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The substance is gaseous although is highly soluble in water. In aqueous environments the substance forms ammonium hydroxide / aqueous ammonia, the aquatic toxicity are therefore generated using aqueous ammonia and related water-soluble ammonium salts.

The toxicity of ammonia to aquatic organisms is highly dependent on physicochemical factors, most notably pH because of its importance in chemical speciation. The acute toxicity of ammonia is also influenced to a lesser degree by temperature, carbon dioxide, dissolved oxygen, and salinity. In aqueous solution, ammonia exists primarily in two forms, un-ionized ammonia (NH3) and ammonium ion (NH4+), which are in equilibrium. As pH increases, the fraction of the total ammonia which is un-ionized increases. It is this un-ionized ammonia which is generally considered to be the primary cause of toxicity in aquatic systems.

Anhydrous ammonia dissolves readily where it ionizes to form ammonium ions. In aqueous ammonium (ammonium hydroxide), the ammonia content also exists predominantly in the ionized form.

The LC50 for fish after 96 h was determined to be 0.75 -3.4 mg/l of unionised ammonia (Thurston, 1983). The LC50 was noted to change with pH. The 96 h LC50 concentrations at pH 6.6, 7.2, 7.7 and 8.7 were 0.5, 1.06, 1.34 and 1.73 mg/l of NH3 (McCormick, 1984). All LC50 values obtained for fish are consistent with the classification; R50 very toxic to aquatic organisms.

The 96 h LC50 for Daphnia was 4.07 mg/L. Adverse effects on Daphnia were noted at 1.3 mg/L (Reinbold and Pescitelli, 1990). In a separate publication, the 48 h LC50 for Daphnia was 2.94 mg NH3N/L (Gersich and Hopkins, 1986).

The 48 h LC50 for algae was 2.94 mg NH3N/L (Tam and Wong, 1996). These LC50 values for algae and Daphnia would classify the test substance under 'toxic to aquatic organisms'.

The large dataset on the acute toxicity of ammonia to fish has recently (2007) been reviewed by the UK Environment Agency, in order to derive Environmental Quality Standards (EQSs) for this substance. The report states that the key study of acute fish toxicity is that of Rice & Bailey (1980) with ammonium sulphate, which identified a 96 -hour LC50 value inOncorhyncus gorbuschkaof 0.067 mg/L (NH3). Thelarge amount of data available for the aquatic toxicity of ammonia, dose not facilitate direct comparison of individual studies, as various temperature and pH conditions were used in individual tests – both of these factors influence the relative proportion of ammonia present in the (more toxic) non-ionised form and consequently also the toxicity. The US EPA (1999) has extensively re-evaluated the existing data on ammonia toxicity by adjusting toxicity values to reflect the temperature and pH- conditions of individual tests, thereby allowing analysis of comparability. Available valid acute toxicity data, were recalculated after adjustment to pH 8, in order to take into account the fact that un-ionised NH3exists in the aquatic environments and that this proportion increases with pH and/or temperature. It is well known that toxicity to aquatic organisms has been attributed to un-ionised ammonia (NH3) species, and NH4+is considered to be non- or significantly less toxic. In the normalisation process the temperature dependence was not considered, since temperature effects are negligible for acute toxicity of ammonia. The pH- and temperature adjusted (and therefore directly comparable) results of all literature data were averaged to species mean acute(chronic) values and genus mean values. A mean species value of 0.89 mg/l un-ionised ammonia was derived for Oncorhyncus mykiss. This also value supports an M-factor of 1 for classification.