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Ecotoxicological information

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For the assessment of the short-term toxicity of thiourea to fish five different studies are available. Species tested were Pimephales promelas, Leuciscus idus (L.), Hong Kong guppy, Danio rerio, and Oryzias latipes. In none of the studies could an LC50 value be obtained that would be close to the regulatory cut-off value of 100 mg/L for classification. Therefore, classification of thiourea for acute toxicity to fish is not warranted.

The 14-d subchronic NOEC of thiourea to fish was determined to be 5000 mg/L.

The 96-hr effect concentrations of thiourea to the green alga Scenedesmus subspicatus Chodat used in the risk assessment of the substance are as follows: EC10 = 0.3 mg/L, and EC50 = 3.8 mg/L.

Conflicting results are available regarding the short-term toxicity of thiourea to daphnia. Several non-standard data of low reliability are available that provide information on the acute toxicity of thiourea towards daphnia. Reported EC50 values were determined after 24 to 96 hours exposure and range between 1.8 mg/L and 4000-6000 mg/L. However, consistent results are reported with regard to the long-term toxicity to daphnia. All 21-d NOEC values (reproduction) are in the range of 0.1 to 1 mg/L. Following the precautionary principle, the lowest reported result of NOEC (21-d) of 0.1 mg/L is used in the assessment of thiourea.

Regarding the toxicity of thiourea to microorganisms, it could be demonstrated in several studies that thiourea is toxic to microorganisms, with nitrification being the most sensitive process. Toxic effects (75 % nitrification inhibition) were reported at thiourea concentrations ranging from 0.76 mg/L to 0.076 mg/L. However, acclimatisation to thiourea occurs over time, resulting in the microorganism's capability of degrading thiourea. In a study conducted by Tomlinson et al. (1966) incremental increase of thiourea concentrations up to 76 mg/l correlated with a gradual increase of the degradation products nitrate and sulphate, indicating complete mineralisation of thiourea. An EC50 of 0.35 mg/l could be established with respect to the inhibitory effect of thiourea to STP microorganisms. This value was chosen for the risk assessment as it is the only result obtained making use of a (modified) standard guideline (ISO 9509).

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