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Toxicity to microorganisms

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Description of key information

NOEC = 2.73 mg/L (mean measured; toxicity control of an OECD 301D)

Key value for chemical safety assessment

Additional information

Since no studies on the toxicity to aquatic microorganisms are available for Methylcyclohexane, a ready biodegradability study is used to derive a NOEC for the toxicity to aquatic microorganisms. If a compound degrades well in a ready biodegradability test, or does not inhibit the degradation of a positive control at a certain concentration, this concentration can be used as a NOEC value. This approach is in accordance with the Guidance on information requirements and chemical safety assessment (Chapter R.7b: Endpoint specific guidance, ECHA 2014b). 

A substance can be assumed to be not inhibitory to aquatic microorganisms, if in the toxicity control of a ready biodegradation test, more than 25% degradation based on [oxygen demand (BOD/ThOD) or CO2 evolution (CO2/ThCO2)] occurred within 14 days (OECD guideline 301). For Methylcyclohexane a biodegradation test according to OECD guideline 301 D is available (Simon, 2015). The test includes a toxicity control, which contains 3.03 mg/L (initial measured)/ 2.42 mg/L (measured at test end) of Methylcyclohexane and 2 mg/L of the reference material (i.e. sodium benzoate). The toxicity control attained 30.7 % degradation after 14 days of incubation. Hence, the substance is not toxic to aquatic microorganisms in the toxicity control. The mean test item concentration of 2.725 mg/L is used as NOEC.

For the structurally similar substance (Cyclohexane, CAS: 110-82-7), a publication is available investigating the toxicity to different trophic and functional groups of sewage treatment plants, such as aerobic heterotrophs, methanogens and Nitrosomonas (Blum and Speece 1991). These data were peer-reviewed by European authorities and used as key information for the assessment of toxicity to aquatic bacteria in the EU Risk assessment of cyclohexane (ECB, 2004).

Results indicate that aerobic heterotrophs, which predominate in activated sludge, are the most sensitive group showing a 15 h-IC50 of 29 mg/L. Anaerobic bacteria (96h-IC50 = 150 mg/L) and specific functional organism groups, such as Nitrosomonas (24 h-IC50 = 97 mg/L) and Photobacterium phosphoreum, used in a Microtox®assay, demonstrated to be less sensitive.

After correction for differences in molar mass (cyclohexane: 84.16 g/mol; methylcyclohexane: 98.19 g/mol), a 15 h-IC50 of 33.8 mg/L is calculated for methylcyclohexane.

The results of both substances show a similar hazard profile towards aquatic microorganisms. As the toxicity control result with Methylcyclohexane results in a slightly lower PNEC value, this result is used as key information for assessing the toxicity towards aquatics microorganisms.