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In aqueous solution, ammonium salts are completely dissociated into NH4+ and a corresponding anion. This equilibrium depends on temperature, pH and ionic strength of the water in the environment. Un-ionized NH3 species exists in the aquatic environments and the fraction (NH3/(NH3 +NH4+)) steeply increases with elevated pH value or temperature. It is well known that toxicity to aquatic organisms has been attributed to un-ionized ammonia (NH3) species, and NH4 + species is considered to be non- or significantly less-toxic (Emerson et al., 1975). However, recent developments in assessing ammonia toxicity clearly show that in contrast to earlier assumptions where un-ionized ammonia was considered to be the toxic component, both the uncharged and charged molecule are toxic. Therefore, a joint toxicity model has been proposed, with ammonia causing most toxicity at high pH values and ammonium ion also contributing to toxicity at lower pH values (U.S. EPA 1999, OECD 2007).

It is generally accepted, that the principal toxic component of ammonium salts such as ammonium chloride or -sulphate is ammonia, rather than the corresponding anion (see also: OECD 2004, SIDS ammonium chloride or OECD 2007 ammonium sulphate). Therefore toxicity values for ammonium salts (such as: ammonium -sulphates, phosphates, carbonates, chlorides or nitrates), where the major toxic component is ammonia, can be considered as equivalent, therefore read-across to those substances is possible. Consequently, this hazard assessment comprises the total topic of ammonia toxicity.

U.S. EPA (1999) published an excellent review on Ammonia toxicity, with special consideration on the pH- and temperature dependence. A huge amount of studies is available on Ammonia toxicity, however a comparability of these toxicity data is difficult, since various temperature- and pH conditions were present during testing. U.S. EPA (1999) re-evaluated exisiting data on Ammonia toxicity by adjusting toxicity values to definite temperature and pH- conditions and by thereby allowing a comparability.

The following dossier is strongly adapted on this approach using fomulas as given by U.S. EPA (1999).

- Acute toxicity data, were adjusted to pH 8. The temperature dependence was not considered, since temperature effects are negligible for acute toxicity of Ammonia.

- Chronic toxicity data were adjusted to pH 8 and T = 25°C.

The pH- and temperature adjusted results of all literature data were averaged to species mean acute(chronic) values and genus mean acute(chronic) values by U.S. EPA (1999).

In contrast to U.S. EPA, who determined the criterion continuous concentration (CCC) and final acute value (FAV) for U.S. surface freshwater, a more determistic approach has been choosen for the derivation of PNECfreshwater within this dossier (an approach also recommended in TGD, 2003). The PNECfreshwater was derived from the lowest species mean chronic value and general accepted assessment factors.

The lowest species mean adjusted total ammonia nitrogen acute/chronic values are presented in the following table:

Test type Species  SMV (mg/L) N  SMV (mg/L) Ammonium sulphate*   

Oncorhynchus mykiss

 LC50 = 11.23 LC50 = 53 mg/L

Prosopium williamsoni

 LC50 = 12.11 LC50 = 57.2 mg/L    

Daphnia magna

 EC50 = 35.76  EC50 = 168.8 mg/L   
 Acute Ceriodaphnia acanthina  EC50 = 25.78  EC50 = 121.7 mg/L    
 Chronic Lepomis macrochirus  EC10 = 1.12  EC10 = 5.29 mg/L  
 Chronic Hyalella azteca  EC10 = 0.66  EC10 = 3.12 mg/L  

SMV = Species mean value

*Acute toxicity results were normalized to pH 8 and Ammonium sulphate. In addition chronic values were normalized to 25°C.

The PNECfreshwater has been derived from the lowest species mean chonic value of Hyallela azteca (EC10 = 0.66 mg/L N). Since NOEC´s of three trophic levels are available, an assessment factor of 10 has been applied.

On basis of the data presented it can be conlcuded that plant species are more tolerant than invertebrates or fishes towards ammonia.

For assessing the toxicity to microorganisms, a read across was performed to ammonium chloride (CAS: 12125 -02 -9). In a short term respiration test, performed similar to OECD guideline 209, an EC20 of 1050 mg/L and an EC50 of 1618 mg/L were determined (values refer to ammonium sulphate, BASF AG 1988). Therefore, inhibition of degradation activity of activated sludge is not anticipated when introduced in appropriate low concentrations.

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