Registration Dossier

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
no hazard identified

Marine water

Hazard assessment conclusion:
no hazard identified

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
100 mg/L
Assessment factor:
10
Extrapolation method:
assessment factor

Sediment (freshwater)

Hazard assessment conclusion:
PNEC sediment (freshwater)
PNEC value:
13 mg/kg sediment dw
Assessment factor:
10
Extrapolation method:
assessment factor

Sediment (marine water)

Hazard assessment conclusion:
PNEC sediment (marine water)
PNEC value:
1.3 mg/kg sediment dw
Assessment factor:
100
Extrapolation method:
assessment factor

Hazard for air

Air

Hazard assessment conclusion:
no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:
PNEC soil
PNEC value:
3.77 mg/kg soil dw
Assessment factor:
100
Extrapolation method:
assessment factor

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
no potential to cause toxic effects if accumulated (in higher organisms) via the food chain

Additional information

Octaphenylcyclotetrasiloxane (CAS No. 546-56-5) has an extremely low estimated water solubility (<1E-05 mg/l at 20°C (insoluble)) and very high estimated log Kow (9.0 at 20°C). It also has a large molecular size (molecular weight 793.19 g/mol) which can inhibit bioavailability by inhibiting the passage of the molecule through the cell membrane.

 

The substance is susceptible to hydrolysis, however the low water solubility and high adsorption potential mean that hydrolysis in the environment is likely to be slow. The final hydrolysis product is diphenylsilanediol (CAS 947-42-2).

The registration substance is a member of the Reconsile Siloxanes Category. Substances in this category tend to have low water solubility, high adsorption and partition coefficients and slow degradation rates in the sediment compartment. For substances with a log Kow of 6.6 and above no effects are seen with aquatic organisms due to the substance low water solubility limiting the effects seen. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

Additional information is given in a supporting report (PFA 2017at) attached in Section 13.

 

On the basis that the substance is insoluble, data waivers are in place for short-term toxicity to fish, aquatic invertebrates and algae. The physicochemical properties of the registration substance indicate that maintaining stable aqueous test solutions will be extremely difficult. Furthermore, the substance will have the tendency to adsorb onto the glassware under experimental conditions. This contributes to technical difficulties making reliable experimental studies very difficult to conduct.

 

In addition:

- Above a log Kow of 6.0 short-term toxicity effects are not expected to occur, because the very limited aqueous solubility prevents toxic concentrations being achieved in exposure media. Similarly, above log Kow of 8, long-term toxicity effects are not expected.1

 

- Regarding environmental exposure: the chemical properties indicate the substance will have the tendency to adsorb onto dissolved organic matter in the natural environment. Therefore, aqueous exposure is unlikely.

 

- The physicochemical properties and very high expected Koc value means that if the substance were to enter a wastewater treatment plant in influent waste waters, it would partition mainly to sludges (a small proportion may be volatilised), with a negligible amount passing to water. The result is that exposure of freshwater or marine aquatic compartments is negligible.

 

1The very high log Kow (9.0) and very low limit of solubility in water mean the substance has very low bioavailability. For substances with these properties, chronic aquatic ecotoxicity is unlikely to be of concern. The principle of log Kow-based cut-off values for toxic effects has been embraced by the ICCA and OECD high Production Volume (HPV) chemical programmes (e.g. UNEP, 2000). The US EPA has also accepted them in its Pollution Prevention Framework (http://www.epa.gov/oppt/p2framework/docs/casestu.htm). The ECOSAR QSAR applies a log Kow cut-off value of 8.0 beyond which chronic toxicity of neutral organics would not be expected because of limitations in bioavailability and uptake of the substance (Clements, 1996). These cut-offs are empirically derived and based on experimental test data.

 

READ-ACROSS JUSTIFICATION

In order to reduce testing read-across is proposed to fulfil up to REACH Annex VIII requirements for the registered substance from substances that have similar structure and physicochemical properties in accordance with the approach set out in the Reconsile Siloxanes Category report (PFA, 2017at).

Substances in this category tend to have low water solubilities, high adsorption and partition coefficients and slow degradation. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

 

Sediment and terrestrial testing are not an Annex VIII requirement for the registration substance. However, the low water solubility, high log Kow and high log Koc of the substance indicate that it will adsorb to organic matter and partition to sludges in a waste water treatment plant, as mentioned above. As aquatic toxicity tests are not feasible, it may be more appropriate to assess the potential toxicity of the substance in the sediment and soil compartments.

