Butene

Short-term toxicity to fish

Description of key information

Members of this category are gases at standard temperature and pressure, therefore conducting an aquatic ecotoxicity test is technically difficult and based on exposure estimates the test may not be relevant. Therefore, the use of a QSAR to predict the ecotoxicity of representative components of these streams is an appropriate technique for aquatic toxicity. In addition, it is difficult to assess the toxicity of UVCB streams using standard test methods. Instead, it is more appropriate to consider the toxicity of representative components of these streams. As such, as the category is made up of C4 hydrocarbons, representative hydrocarbon C4 substances butene (CAS No. 25167 -67 -3) but-1 -ene (CAS 106 -98 -9) and 2 -methylpropene 115 -11 -7 (Cas No. 115 -11 -7) have been used as read-across.

The estimated EL50 TLM calculated values for short term toxicity to fish are: for butene / but-1 -ene 14.758 mg/L, for 2 -butene 17.173 mg/L and for 2 -methylpropene 16.805 mg/L. These results are used as key read across. The lowest value has been taken forward as the key value for chemical safety assessment.

The calculated freshwater fish 96 hour LC50 for butenes range from 29 to 38 mg/L. This range was derived from the members of the butenes category: but-1-ene (CAS 106-98-9), 2-butene (CAS 107-01-7) and 2-methylpropene (CAS 115-11-7) using QSAR estimates as supporting read-across (Nabholz and Mayo-Bean 2009).

The use of ECOSAR is appropriate for this Category as these streams falls within the applicability domain of the model. The calculated 96 hour LC50 values for representative substances within this Category showed a short term toxicity to fish range of 19 - 27.9 mg/L These values have been used as supporting read across for this category.

Key value for chemical safety assessment

Fresh water fish

Fresh water fish

Effect concentration:

14.758 mg/L

Additional information

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.

Measured data are not available for butenes for this endpoint. Due to the practical difficulties associated with the ecotoxicity testing of gases (i.e. maintaining exposure concentrations), the use of QSAR toxicity estimatesis an appropriate alternative. The ECOSAR model is a reliable and appropriate QSAR model to apply to butenes 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 the baseline toxicity.

A QSAR (ECOWIN v1 ECOSAR, Nabholz and Mayo-Bean 2009) has been used to give an indication of the toxicity of butenes to freshwater fish and used in supporting read across. The calculated 96 hour LC50 are 32.5 mg/L for but-1-ene, 38.2 mg/L for 2-butene and 28.9 mg/L for 2-methylpropene.

The TLM model has been used as key QSAR. The target lipid model (TLM) is a framework that relates toxicity to the physicochemical properties of  a nonpolar organic constituent. It is a well documented and peer reviewed model that is widely used across the petrochemicals industry. This substance fits within the criteria of the model and there are no reservations about the validity of the model runs. It is expected that this data is reliable with restrictions.

The endpoint calculated is the HC5 - The narcosis Target Lipid model (TLM) is used to develop a species sensitivity distribution, from which the HC5 is calculated and covers aquatic organisms. The range of organisms (47 species) used in the training set are known and include plants, invertebrates and fish. The narcosis target lipid model was developed to predict the toxicity of chemicals to aquatic organisms that act via narcosis. It is based on the hypothesis that target lipid is the site of toxic action within the organism, that octanol is the appropriate surrogate, and that target lipid has the same physical–chemical properties in all organisms. The TLM is used to derive the predicted-no-effect concentrations (PNECs) using the hazardous concentration to 5% species (HC5) statistical extrapolation procedure.

The approach describes:

•       The theory and how to predict the acute toxicity to these organisms;

•       Development of an Acute to Chronic ratio [ACR] for only hydrocarbons and for 29 data points; and

•       Statistical analysis and conversion into an HC5.