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Short-term toxicity to fish

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

Key value for chemical safety assessment

Additional information

Five different studies are available for the assessment of the acute fish toxicity of thiourea. Two of these studies were rated reliable with a reliability code of Klimisch I (TNO, 1988) and Klimisch II (Curtis & Ward, 1988), and were therefore used as key studies in the chemical safety assessment. The remaining studies were rated unreliable and are used as supporting information in the assessment. Some information was not further included in the assessment due to considerable doubt on the correctness of reported results (“disregarded studies”).

In a 96-h acute toxicity study, Curtis & Ward (1981) determined the toxicity of thiourea to the freshwater fish Pimephales promelas (fathead minnow) at five concentrations (spacing factor 0.6; maximum concentration: 600 mg/L; control: 0 mg/L) under static conditions. The 96-h LC0 was determined to be ≤ 600 mg/L as no mortality occurred at any of the concentrations tested.

In 1988, TNO conducted a acute toxicity study for thiourea with the freshwater fish species Leuciscus idus (L.) ("Goldorfe") according to DIN 38412 "Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung" (DEV-11. Lieferung 1982) L15. The total exposure duration was 48 h. After 48 h, no mortality or other signs of sublethal effects could be observed at 10,000 mg/L which was the highest concentration tested. The 48h LC50 value was found to be greater than 10,000 mg/L.

Although the test duration of 48 h used in this study conducted by TNO (1988) is shorter than the recommended standard test duration of 96 h, this information is used in the chemical safety assessment as the study is considered reliable and the highest concentration tested – at which at the same time no lethal or sublethal effects could be observed - is a factor of 100 higher than the regulatory cut-off value of 100 mg/L for classification.

The results from the two key studies (Curtis & Ward, 1981; TNO, 1988) clearly demonstrate that thiourea cannot be classified as toxic towards fish. This information is further supported by the following information:

In 1977, experiments on the acute toxicity of thiourea to fish were conducted at the SKW Trostberg laboratory using Hong Kong Guppy (guideline or method not reported; Klimisch III). At 6,000 mg/L Hong Kong Guppies showed 100% mortality after 14 h of exposure, no animals died at 5,000 mg/L, however the total exposure duration was not reported for this dose level.

In 1981, Danio rerio fish were exposed to thiourea in a study conducted by Korte & Greim under static through conditions for a 48-h. The 48-h LC50 was found to be 16,000mg/L, (LC50 = 16,000 mg/L, LC100 = 20,000 mg/L).

The following information is available for thiourea but is not further considered in the chemical safety assessment (disregarded studies”):

The BUA Stoffbericht 179 (1996) cites results on the acute toxicity (48 h) of thiourea for Oryzias latipes from "Japan Chemical Industry Ecology - Toxicology and Information Center. 1992. Biodegradation and Bioaccumulation Data of Existing Chemicals Based on the CSCL Japan". The 48-h LC50 toxicity of thiourea for Oryzias latipes is cited to be below 1000 mg/L. This result is considerably lower than the other reported LC50. This could be attributed to a specific species sensitivity of Oryzias latipes. However, there is doubt that this result is cited correctly in the BUA Stoffbericht 179, because also the result of Korte & Greim (1981) for the 48-h LC100 for Danio rerio was falsely cited as being below 20,000 mg/L. Unfortunately, the original reference for Oryzias latipes was not available for review.

In 1977, the SKW Trostberg laboratory also conducted experiments on the acute toxicity of thiourea to fish were conducted at using Leuciscus idus (guideline or method not reported; Klimisch III). 100 % mortality was found for Leuciscus idus after 8 h and 2 h exposure at 5,000 mg/L and 6,000 mg/L, respectively. No animals died at 4,000 mg/L (however, total exposure duration not reported for this dose level). This result for Leuciscus idus is in conflict with the results from the TNO study (1988) which was rated to be reliable (Klimisch I). Therefore, the result of SKW Trostberg (1977) is not further considered in the chemical safety assessment.