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Long-term toxicity to fish

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Description of key information

A long-term NOEC of 50 mg (SO3)2-/L for fish is put forward for the environmental classification and effects assessment of sulfite/disulfite compouds.

Key value for chemical safety assessment

EC10, LC10 or NOEC for freshwater fish:
50 mg/L

Additional information

Reliable chronic toxicity data were reported in GLP-OECD Guideline 210 early-life stage (ELS) study, using sodium sulfite (Na2SO3) as test substance.

Five different substance concentrations were evaluated, but no adverse effects on hatching , post–hatch mortality, health, length and dry weight of surviving fish (Danio rerio) were observed. Consequently, a chronic 34d-NOEC of ≥316 mg test substance for fish (i.e., ≥200.5 mg SO32- or ≥80.4 mg S/L) is proposed for this endpoint.

It should be noted, however, that the calculated NOEC for fish is almost a factor of 2 higher than the acute value of 149.6 mg SO32-/L. Indeed, upon addition of high concentrations of sodium metabisulfite, the formation of sulfite (fom disulfite) results in a pH-drop causing mortality. This physicochemical process did not occur in the chronic ELS test where sodium sulfite was used as test substance.

The chronic NOEC-value, however, is also higher than the acute value that has been reported for the same substance, i.e., Na2SO3. This is due to the fact that oxygen depletion caused acute toxicity, whereas the test design of the ELS test (aerated test media, using sodium sulfite as test compound) prevents mortality of exposed fish due to oxygen depletion.

A more realistic long-term NOEC is therefore an acute exposure concentration that:

-      does not reduce the pH of the test media below a level causing adverse effects

-      that does not reduce oxygen levels below unsafe levels at any point during the test.

An acute concentration of 100 mg Na2S2O5 did not cause a pH-drop in the test medium (pH range 6.1-6.8). This concentration corresponds to 84.1 mg SO32-/L. Evaluating the oxygen levels in the acute tests that were conducted with sulfite compounds, significant O2-depletion (O2) upon addition of the test substance (t=0) only occurred at SO32- concentration levels higher than 50 mg SO32-/L (test with K2SO3).

Based on these findings, a more realistic long-term NOEC of 50 mg SO32-/L is put forward for the environmental classification and effects assessment of sulfite/disulfite compouds.