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Short-term toxicity

A large number of acute toxicity tests on several species are available for o-cresol. For assessment, only the most reliable and sensitive studies are considered.

The most sensitive fish species in acute toxicity tests belong to the salmonids. The static test was conducted over a period of 96 hours on the toxicity of o-cresol to three trout species. Among the fish species tested Salmo trutta (brown trout) was the most sensitive exhibiting a LC50 of 6.2 mg/l.

Concerning short term toxicity of o-cresol towards aquatic invertebrates, the most sensitive test is a static immobilisation test with Daphnia pulex. Determination was carried out in analogy to a Dutch standard method which is similar to OECD 202. The lowest nominal EC50 of 9.6 mg/l was obtained after an exposure period of 48 hours.

Describing toxicity to aquatic algae the lowest toxicity value obtained with a blue-green algae is taken into account for assessment. The toxicity of o-cresol to the blue-green algae Microcystis aeruginosa (Cyanaophyceae) was determined in a cell multiplication inhibition test according to the guideline DEV L9. The relevant 8d-NOEC (EC3) is 6.8 mg/L .

Further, with a growth inhibition test according to a dutch national standard method NEN-6506 (1980) on alga Selenastrum sp. a 96-hour EC50 of 100 mg/L based on nominal concentrations was determined for o-cresol.

Long-term toxicity

There are no reliable data on chronic toxicity towards fish and aquatic invertebrates available using o-cresol as test substance.

In order to get a comprehensive data set for o-cresol a read-across from p-cresol is applied, in accordance with the following justification:

Data from substances who’s physicochemical, toxicological and ecotoxicological properties are likely to be similar or follow a regular pattern as a result of structural similarity may be used in a read-across approach in order to avoid unnecessary animal testing. It can be stated that the 3 cresols act as a prime example of substances that are suitable for read-across. Cresols are isomers and, thus ideally fulfill the recommended criteria of structural similarity. In its chemical structure, a cresol molecule has a methyl group substituted onto the benzene ring of a phenol molecule, by different arrangement of the -CH3 groups are three structural isomers possible. (ortho-cresol, meta-cresol and para-cresol). Of particular importance to environmental effects are the values for partition coefficient (log Kow), vapour pressure, water solubility and dissociation constant. The values of the isomers are very close together, resulting in the same environmental fate and behaviour. Further, with regard to the bioderadation behavior, all 3 cresols are readily biodegradable. Concerning aquatic toxicity of the cresols on aquatic species, a large number of experimental results from tests with fish, invertebrates and algae are available, indicating a similar toxicity of all isomers, with p-cresol being slightly more toxic in acute tests: Based on the similarities in the results mentioned above the read-across approach is therefore scientifically justified.

Studies resulting in the most sensitive NOEC are used for read-across.

Chronic toxicity of p-cresol to fish was tested with Pimephales promelas in an Early-Life Stage Toxicity Test equivalent to OECD Guideline 210. The 32d NOEC is 1.35 mg/L.

Concerning long-term toxicity of p-cresol to aquatic invertebrates a semi-static test according to the preliminary guideline proposal of the German Umweltbundesamt from1984 was conducted. After 21 days of exposure a NOEC of 1 mg/l was determined.

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