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Toxicity to aquatic algae and cyanobacteria

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Description of key information

In an algal inhibition test (used organisms: Pseudokirchneriella subcapitata) the 72h-EC50 (growth rate) and 72h-NOEC (growth rate) of t-butyl peroxy isobutyrate in a static system were determined to be 6.13 mg/L and 0.28 mg/L, respectively.

The 72 h-EC50 (growth rate) and 72h-NOEC (growth rate) value corresponding to a solution containing 75 % t-butylperoxyisobutyratewere estimated to be 8.17 mg/L and 0.37 mg/L, respectively.

Key value for chemical safety assessment

EC50 for freshwater algae:
8.17 mg/L
EC10 or NOEC for freshwater algae:
0.37 mg/L

Additional information

The determination of the growth inhibition of the freshwater algae Pseudokirchneriella subcapitata (previously known as Raphidocelis subcapitata and Selenastrum capricornutum) exposed to t-butyl peroxy isobutyrate for a duration of 72 hours was performed following the EU method C.3 and OECD guideline no. 201. Algae were exposed to a range of concentrations of the test substance dissolved in dilution water. The toxic effect measured during the assay was the inhibition of cellular multiplication over a time period of 72 hours.

The validity criteria of the study were not all respected: the increase in cell density, measured in the control solution between the end and the beginning of the test, was greater than a factor of 16. Nevertheless the study is considered to be sufficient for hazard assessment.

The No Observed Effect Concentration (NOEC), corresponding to the highest tested concentration where no significant effect was observed in comparison to the control, was determined using a statistical analytical method (Dunnett test). The obtained EC50 (growth rate) and NOEC (growth rate) were 6.13 mg/L and 0.28 mg/L respectively and the obtained EC50 (cell growth) and NOEC (cell growth) were 1.84 mg/L and 0.28 mg/L respectively. The EC50 and NOEC have been calculated using the geometric average values of initial and final concentrations of the test substance (actual ingredient). Concentrations of the test substance (actual ingredient) were measured by gas chromatography coupled with mass spectrometry. Initial and final concentrations were equivalent to nominal concentrations. The appearance of the test solutions was visually checked at the beginning and at the end of the test. Solutions were found to be clear, colourless, became not cloudy over the period of the test. No precipitation was observed at the end of the test. Microscopic observation confirmed that the algae appeared normal at the end of the test: the normal form of the unicellular algae is a crescent shaped cell with an average length of 5-10 µm.

Since the substance t-butyl peroxy isobutyrate will be registred as 75 % solution, the effect concentration (72 h-EC50) and 72 h- NOEC were extrapolated for a solution containing 75 % t-butyl peroxy isobutyrate (starting from the 72 h-EC50 (growth rate) and 72h-NOEC (growth rate) value for 100 % t-butyl peroxy isobutyrate). The 72 h-EC50 (growth rate) and 72h-NOEC (growth rate) value corresponding to a solution containing 75 % t-butyl peroxy isobutyrate were estimated to be 8.17 mg/L and 0.37 mg/L, respectively. These values will be used as key value for chemical safety assessment and for classification and labelling.