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Toxicity to microorganisms

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Description of key information

Regarding all available test on toxicity to microorganisms, the most relevant test resulting in the lowest PNEC value is used for assessment.
Nitrification inhibition by o-cresol was measured by a similar method yielding an EC75 of 12.8 mg/L during a 2 -4 h incubation period . In a cell multiplication inhibition test conducted with Entosiphon sulcatum a 72h-NOEC of 17 mg/l is determined. For Pseudomonas putida a NOEC of 33 mg/l was obtained in a cell multiplication inhibition test during 16 hours of exposure. In order to protect the efficiency of sewage treatment plants, inhibition of nitrification is identified to be the most sensitive test system.

Key value for chemical safety assessment

EC50 for microorganisms:
12.8 mg/L

Additional information

No standard tests concerning the respiration inhibition on microorganisms in sewage treatment systems are available. However, several tests are available to evaluate the toxicity of o-cresol to microorganisms.

The influence of o-cresol on the nitrification process in domestic wastewater is described in an experiment. A purified activated sludge was incubated for 2 to 4 hours to the wastewater (BSB5 = 250 mg/l, 50 to 80 mg NH4-N/l) fortified with different concentrations of o-cresol. The effect on the inhibition of the nitrification was determined by photometric analysis of nitrite and nitrate. An EC75-value of 12.8 mg/l is given for the inhibition of the reaction step of ammonia to nitrite.

Concerning toxicity towards single species, the toxicity of o-cresol to Pseudomonas putida was assessed in a cell multiplication inhibition test according to Bringmann. Bacteria were exposed for 16 hours at a temperature of 25°C to the test substance. A toxicity threshold concentration of 33 mg/l was determined after 16 hours exposure. The result is equivalent to an EC3 or a NOEC. The endpoint biomass is measured by turbidity determination with a photometric detector.

The protozoa Entosiphon sulcatum was exposed to o-cresol for 72 hours using the cell multiplication inhibition test method. A toxicity threshold of 17 mg/l was obtained which is equivalent to a NOEC or an EC3.

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