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The lowest LC50 value after a 96 h continuous exposure of fish juveniles to ozone reported in the scientific literature was ca 0.008 mg/L. Ozone has to be classified as acute aquatic toxicity category 1. The lowest NOEC value after a 3 months partial chronic testing of freshwater fish juveniles to ozone reported in the scientific literature was 2 µg/L. For marine fish only a 21-d study is available, where fish were exposed to ozone and ozone produced oxidants. The derived NOEC is 60 µg/L. Both values are below the threshold of 1 mg/L hence ozone is classified as chronic aquatic toxicity category 1.

Additional information

No studies which were performed according to internationally accepted guidelines (OECD, EC, EPA etc…) for evaluation of the long term toxicity of ozone to fish were found in the public domain or in the archives of the applicant. However, the following studies were found and summarised:


Wedemeyer et al. (1979) exposed juvenile 10–13 cm rainbow trout (Salmo gairdneri) to dissolved ozone at 2 µg/L and at 5 µg/L in soft water at 10 °C for 3 months. The lowest exposure level (2 µg/L) caused no significant biological damage while 5 µg/L caused some gill pathological changes and reduced feeding behaviour. Mortality was not observed. Accordingly, 2 µg/L was suggested as a provisional maximum safe exposure level, pending completion of life cycle studies.

Paller and Heidinger (1980) exposed fish for 7 days to ozone varying in concentration from 0.01 to 0.06 mg/L and for 42 days to a mean concentration < 0.01 mg/l. 100 % mortality was observed in the 0.01 to 0.06 mg/L exposed animals, while > 50 % mortality was observed in the < 0.01 mg/L exposed animals. In the latter group ozone concentrations occasionally peaked at 0.02 mg/L, at which signs of toxicity were seen (irregular respiration, decreased activity and cessation of feeding).

Based on the data of Wedemeyer et al. (1979) it is concluded that it is likely that the NOEC for ozone in fish is ca 2 µg/L hence clearly less than 0.01 mg/L. Consequently, ozone, being a substance which is rapidly degraded, should be classified for Chronic aquatic toxicity category 1. However, it should be considered that the half-life of ozone in surface water is very short and that for this reason this classification is possibly exaggerated.


Two studies showing effects of repeated ozone treatment on saltwater fish or invertebrates were found in the public domain. In seawater, ozone is rapidly quenched in reaction with bromide ions and the observed toxic effects are due to “ozone produced oxidants” (OPOs).

1) Exposure of juvenile turbot to OPO concentrations of 0.15 mg/L for 21 days induced irreversible changes in the gills. Together with evidence from an earlier study by the same authors (Reiser et al., 2010), it was concluded by Reiser et al. (2011) that an OPO concentration of ≤ 0.06 mg/L can be suggested as a safe level for the rearing of juvenile turbot (21 d NOEC = 0.06 mg/L).

2) The second study (Schröder et al., 2010) reported that long-term exposures of juvenile Pacific white shrimp (L. vannamei) to "ozone produced oxidants" (OPOs) at 0.10 and 0.15 mg/L induced incidence of soft shell syndrome which led to mortalities due to cannibalism. Juvenile L. vannamei tested against 0.06 mg/L of ozone-produced oxidants survived the 21 day exposure and did not show any observable impairment (21 d NOEC = 0.06 mg/L).