Ammonia, anhydrous

Endpoint summary

Additional information

 

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.

Data are available for fish, invertebrates and algae. Further data are available for sediment dwellers. The data cover freshwater and (to a smaller extend) marine species. The lowest acute values (LC50/EC50) were obtained for fish. The same is true for long-term data: the lowest NOEC values were obtained for fish. Hence the assessment will focus on fish.

The LC50 for fish after 96 h was determined to be 0.083 -3.4 mg/L of unionised ammonia (Thurston, 1983). In fish long-term tests the lowest LOEC was determined to be 0.027 mg NH3/L. The NOEC was estimated to be 0.0135 mg/L. Further data for various species range from 0.05 to 7.44 mg NH3/L

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 in Oncorhyncus gorbuschka of 0.083 mg NH3/L. The large amount of data available for the aquatic toxicity of ammonia, does 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 NH3 exists 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.