Registration Dossier

Ecotoxicological information

Endpoint summary

Administrative data

Description of key information

Additional information

Reliable short-term toxicity test results are available for the registration substance for freshwater fish (Brachydanio rerio), invertebrates (Daphnia magna) and algae (Scenedesmus subspicatus). LC50 or EC50 values for the three organisms were 88, 80 and 50 mg/L respectively. A NOEC of 7.5 mg/L was determined in the algal test. It is likely that the test organisms were predominantly exposed to the hydrolysis products of the substance.

Testing for long-term toxicity to aquatic organisms is not considered necessary because:

- The substance is used under highly controlled conditions and, therefore, exposure of the aquatic environment is low (see Section 9 for further details).

- Risk characterisation ratios (RCR) based on existing data are <1.

- The silanol hydrolysis product, trimethylsilanol, is not acutely toxic to aquatic organisms at loading rates of 100 mg/l.

- Chronic data are avilable for the other hydrolysis product, ammonia.

No reliable data is available for the toxicity to microorganisms; an EC50 of 6670 mg/l is read-across from the hydrolysis product trimethylsilanol. No reliable data are available for toxicity of ammonia to microorganisms.

Given the rapid hydrolysis of hexamethyldisilazane, it is appropriate to carry out risk characterisation on the basis of the properties of the two hydrolysis products. Therefore, the PNECs based on data for trimethylsilanol and ammonia are used.

Reliable short-term aquatic toxicity tests results are available for trimethylsilanol (CAS 1066-40-6). A 96-hour LC50 of 271 mg/l (measured concentration) has been determined for the effects of trimethylsilanol on mortality of Oncorhynchus mykiss. A NOEC of 128 mg/l has been determined in the same test.

A 48-hour EC50 of 124 mg/l (measured concentration) has been determined for the effects of trimethylsilanol, on mobility of Daphnia magna. A NOEC of <60 mg/l has been determined in the same test.

A 72-hour EC50 value of >1053 mg/l and NOEC of 70 mg/l have however been determined for the effects of trimethylsilanol on growth rate of Selenastrum capricornutum (new name:Pseudokirchnerella subcapitata) based on initial measured concentrations of the test substance. When expressed relative to mean measured concentrations the equivalent values are >750 mg/l and 50 mg/l.

Ammonia ecotoxicity data have been reviewed by various regulatory programmes. Toxicity of ammonia is dependent on the degree of ionization and, therefore, is highly pH dependent and also dependent on temperature.

Reliable long and short-term test results are available for ammonia sourced from the Environment Agency Proposed EQS for Water Framework Directive Annex VIII Substances: Ammonia (unionised) 2007.

In the short-term tests the most sensitive species to ammonia was found to be fish (Oncorhynchus gorbusha - pink salmon), with an LC50 of 0.083 mg unionised NH3/l (Rice and Bailey, 1980). Long-term test results also showed fish as the most sensitive species with a NOEC of 0.014 mg unionised NH3/l for rainbow trout (Oncorhynchus mykiss) (Solbe et al, 1989).