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Toxicity to microorganisms

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Description of key information

The key study carried according to OECD Guideline provides a 3h-EC50 value of 74 mg/L.

Key value for chemical safety assessment

EC50 for microorganisms:
74 mg/L

Additional information

Two experimental studies (first in 1992; second in 1994) reported results of acute toxicity tests of the mixture of the registered substance in water to bacteria from activated sludge. Both studies were performed according to OECD Guideline No 209 (1984). A 3 -hour range-finding test and a 3 -hour definitive test were carried out in the study performed in1994. A 30 -minute definitive test and a 3 -hour definitive test were carried without range-finding test of the first experiment (1992). Inoculums were sampled from the same location namely the aeration stage of Severn Trent Water PLC sewage Treatment Plant.

Validity criteria

In both studies, the 3h-EC50 value for the positive control (3,5-dichlorophenol) was found in the range 5-30 mg/L which is one validity requirement from the OECD Guideline No 209.

In the definitive test from 1994, the respiration rates from the two controls differ from each other of 18%. This value, being higher than 15%, is a deviation from the OECD Guideline No 209. In addition, no dose-response relationship was depicted by the author. The range-finding test provides a 3h-EC50 value of 260 mg/L (based on nominal concentration).

In both tests performed in 1992, the requirements of the control respiration rate were fulfilled.

Inconsistency of results

One inconsistent result was observed between the range-finding test and the definitive from the first experiment (1992): at 100 mg/L, the percentage of inhibition was 20% whereas no inhibition was observed in the main test.


Following the review of both study reports, the second test (1994) was qualified as reliable with restrictions (2) and the first study (1992) as reliable without restrictions. Then the study in 1992 was flagged as key study for this endpoint with EC50 -3h=74 mg/L.