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In water, the tetrahydrofuran - boron trifluoride complex rapidly decomposes to form tetrahydrofuran and boron trifluoride dihydrate. The latter reacts further to boric acid and fluoroboric acid, which finally hydrolyses to yield hydrofluoric acid/ fluoride ions (BUA Report 261, 2005). Due to the reactivity of the tetrahydrofuran – boron trifluoride complex in water, the assessment of aquatic toxicity is based on the properties of the decomposition/hydrolysis products. For this purpose all public data described in the EU Risk Assessment Report for boric acid was used. Fluoride was not included in the assessment, since it is not assumed to significantly contribute to the toxic effect.The EU Risk assessment report on hydrofluoric acid indicates for fluoride ions a PNEC of 0.9 mg/L (or 3.3 mg/L derived with SSD), whereas a PNEC of 0.56 mg/L (or 1.9 mg/L derived with SSD) was estimated for boric acid. We can, therefore, assume that the toxicity of BF3 is dominated by the toxicityof H3BO3.

Short-term fish studies are available for all main hydrolysis products. Tetrahydrofuran showed very low fish toxicity, with EC50 values of 2160 mg/L and 1930 mg/L, based on mortality and behaviour respectively (Brooke et al., 1984). With boron trifluoride dihydrate, the observed effects were due to the decreased pH only. No mortality or sublethal effects occured in the neutralised sample at 100 mg/L (BASF AG, 1988). The most reliable literature studies with boric acid and fluoroborate report LC50 values between 125 and 600 mg/L (Hamilton, 1997; Curtis and Ward, 1981).

Bringmann and Kuhn reported studied the toxicity of tetrahydrofuran on Daphnia magna and reported an EC50 value of 5930 mg/L for 24 hours exposure (Bringmann and Kuhn, 1982).

With the similar substance diethyl ether – boron trifluoride no effects on Daphnia magna were observed in the neutralised sample at 100 mg/L (BASF AG, 1992). In the EU Risk Assessment Report for boric acid, 24 – 48 h EC50 values for Daphnia magna range from 73 to 226 mg B /L. Expressed as concentration of tetrahydrofuran – boron trifluoride, all values are above 100 mg/L.

No growth inhibition of Scenedesmus subspicatus was observed with boron trifluoride dihydrate at the highest concentration tested 500 mg/L (BASF AG, 1991). In the EU Risk Assessment Report for boric acid, only an EC50 value of 52 mg B /L is reported, which corresponds to 673 mg/L tetrahydrofuran – boron trifluoride. For tetrahydrofuran, Bringmann and Kuhn reported a toxic threshold concentration of 3700 mg/L (Bringmann and Kuhn, 1976).

Long-term data is available for boric acid and borax (disodium tetraborate). Reliable NOEC values for fish and daphnia range between 5 and 27 mg B /L.

In the 33 h fish early life stage study with tetrahydrofuran, a NOEC value of 216 mg/L was obtained (Call et al., 1985).