Dodecamethylcyclohexasiloxane

Endpoint summary

Additional information

Reliable data are available for toxicity to aquatic algae. A 72-hour EC₅₀ value of >2.0 μg/L and NOEC of ≥2.0 μg/L have been determined for the effects of the test substance on growth rate of Pseudokirchneriella subcapitata.

There are no data for short-term toxicity to fish. However, in accordance with Column 2 specific rules for adaptation from Column 1 of Annex VIII of the REACH Regulation, the study does not need to be conducted because a long-term aquatic toxicity study on fish is available for an appropriate read-across substance.

There are no data for short-term toxicity to aquatic invertebrates. However, in accordance with Column 2 specific rules for adaptation from Column 1 of Annex VII of the REACH Regulation, the study does not need to be conducted because a long-term aquatic toxicity study on invertebrates is available.

Reliable test results are available for long-term toxicity to fish. A read-across 90-day NOEC of ≥14.4 μg/l has been determined for the effects of the structurally similar substance decamethylcyclopentasiloxane (D5, CAS 541-02-6) on early-life stages of Oncorhynchus mykiss.

Reliable test results are available for long-term toxicity to aquatic invertebrates. A 21-day NOEC of ≥4.6 µg/L has been determined for the effects of the test substance on survival, growth and reproduction of Daphnia magna.

The highest exposure concentrations utilised in the short- and long-term tests were close to the functional solubility of the test substance in the exposure media. It is therefore concluded that the substance is not toxic to fish, invertebrates and algae following short- and long-term exposure at its functional water solubility limit.

For aquatic sediment, NOEC values are available from three long term tests with two species representing different living and feeding conditions (Lumbriculus variegatus and Chironomus riparius). Assessment of the registration substance and analogous siloxanes sediment toxicity data set has revealed some apparently inconsistent and contradictory results.

In many different studies across the range of siloxanes with sediment toxicity data available, the observations indicate in general no effects, or an organic carbon normalised NOEC ≥30 mg/kg dwt.

This would be in-line with a general narcotic mode of action, observed for other structural classes and effects in pelagic species. In some cases, however, a significantly higher toxicity for a substance is exhibited, resulting in very low NOEC values. Furthermore, in each case where significantly higher toxicity was observed, an alternative study is available in which much lower toxicity was measured in the same species for the identical substance using natural sediment and with lower pH (<8). Consideration of the possible contributing factors that could have caused high toxicity in these studies, led to an understanding of the importance of certain factors of the test design, particularly the use of artificial sediment with a peat-based carbon source, and elevated pH in the test system.  

As a result, the reliability of the Chironomus riparius study for the registration substance with artificial sediment (with peat based carbon source) and high pH is considered not assignable.

The study with test species Lumbriculus variegatus using natural sediment reported a LOEC of >420 mg/kg dw sediment and a NOEC of ≥420 mg/kg dw sediment. The study with Chironomus riparius, conducted in natural sediment, reported a NOEC of ≥620 mg/kg dw. 

A result for effects in natural sediment on the invertebrate amphipod Hyalella azteca is read across from decamethylcyclopentasiloxane (D5, CAS 541-02-6). The result from that test is a 28 d EC₅₀ value of >130 mg/kg sediment dry weight for growth rate and a NOEC value of 130 mg/kg sediment dry weight (135 mg/kg dw normalised to 5% OC) for growth rate.

 

The NOEC value normalised to 5% OC of 135 mg/kg dw sediment for effects of the read-across substance D5 on Hyalella azteca is used as the key value for the CSA.