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Ecotoxicological information

Toxicity to microorganisms

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Description of key information

The 15 hour EC50 (inhibition of oxygen uptake) is 29 mg/L for aerobic heterotrophs.

Key value for chemical safety assessment

EC50 for microorganisms:
29 mg/L

Additional information

A proprietary study was available for this endpoint however the result reported in the study does not agree with those reported in a non-standard test by Blum and Speece (1991) and other study results reported in the EU RAR and for this reason the result of this study is considered to be highly uncertain. In addition, the proprietary study used a concentration of 50 times the solubility of cyclohexane as the test concentration. This concentration is not likely to be a concentration at which microorganisms at STPs would be exposed to.

The results from the Blum and Speece (1991) study reported an EC50 of 29 mg/L which is a similar result to a study published by Rogerston et al where the intermediate concentration causing cell death to a protozoan was 24.2 mg/L. This result supports the results of the Blum and Speece (1991) study. Other studies reported EC50 values up to a maximum of 156 mg/L. Clearly the EC 50 value of >1000 reported for the proprietary study maybe considered to be an outlier.

The selected 15 hour EC50 (inhibition of oxygen uptake) is 29 mg/L for aerobic heterotrophs. The test does not follow standard guidelines with higher temperature and longer exposure than standard test conditions, and no positive controls. Most importantly, the tested concentrations, raw results and methods of data analysis are not reported. However, this study was used as the key study for this endpoint in the EU Risk Assessment Report for Cyclohexane (2004) and for this reason we have used it as the key study. Also other reported values support the selected value.