Hexamethylcyclotrisiloxane

Endpoint summary

Additional information

Reliable short-term toxicity tests are available with the registered substance, hexamethylcyclotrisiloxane (D3, CAS 541-05-9; EC No. 208-765-4), for fish (Oncorhynchus mykiss), invertebrates (Daphnia magna) and algae (Pseudokirchneriella subcapitata).

The relevant values are:

Fish: LC₅₀ >1.6 mg/l nominal concentration (highest concentration tested, LoS) based on mortality.

Daphnia: EC₅₀ >1.6 mg/l nominal concentration (highest concentration tested, LoS) based on mobility.

Algae: EC₅₀ >1.6 mg/l, NOEC ≥1.6 mg/l nominal concentration (highest concentration tested, LoS) based on yield and growth rate.

The fish and Daphnia tests were carried out at concentrations of maximum 1.6 mg/l (limit of D3 solubility) under flow through conditions. It is likely that the flow through tests exposed organisms to some D3 but predominantly the initial hydrolysis product 1,1,3,3,5,5-hexamethyltrisiloxane-1,5-diol (CAS 3663-50-1; EC No. 222-920-3; L3-diol), due to the rapid hydrolysis rate of D3 to the L3-diol. It is unlikely that there would be much exposure to 1,1,3,3-tetramethyldisiloxane-1,3-diol (CAS 1118-15-6; EC No. 214-258-9; L2-diol) or dimethylsilanediol (DMSD, CAS 1066-42-8; EC No. 213-915-7) as there was approximately 90% replacement of the water every 6 hours and the pH was kept neutral, therefore there would be very little time for further hydrolysis of the L3-diol.

No effects were seen in the tests with the fish and Daphnia, therefore D3 and the L3-diol were not toxic up to the limit of solubility of the parent substance (D3, 1.6 mg/l).

During the static algae test, the parent substance would rapidly hydrolyse to the intermediate hydrolysis product, the L3-diol. The algae would initially be predominantly exposed to the L3-diol. However, due to the static conditions of the test and with the increase in pH seen during the test, the algae is likely to have been exposed to the L2-diol and DMSD as well as the L3-diol, especially during the latter part of the test.

No effects were seen in the test, therefore it can be said that the L3-diol, L2-diol and DMSD are not toxic to algae at the limit of solubility of the parent substance (D3, 1.6 mg/l).

No long-term toxicity to aquatic invertebrates data are currently available with the registered substance. A chronic Daphnia reproduction test in accordance with OECD TG 211 is proposed with the initial hydrolysis product, L3-diol, due to the rapid hydrolysis half-life of the parent substance (23 minutes at pH 7 and 25°C). The need for further vertebrate testing in fish will be reviewed once the result of this study is available.

 

Reliable short-term toxicity tests are available with the final hydrolysis product, dimethylsilanediol (DMSD, CAS 1066-42-8; EC No. 213-915-7), for fish (Oncorhynchus mykiss), invertebrates (Daphnia magna) and algae (Pseudokirchneriella subcapitata).

The relevant short-term values are:

Fish: LC₅₀ >126 mg/l measured concentration (arithmetic mean) (highest concentration tested) based on mortality.

Daphnia: EC₅₀ >117 mg/l measured concentration (arithmetic mean) (highest concentration tested) based on mortality/immobility.

Algae: EC₅₀ >118 mg/l, NOEC ≥118 mg/l measured concentration (arithmetic mean) (highest concentration tested) based on yield and growth rate.

Reliable long-term toxicity tests are also available with DMSD. The relevant long-term values are:

Daphnia: 21-day NOEC ≥12.6 mg/l (measured concentration (TWA)) (highest concentration tested) based on survival, growth and reproduction.

Fish: 32-day EC₁₀ of >12 mg/l has been determined for the effects of dimethylsilanediol on larval development of Pimephales promelas based on mean measured concentrations

DMSD is stable under the conditions of the tests. No effects were seen in the tests performed with DMSD.