2-methylbutane

Short-term toxicity to fish

Link to relevant study record(s)

Reference

Endpoint:

short-term toxicity to fish

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'.

Test organisms (species):

Oncorhynchus mykiss (previous name: Salmo gairdneri)

Test type:

other: QSAR

Water media type:

other: QSAR

Limit test:

no

Total exposure duration:

96 h

Reference substance (positive control):

not required

Key result

Duration:

96 h

Dose descriptor:

LL50

Effect conc.:

34.3 mg/L

Nominal / measured:

estimated

Conc. based on:

other: QSAR prediction

Basis for effect:

mortality (fish)

Remarks on result:

other: QSAR predicted value

Validity criteria fulfilled:

yes

Conclusions:

The PETROTOX-estimated freshwater 96-h LL50 (Lethal Loading Rate 50%) value for Oncorhynchus mykiss is 34.3 mg/L based on mortality.

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 96-h LL50 (Lethal Loading Rate 50%) value for Oncorhynchus mykiss is 34.3 mg/L based on mortality.

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

The following supporting studies were also input as endpoint records. These studies are for the analogue substances pentane and cyclopentane and are summarised below.

The study from Exxon (1997) examined the short-term toxicity of the test substance Pentane to Oncorhynchus mykiss. The study deviated slightly from the standard guidelines. The guidelines state that the test solution be prepaired by dilution of a stock solution. Due to the volatility of the the test material it was deemed more appropriate to dose each test chamber individually to prevent loss of test material. The study is otherwise considered reliable for assessment and the LC 50 (96 hours) for n-pentane is considered to be 4.26 mg/l (± 95 % C.I 3.60 - 5.04 mg/l).

Two QSAR studies have been reported for Cyclopentane. The toxicity of Cyclopentane to saltwater and freshwater fish have been estimated using ECOWIN v.1:

• 96hr LC50 has been estimated to be 9.184 mg/l for saltwater fish (species not reported)
• 96hr LC50 has been estimated to be 7.128 mg/l for freshwater fish (species not reported)

The study from Morrow (1975) examined the short-term toxicity of the test substance Pentane to Oncorhynchus kisutch. There was no analytical confirmation of test concentrations. The method of application of the test substance is likely to have resulted in losses in the test system due to volatilization given that aeration was provided and there was headspace. No effects were seen at the nominal concentrations tested. This study was also reviewed in the EU RAR and deemed unsuitable for assessment. This study has therefore not been used in this assessment.