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Long-term toxicity to aquatic invertebrates

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Description of key information

The value of 5.90 mg S2O32-/L is put forward as a worst-case estimate for the environmental classification and effects assessment of thiosulfate substances.

Key value for chemical safety assessment

EC10, LC10 or NOEC for freshwater invertebrates:
5.9 mg/L

Additional information

No reliable data on chronic toxicity of ammonium thiosulfate to invertebrates were identified. Reliable chronic toxicity data, however, were reported for sulfite/disulfite substances with the cladoceran Daphnia magna as test species (BASF, 1990). Test substance was sodium metabisulfite (Na2S2O5).

The following arguments are put forward that support a read-across approach of data generated with sulfites/disulfites:

  •  thiosulfates oxidizes to sulfites (and subsequently to sulfate) under normal environmental conditions (e.g., through microbial activity);
  •  the acute toxicity of thiosulfates and sulfites/disulfites are both related to their impact on physicochemical parameters (decrease on the pH through formation of H2SO4, oxygen depletion due to oxidation of these reducing sulfur substances);
  • acute invertebrate toxicity data for sulfites/disulfites were lower than those that were identified for thiosulfates. The key acute value for sulfites was 74.9 mg SO32-/L, whereas the lowest value using a thiosultate as test substance was 248.2 mg/L as SO32 -. Both data were generated for the same test species (Daphnia magna). Similar findings were observed for other trophic levels. It is therefore concluded that read-across from chronic sulfite toxicity data can be considered as a worst-case assumption.

  

The reported 21d-NOEC (endpoint: reproduction rate and mortality) was >10 mg test material/L, which corresponds to >3.37 mg S/L, >8.41 mg SO32-/L or >5.90 mg S2O32-/L. Physicochemical properties of the test media were monitored during the test period, and both pH (7.5-8.0) and oxygen content (>7 mg/L) met the validity criteria set for this type of test.

 

The value of 5.90 mg S2O32-/L is put forward as a worst-case estimate for the environmental classification and effects assessment of thiosulfate substances.