2-methylbutane

Short-term toxicity to aquatic invertebrates

Link to relevant study record(s)

Reference

Endpoint:

short-term toxicity to aquatic invertebrates

Type of information:

(Q)SAR

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification

Justification for type of information:

The registered substance is a complex mixture whose constituents within the mixture have variable physicochemical properties. Therefore, a modeling framework is needed to account for the bioavailability of the constituents in the mixture, which was then used to predict the toxicity of the substance as a whole. A computer-based program, PETROTOX (v4.0), was used to calculate aquatic toxicity. The program is an extension of the three-phase partitioning (to calculate the partitioning of constituents in air, water, and non-aqueous phase liquid phases) and toxicity models used to predict the toxicity of complex substances, and incorporates the Target Lipid Model (TLM), which has been validated for applications to individual narcotic chemicals and mixtures. Composition information is entered as separate hydrocarbon blocks according to carbon number range and chemical classes, which comprise the substance.Representative constituents input to model located in the attachment "PETRORISK and PETROTOX block allocation" in Section 13.2 of IUCLID.

The TLM is a framework that relates toxicity to the physicochemical properties of a nonpolar organic constituent. The TLM is used to calculate the toxicity of the dissolved constituents to aquatic organisms. Toxicity of complex mixtures with common toxic modes of action is determined using toxic units (TU) that normalizes aqueous concentrations by their inherent toxicity. To compute a TU for each component, the estimated concentration, Cw, from the partitioning model is divided by the aqueous effect concentration (e.g., LC50 or NOEC). The individual toxic units are then summed to compute the toxicity of the mixture. The lethal or chronic loading is determined by iterating until a loading is determined that yields one TU.

The model contains a database of TLM parameters for more than 70 individual organisms including several fish, daphnid, and algal endpoints. The model contains a database of TLM parameters for more than 70 individual organisms including several fish, daphnid, and algal endpoints. The TLM framework was validated previously for acute and chronic aquatic toxicity endpoints. As such, modeled data are considered fit for purpose for classification and labelling and risk assessment.

Principles of method if other than guideline:

The aquatic toxicity was estimated by a QSAR, the PETROTOX computer model (v 4.0). This model combines a partitioning model used to calculate the aqueous concentration of hydrocarbon components 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. For model description and justification of QSAR prediction: see field 'Justification for type of information'.

Analytical monitoring:

not required

Test organisms (species):

Daphnia magna

Test type:

other: QSAR

Total exposure duration:

48 h

Reference substance (positive control):

not required

Key result

Duration:

48 h

Dose descriptor:

EL50

Effect conc.:

59.9 mg/L

Nominal / measured:

estimated

Conc. based on:

other: QSAR prediction

Basis for effect:

mobility

Remarks on result:

other: QSAR predicted value

Validity criteria fulfilled:

yes

Conclusions:

The PETROTOX-estimated freshwater 48-h EL50 (Effect Loading Rate 50%) value for Daphnia magna is 59.9 mg/L based on mobility.

Executive summary:

The aquatic toxicity was estimated using the PETROTOX computer model (v4.0), 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 nonpolar 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 PETROTOX-estimated freshwater 48-h EL50 (Effect Loading Rate 50%) value for Daphnia magna is 59.9 mg/L based on mobility.

Description of key information

The aquatic toxicity was estimated using the PETROTOX computer model (v4.0), 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 nonpolar 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.

Additional information

Two studies for 2 -methylbutane and Pentane have been provided to support the QSAR data.

The 48-hour EC50 of 2-methylbutane to Daphnia magna has been reported to be 2.3mg/l (based on initial measured concentration ). However, the true EC50 value maybe higher since loss of the starting material was determine to be about 50% during the 48hr exposure period. The 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 the key study in the EU risk assessment report for n-pentane and can be considered acceptable for assessment.

The 48-hour EC50 of Pentane to Daphnia magna has been reported to be 2.7 mg/l (based on initial measured concentration). However the true EC50 value may be higher since loss of the starting material was determined to be approximately 76 % during the 48 hr exposure period. The version of the study reviewed lacked the annex containing the key details on the test conditions and therefore these could not be reviewed and generally validated against the 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.