Reaction mass of decamethyltetrasiloxane; dodecamethylpentasiloxane; hexamethyldisiloxane; octamethyltrisiloxane

Endpoint summary

Additional information

No data are available for toxicity of the whole substance to terrestrial organisms.

Available data have been read-across from each constituent of the multiconstituent substance.

Constituent HMDS

A 28-day test of the effects of Constituent HMDS (hexamethyldisiloxane, CAS 107-46-0), on nitrate formation rate of soil microflora has been conducted in accordance with OECD TG 216. However, the substance could not be maintained in the test system, as demonstrated by the analytical evidence: measurements in all concentrations were below the Limit of Quantification (LOQ) by day 3 of the test.

Additionally, testing for toxicity to other terrestrial organisms (earthworm reproduction test, OECD TG 222; terrestrial plant test, OECD TG 208) was also requested by ECHA for HMDS. However, a stability/recovery test under OECD TG 222 conditions with the related test substance octamethyltrisiloxane (L3, CAS 107-51-7; EC 203-497-4) demonstrated significant loss of test item from the test system, ascribed to volatilisation losses. The definitive test was therefore not carried out. A nitrogen transformation test (OECD TG 216) carried out with L3 also demonstrated significant loss of test material.

 

Constituent L3

Testing for toxicity to terrestrial organisms has been carried out with soil micro-organisms. A stability/recovery test under OECD 222 (earthworm reproduction test) conditions has also been carried out.

A 28-day test of the effects of L3 on nitrate formation rate of soil microflora has been conducted in accordance with OECD TG 216. However, the substance could not be maintained in the test system even during the substrate preparation phase prior to the start of the test, as demonstrated by the analytical evidence: measurements at day 0 in all concentrations were below the Limit of Quantification (LOQ). LOQ ranged from 0.198 to 0.202 mg/kg, and measured concentrations remained <LOQ throughout the rest of the test. The soil micro-organisms, therefore, would not have been exposed to the test item for long enough or at high enough concentrations to assess its toxicity. Statistically significant effects (positive, non-inhibitory) on nitrate concentration and nitrate transformation rate were observed in this study: replicates at equivalent test concentration showed good repeatability. No dose-response relationship was apparent, and in view of the <LOQ recoveries of test material, it is not appropriate to ascribe this to a lack of effects of L3. The results of this test cannot be used for derivation of PNECs and risk assessment.

The stability/recovery test under OECD 222 conditions also demonstrated significant loss of test item from the test system, ascribed to volatilisation losses. Based on these experimental findings, the registrants believe that it is not technically feasible to proceed with the definitive OECD 222 test on the basis that the test substance is too volatile to maintain adequate concentrations in the test system. The proposed terrestrial plant test (OECD 208) is also considered to be not technically feasible on this basis.

Terrestrial testing will be waived as maintaining the volatile test substance in the system is not feasible. This is based on experience with the soil stability and terrestrial tests conducted with HMDS and L3, and using a threshold Kair-soil >1 in order to determine the feasibility of testing.

 

Constituent L4

No terrestrial toxicity data are available for L4; a category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017).

A stability/recovery test under OECD TG 222 conditions performed with L4 demonstrated significant loss of test item from the test system over a five-week period (37% remaining radioactivity after 35 days), ascribed to volatilisation losses. However, it is considered that it is possible that measurable concentrations will remain in the soil at the end of the eight-week test period for the definitive OECD TG 222 study. An OECD TG 222 study with L4 is therefore under way. The need for further terrestrial toxicity tests (OECD TG 208) will be reviewed once the results of this study are available.

An OECD TG 216 toxicity to soil microorganisms study is also proposed for L4.

Until these data become available PNECsoil for L4 has been derived based on read across from decamethylcyclopentasiloxane D5.

Terrestrial toxicity data are read-across from D5 (decamethylcyclopentasiloxane, CAS: 541-02-6), a siloxane with similar physico-chemical properties to L4. The data are considered to be suitable for deriving an interim PNECsoil for L4. The testing has been carried out with species that are representative of three taxonomic groups; earthworms, springtails and plants.

·   A 28-day LC₅₀ value of >4074 mg/kg dry weight and a 56-day NOEC of ≥4074 mg/kg dry weight have been determined for the effects of D5 on mortality and reproduction and growth respectively of the earthworm Eisenia andrei (Velicogna et al., 2011).

