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The available data indicate that NaOH concentrations of 20 to 40 mg/L may be acutely toxic to fish and invertebrates (single species tests). Data on pH increases due to the addition of these amounts of NaOH in the used test waters are lacking. In waters with a relatively low buffering capacity, NaOH concentrations of 20-40 mg/L may result in a pH increase with one to several pH units (EU RAR, 2007;section, page 30).

The OECD SIDS (2002) assigned a low code of reliability ('invalid' or 'not assignable') to all available tests, as in general the tests were not conducted according to the current test guidelines (EU RAR, 2007; section, page 30). Furthermore, in many tests reports there were no data on pH, buffer capacity and/or test medium composition, although this is essential information for toxicity tests with NaOH. This is the most important reason why most of the tests were considered 'invalid'. Despite this lack of valid data, there is no need for additional aquatic toxicity testing with NaOH, as all available tests resulted in a rather small range of toxicity values (acute toxicity tests: 20 to 450 mg/L; chronic toxicity test: > or = 25 mg/L) and there are sufficient data on the pH ranges that are tolerated by major taxonomic groups.

Moreover, a generic PNEC cannot be derived from single-species toxicity data for NaOH, as the pH of natural waters as well as the buffer capacity of natural waters show considerable differences and aquatic organisms/ecosystems are adapted to these specific natural conditions, resulting in different pH optima and pH ranges that are tolerated (EU RAR, 2007; section, page 30). According to the OECD SIDS (2002), a lot of information is available about the relationship between pH and ecosystem structure and also natural variations in pH of aquatic ecosystems have been quantified and reported extensively in ecological publications and handbooks.