Reaction mass of 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane and 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane

Toxicity to aquatic algae and cyanobacteria

Description of key information

96-hour ErC₅₀: >12 µg/L and NOEC ≥12 µg/L (initial measured concentration) for the effects of D5 on growth rate of Selenastrum capricornutum (new name Pseudokirchneriella subcapitata) read-across to Constituent 1, Vi5-D5. No effects were observed up to the water solubility limit of the test substance.

70.5-hour EL₅₀: >988 mg/l and NOELR ≥988 mg/l (nominal loading rate WAF) (highest concentration tested) for the effects of Vi4D4 (Constituent 2) on growth rate of the marine algal species Skeletonema costatum. No effects were observed up to the water solubility limit of the test substance.

Additional information

There are no reliable algal toxicity data available for the registration substance as a whole, therefore good quality data for Constituent 2 (Vi4-D4) and read-across from an appropriate structural analogue for Constituent 1 (Vi5-D5) have been used.

 

The following data have been read-across from decamethylcyclopentasiloxane (D5, 541-02-6) for Constituent 1, Vi5-D5:

A 96-hour ErC₅₀ of >12 µg/L (initial measured concentration) has been determined for the effects of D5 on growth rate of Selenastrum capricornutum (new name Pseudokirchneriella subcapitata), in accordance with OECD guideline 201. A NOEC of ≥12 µg/L has been determined in the same test (Springborn Laboratory, 2001). In view of the test media preparation method of preparing a saturated stock solution in acetone, and the slow hydrolysis rate, it is likely that the test organisms were exposed predominantly to the highest achievable concentrations of the parent substance, even though the pH rose during the test. No effects were observed up to the water solubility limit of the test substance.

 

The following data are available for 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4-D4, CAS 2554-06-5):

A 70.5-hour EL₅₀ of >988 mg/l and NOELR of ≥988 mg/l (nominal loading rate WAF) (highest concentration tested) have been determined for the effects of Vi4D4 on growth rate of the marine algal species Skeletonema costatum, in accordance with ISO/10253 (TNO, 2003). These values are above the water solubility limit of the substance (0.0073 – 0.0088 mg/l at 23°C) and indicate that toxic effects on algae are unlikely to occur at this concentration. For monoconstituent substances it would not normally be appropriate to test at concentrations above the solubility limit. However, the results as reported do provide strong evidence of the absence of toxicity at the solubility limit of the substance following short-term exposures.

In view of the hydrolysis half-life of the test substance, the method of preparation of the test solution (20 hours stirring at approximately pH 8) and the static exposure regime, it is likely that the test organisms were exposed to a mixture of parent substance and hydrolysis product.

 

Refer to the Section 6 Ecotoxicological Information discussion for the approach to chemical safety assessment for this registration substance, and justification for read-across used.