Thymol

Toxicity to soil microorganisms

Administrative data

Endpoint:

toxicity to soil microorganisms

Data waiving:

exposure considerations

Justification for data waiving:

the study does not need to be conducted because direct and indirect exposure of the soil compartment is unlikely

Cross-reference

Cross-reference 1

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Ready biodegradation screening test:

EU Method C.4 -E (equivalent to OECD 301 D), test substance concentration 0.8 mg/L, ThOD 2.77 mg/mg TS/L, 60 % threshold passed: 83% biodegradation in 28 days; readily biodegradable (Currenta, 2010)

Inherent biodegradation screening test:

equivalent to OECD Guideline 302B, initial concentration 200 mg/L COD: 94.6% DOC removal in 5 days; inherently biodegradable (or even redy biodegradable based on the evaluation criteria cited in the report) (Pitter, 1976)

No further simulation tests are considered necessary, since thymol is readily biodegradable.

Biodegradation in water:

readily biodegradable

Type of water:

freshwater

Ready biodegradation screening test:

In the key study on ready biodegradation according to EU Method C.4 -E (Currenta, 2010) it was concluded that the test item thymol was ready biodegradable with meeting the 10-day window.

The initial test substance concentration in this study was 0.8 mg/L, and therefore lower than the recommended concentration outlined in EU Method C.4 -E of 2 -10 mg/L. Also the test substance concentration based on the theoretical oxygen demand (ThOD) of 2.770 mg per mg DM per liter was outside the range recommended in the guideline (5-10 mg/mg/L). As not explicitly stated in the report (Currenta, 2010) it can be assumed that this lower test substance concentration was chosen based on an initial experiment (Bayer, 1986, re-written Currenta, 2009) on ready biodegradation according to OECD Guideline 301 D with concentrations of 0.8 mg/L and 2.4 mg/L showing only 57% biodegradation of thymol at 2.4 mg/L. Although at 0.8 mg/L about 61% biodegradation was observed, it is not possible to conclude whether or not 10-day window criterion was fulfilled based on the available data. Toxicity to microorganisms is a valid explanation for the inconclusive results of the Bayer study, 1986 and the observed non-biodegradation in a MITI study from 1996. In this study the test concentration (100 mg/L) used was ca. 3 times higher than the 3h-EC50 of thymol (39.6 mg/L) observed on activated sludge (Currenta, 1986) and a respiration inhibition on activated sludge is suspected. Thus it is concluded that the lower test concentration in the key study for ready biodegradation (Currenta, 2010) is appropriate to meet the requirements outlined in EU Method C.4-E taking into account an inhibitory toxic effect on effluent microorganisms and is therefore suitable as key study.

Cross-reference 2

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Hansch et al. reported in 1995 in the article " Exploring QSAR. Hydrophobic, Electronic, and  Steric Constants." ( ACS Professional Reference Book. American Chemical Society, Washington, DC.) that the log Pow of thymol is determined experimentally to be 3.3.

The log octanol-water partition coefficient (log Kow) for thymol was predicted using the QSAR model KOWWIN v1.68 implemented in the Estimation Program Interface (EPI) Suite v 4.1 (US EPA 2010). The log Kow was predicted to be 3.52. The predicted value falls within the applicability domain of the model.

Log Kow (Log Pow):

3.3

at the temperature of:

20 °C

Since one result found in handbook data may not adequately fulfill the requirements for data quality for this endpoint, it was completed by QSAR estimation with a scientifically validated and reliable model. The substance falls within the applicability domain of the model, results are adequate for risk assessment and the applied method is adequate and reliably documented.

Handbook data and QSAR estimation are evaluated in a weight-of-evidence approach and experimentally derived handbook data are shown to be comparable to the estimation. Since experimental data are expected to be even more reliable than calculated ones these are used as key data and taken forward to risk assessment.

Furthermore, using the slightly lower experimental log Kow from handbook data has no significant influence on risk assessment, e.g. RCR <1 or PBT assessment. Furthermore no temperature was given in the handbook data and in the calculation model, but room conditions (20 - 25 °C) can be assumed. For the chemical assessment the temperature was set to 20 °C.

Cross-reference 3

Reason / purpose for cross-reference:

data waiving: supporting information

Reference

Data on aquatic effects of thymol are available on three trophic levels (fish, daphnia and algae) for short-term toxicity and on two trophic levels (daphnia and algae) for long-term toxicity. The most sensitive short-term toxicity was observed for fish and daphnia with 96h-LC50 for fish of 4.67 mg/L and 48h-EC50 for daphnia of 4.46 mg/L, respectively. The most sensitive long-term effect was to algae with 72h-NOEC of 1.9 mg/L. Meanwhile a 3h-EC50 of 39.6 mg/L to activated sludge was also reported as the toxicity of thymol to microorganisms.

The effect concentrations of the key results reported in this chapter are nominal concentrations. The test substance concentrations were confirmed to be stable within +/- 20% of initial nominal concentration via analytical measurement. Although the test concentrations of thymol were not analytically confirmed in most of the supporting studies, the nominal concentrations are also expected to remain constant throughout the experiments as thymol is proven by the analytical determination in the key studies to be stable and no other fate or loss of the substance is expected.