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Available studies were performed with sodium fluoride (NaF); read-across is appropriate as both NaF and sodium hydrogen difluoride are highly water soluble and therefore will dissociate into their constituent ions in the aquatic environment.

The same is true for hydrogen fluoride (HF) for which data is available from the HF EU RAR.

The toxicity of these substances is essentially due to the fluoride ion. The EU RAR notes a clear relationship between the aquatic toxicity of sodium fluoride (and hence sodium hydrogen difluoride) and water hardness. Tests performed in soft water (<50 mg CaCO3/L) showed greater toxicity than those performed in hard water (>50 mg CaCO3/L) due to the precipitation of fluoride as CaF2. All endpoints are expressed in terms of concentrations of the fluoride ion (F-).

Short-term toxicity to fish

LC50 value of 107.5, 92.4, 118.5, 105.1, 64.1 ppm at 96, 120, 144, 168 and 192 h respectively are reported for rainbow trout (Camargo & Tarazona, 1991). The same authors report LC50 value for brown trout of 164.5, 135.6, 118.5, 105.1 and 97.5 ppm after 96, 120, 144, 168 and 192 h respectively. The EU RAR for hydrogen fluoride reports additional LC50 values of 299 mg/L (48h in Leuciscus idus); 51 mg/L in (96h in Onchorynkus mykiss) and 340 mg/L (96h in Gasterosteus aculeatus). The RIVM Integrated Criteria Document reports additional data, with LC50 values ranging from 128-460 mg/L (Sloof et al, 1988).

Long-term toxicity to fish

In a 21 day test with Oncorhynchus mykiss, reviewed in the ICD and EU RAR, an LC5 value of 4 mg/L is reported (actual concentration). This value is considered to be equivalent to the NOEC for mortality. The test was conducted in very soft (12 mg CaCO3/L) natural water with daily renewal of the test water.

Short-term toxicity to aquatic invertebrates

Camargo & Tarazona (1991) report that benthic larvae are sensitive to the concentration of fluoride, with EC50 values ranging from 26-48 mg/L (actual concentration); however the study was performed in soft water. The EU RAR reviews and summarises the available data on short-term toxicity to aquatic invertebrates. The reported EC50 values for Daphnia sp. range from 97-352 mg/L and are based on nominal concentrations; EC50 values of 10.5-39 mg/L are reported for marine invertebrates. The ICD summarises the available data on the short-term toxicity of sodium fluoride to aquatic invertebrates and reports EC50 values of 109-340 mg/L; EC50 values of 30-500 mg/L are reported for marine invertebrates.

Long-term toxicity to aquatic invertebrates

The EU RAR summarises the effects of two reproductive studies of sodium fluoride on Daphnia magna. The two studies report NOEC values of 3.7 and 14.1 mg/L, with an arithmetic mean of 8.9 mg/L. The ICD reports EC50 values in the range of 10-48 mg/L.

Toxicity to algae

The EU RAR reviews and summarises the available data on the toxicity of sodium fluoride to freshwater and marine algae species. The EC50 values for freshwater algae are reported to range from 43 to 122 mg/L. For marine algae the EC50 was 81 mg/L in a single study with Skeletonema costatum. NOEC values of 50-249 mg/L and 50 200 mg/L are reported for freshwater and marine algae, respectively.

Toxicity to microorganisms

An OECD 209 guideline-compliant Bayer study reports a 3 -hour NOEC of 510 mg/L for activated sludge. The EU RAR summarises and reviews the other available data on the toxicity of fluoride to aquatic microorganisms; NOEC values of between 7.1 -226 mg/L are reported.