·   A 28-day LC₅₀ value of >41 mg/kg dry weight and a 56-day NOEC of 41 mg/kg dry weight (highest concentration tested) (mean measured concentrations) have been determined for the effects of D5 on survival and weight change, and reproduction, respectively, of the earthworm Eisenia andrei (Smithers Viscient, 2015).

·   A 28-day LC₅₀ value of 813 mg/kg dry weight and a 28-day IC₅₀ value of 767 mg/kg dry weight have been determined for the effects of D5 on mortality and reproduction of the springtail Folsomia candida. A NOEC of 377 mg/kg dry weight has been determined by the reviewer on the basis of a visual examination of the data for both mortality and reproduction (Velicogna et al., 2011).

·   A 14-day IC₅₀ value of 209 mg/kg dry weight has been determined for the effects of D5 on root dry mass of Barley (Hordeum vulgare). IC₅₀/EC₅₀ values for effects on seedling emergence, root and shoot length and shoot dry mass determined in the same test were ≥248 mg/kg dry weight. 14-day EC₅₀ values of >4054 mg/kg dry weight have been determined for the effects of D5 on seedling emergence, root and shoot length and root and shoot dry mass of Red Clover (Trifolium pratense). NOECs were not determined in the tests (Velicogna et al., 2011).

The studies are considered to be non-standard; they have been conducted by spiking surrogate biosolids mixed into natural soil rather than by direct spiking of soil.

L4 and D5 share similar physico-chemical properties but are not close structural analogues (linear and cyclic siloxanes).

Read-across of the terrestrial toxicity data for D5 to L4 is considered to be suitable to derive an interim hazard and risk assessment under REACH for L4.

 

Constituent L5

A stability/recovery test under OECD TG 222 conditions performed with the structurally-related substance (L4) demonstrated significant loss of test item from the test system over a five-week period (37% remaining radioactivity after 35 days), ascribed to volatilisation losses. However, it is considered that it is possible that measurable concentrations will remain in the soil at the end of the eight-week test period for the definitive OECD TG 222 study. L5 is expected to be more stable in soil, therefore an OECD TG 222 study is under way. The need for further terrestrial toxicity tests (OECD TG 208) will be reviewed once the results of this study are available.

An OECD TG 216 toxicity to soil microorganisms study is also proposed for Constituent L5.

Until these data become available PNECsoil has been derived based on read across from decamethylcyclopentasiloxane D5.

 

Terrestrial toxicity data are read-across from D5 (decamethylcyclopentasiloxane, CAS 541-02 -6), a siloxane with similar physico-chemical properties to L5. The data are considered to be suitable for deriving an interim PNECsoil for L5. The testing has been carried out with species that are representative of three taxonomic groups; earthworms, springtails and plants.

·   A 28-day LC₅₀ value of >4074 mg/kg dry weight and a 56-day NOEC of ≥4074 mg/kg dry weight have been determined for the effects of D5 on mortality and reproduction and growth respectively of the earthworm Eisenia andrei (Velicogna et al., 2011).

·   A 28-day LC₅₀ value of >41 mg/kg dry weight and a 56-day NOEC of 41 mg/kg dry weight (highest concentration tested) (mean measured concentrations) have been determined for the effects of D5 on survival and weight change, and reproduction, respectively, of the earthworm Eisenia andrei (Smithers Viscient, 2015).

·   A 28-day LC₅₀ value of 813 mg/kg dry weight and a 28-day IC₅₀ value of 767 mg/kg dry weight have been determined for the effects of D5 on mortality and reproduction of the springtail Folsomia candida. A NOEC of 377 mg/kg dry weight has been determined by the reviewer on the basis of a visual examination of the data for both mortality and reproduction (Velicogna et al., 2011).

·   A 14-day IC₅₀ value of 209 mg/kg dry weight has been determined for the effects of D5 on root dry mass of Barley (Hordeum vulgare). IC₅₀/EC₅₀ values for effects on seedling emergence, root and shoot length and shoot dry mass determined in the same test were ≥248 mg/kg dry weight. 14-day EC₅₀ values of >4054 mg/kg dry weight have been determined for the effects of D5 on seedling emergence, root and shoot length and root and shoot dry mass of Red Clover (Trifolium pratense). NOECs were not determined in the tests (Velicogna et al., 2011).

The studies are considered to be non-standard; they have been conducted by spiking surrogate biosolids mixed into natural soil rather than by direct spiking of soil.

L5 and D5 share similar physico-chemical properties but are not close structural analogues (linear and cyclic siloxanes).

Read-across of the terrestrial toxicity data for D5 to L5 is considered to be suitable to derive an interim hazard and risk assessment under REACH for L5.