Octamethylcyclotetrasiloxane

Endpoint summary

Additional information

Reliable results from short-term toxicity tests are available for fish, invertebrates and algae. The LC₅₀ or EC₅₀ values for the three trophic levels are as follows:

Fish

96 h LC₅₀: >22 µg/l (Oncorhynchus mykiss) (Springborn Laboratories (1990a)).

Invertebrates

48 h EC₅₀: >15 µg/l (Daphnia magna) (Springborn Laboratories (1990c)).

Algae

96 h ErC₅₀: >22 µg/l and ErC₁₀: ≥22 µg/l (Pseudokirchneriella subcapitata) (Springborn Laboratories (1990f)).

 

NOECs of ≥4.4 µg/l and ≥15 µg/l have been determined in long-term tests with fish (early-life stages of Oncorhynchus mykiss) and invertebrates (reproduction, the population relevant endpoint, of Daphnia magna).

The 93-day study with Oncorhynchus mykiss reported a NOEC value of ≥4.4 µg/l as no effects were seen at the highest concentration tested. Therefore, the overall long-term NOEC for fish is considered to be ≥4.4 µg/l. However, to better define the potential NOEC, modelling (Mackay et al., 2015) to estimate fish critical body burden (CBB) levels and compare those CBB levels to those associated with a narcotic mode of action (MOA), under which the D4 and other volatile methyl siloxanes materials are proposed to operate (Redman et al., 2012, Mackay et al., 2015) was conducted. These results indicate that D4 dose levels up to 12 µg/l could have been successfully used in the D4 93-day trout ELS study without adverse effect.

 

It is important to note that although the summarised results above, and included in detail below, are considered reliable, there are some uncertainties associated with testing a substance such as D4 that must be considered. D4 is a substance of very low solubility in water (56 micrograms per litre in pure water), and its solubility in test media is likely to be lower than its solubility in pure water. In addition, D4 is volatile with a high Henry’s law constant. Often, in order to meet the requirements of the testing guidelines, extra measures must be used to maintain D4 in the test system. These include using closed, sealed systems that have no headspace, test systems prepared using saturated stock solutions (prepared at ambient temperatures) which can lead to an excess of the substance at the point of addition and, when added to a test system operating at the lower temperature of 12 °C, or solvent addition. Analytical measurement of test solutions is often carried out using GC-MS a method that does not necessarily distinguish between dissolved and dispersed test material. As D4 is a clear liquid, undissolved test material may not be obvious therefore often the solubility in the test media is uncertain.  Lastly, in Sousa, J.V. et al.(1995), although the analytical method was validated prior to use in the study (as reported in Appendix IV), the validated concentration range was 5 to 50 μg/L.  The mean measured concentrations of the two lowest doses from the definitive study were less than 5 μg/L. Additionally, only 1 of the 15 QC samples prepared and analysed between Day 0 and Day 14 was as low as 5 μg/L; the rest were 10 μg/L or greater. KC 2 has been assigned to reflect these uncertainties.

Two reliable studies have been selected with sediment species, reporting 28 day NOEC values of 13 and 44 mg/kg dry weight for effects of the registered substance on Lumbriculus variegatus and Chironomus riparius. A NOEC of 130 mg/kg dw has been read-across from D5 for effects on survival of Hyalella azteca.