspace, a standard test species (Daphnia magna) and the standard exposure duration (48hrs). Two marine crustaceans were also used in the study, Chaetogammarus marinus and Mysidopsis bahia. The tests with marine crustaceans were carried out with only one animal per container (10 individuals per concentration). LC₅₀values for both species were based on the initial measured concentration. C marinus LC 50 (96 hours) = 3.2 mg/l (initial measured concentration) and M bahia = 3.2 mg/l (initial measured concentration). The most sensitive result was reported for Daphnia magna. The Daphnia magna EC₅₀(48 hours) for 2 -methylbutane = 2.3 mg/l (based on initial measured concentration).

 

The true EC50 value maybe higher since loss of the starting material was determinedto be about 50% during the 48hr exposure period.This point is highlighted in the report itself butthe version of the study reviewed lacked the annex containing key details on the test conditions and therefore these could not be reviewed and generally validated against OECD criteria.However the study was selected as key study in the EU Risk assessment report for n-pentane and can be considered acceptable for assessment. Reanalysis of the original data is reported in the EU RAR. They state that reanalysis of the data using mean exposure concentration would probably give somewhat lower values by a factor of 0.88 -0.51. The Adema report does not allow exact recalculation but based on the available information the EC₅₀/LC₅₀would be 2.4, 1.4 and 1.8mg/l for D. magna, C.marinus and M.bahia respectively.

 

This is supported by a QSAR calculation, with the 48 hr LC50 estimated to be 8.06 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.

This result would not indicate that 2 -methylbutane would meet the criteria for toxic in the PBT assessment. However, the 48 hour EC50 of 2.3 mg/l would result in a classification of acute category 2 with the hazard phrase 'toxic to aquatic life'.

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.

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