Read-across of terrestrial effects data within the Siloxanes Category

The hypothesis for read across of terrestrial ecotoxicity evidence within the Siloxanes Category is based on structural considerations and physicochemical properties. To fulfil the requirements of REACH, a conservative approach is made by reading across on a nearest-neighbour basis the reliable data within the Category. Further terrestrial testing has already been proposed for silsesquioxanes, phenyl (CAS 70131-69-0) as part of an integrated testing strategy within the Category. Silsesquioxanes, phenyl is a reaction mass of high molecular weight siloxane analogues with phenyl substituents on the Si atoms. For octaphenylcyclotetrasiloxane, the nearest neighbour category member with reliable existing data is considered to be decamethylcyclopentasiloxane (D5) based on molecular weight, log Kow value as well as structural considerations (PFA, 2017at). The approach to chemical safety assessment for octaphenylcyclotetrasiloxane will be reconsidered once further data becomes available, particularly the proposed studies with silsesquioxanes, phenyl (CAS 70131-69-0).

Read-across from D5 (Terrestrial toxicity – Interim hazard and risk assessment)

The registration substance, octaphenylcyclotetrasiloxane and the read-across substance decamethylcyclopentasiloxane (D5, CAS 556-67-2) are both cyclic siloxanes with 4 and 5 silicon atoms respectively connected by oxygen and having phenyl and methyl side group substituents respectively. Octaphenylcyclotetrasiloxane and D5 have similar physicochemical properties: high molecular weight (793 g/mol and 370 g/mol respectively), low solubility (both insoluble at <1E-05 mg/l and 1.7E-03 mg/l respectively), high log Kow (9.0 and 8.0 respectively) and high log Koc. Both substances have negligible biodegradability and hydrolyse very slowly.

No effects at the limit of solubility have been reported in short-term and long-term studies in other trophic levels conducted with both substances. Given the similar properties and structural similarities, it is considered valid to read-across data from D5 for an interim hazard and risk assessment.

Read-across of sediment effects data within the Siloxanes Category

The hypothesis for read across of sediment ecotoxicity evidence within the Siloxanes Category is that no structure-based or property-based pattern is evident from the Category data set of existing studies, although patterns are identifiable associated with extrinsic aspects of test design to which effects may be attributed. A single overall interpretation is made across the Category. To fulfil the requirements of REACH, a conservative approach is made by reading across on a nearest-neighbour basis the reliable data within the Category. For octaphenylcyclotetrasiloxane, the nearest neighbour category members with reliable existing data are considered to be decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) based on molecular weight and log Kow value as well as structural considerations (PFA, 2017).

Consideration of low functionality side chains and reactive groups

Phenyl group: The ecotoxicity of phenyls is related to non-polar narcosis. The ecotoxicity of substances with phenyl groups increases with increasing log Kow. For example, trichlorophenylsilane (CAS 98-13-5) has a very fast hydrolysis rate and the log Kow of the hydrolysis product is -0.2. Short-term toxicity data with trichlorophenylsilane determine no effects up to 100 mg/l (Springborn Smithers 2009a, b and c).

 

References:

Clements (1996). Estimating toxicity of industrial chemicals to aquatic organisms using structure-activity relationships. Edited by: Richard G. Clements. Contributors: R.G. Clements, J.V. Nabholz, M. Zeeman, Environmental Effects Branch, Health and Environmental Review Division, Office of Pollution Prevention and Toxics, U.S. Environmental Protection Agency, Washington, DC 20460, August 30, 1996.

 

UNEP (2000). OECD SIDS Alfa Olefins (CAS N°:592-41-6, 111-66-0, 872-05-9,112-41-4, 1120-36-1). UNEP Publications.

Conclusion on classification

The substance has no reliable ecotoxicity data: testing is waived due to the insolubility of the test substance meaning it is technically not feasible to perform the studies and toxic effects are not likely to be expressed at the limit of solubility.

For low solubility, highly adsorbing substances, the safety net classification Category Chronic 4 may apply. However, in this instance the bioavailability of the substance is expected to be limited by the large molecular size (molecular weight 793.19) inhibiting the passage of the molecule through the cell membrane. For the purposes of assessment, the log Kow of the substance is limited to 9.0, however it is highly likely that the log Kow >10. The very high log Kow and very low limit of solubility in water mean the substance has very low bioavailability. For substances with these properties, chronic aquatic ecotoxicity is unlikely to be of concern.

An aquatic / sediment steady-state BCF value of 284 l/kg has been read across from a structural analogue. The substance does not meet the screening criteria nor the definitive criteria for bioaccumulation.

 

It is therefore proposed that octaphenylcyclotetrasiloxane (CAS 546-56-5) should not be classified in the EU for acute or chronic toxicity.

 

According to Regulation (EC) No 1272/2008 (CLP):

Aquatic Acute: Not classified

Aquatic Chronic: Not Classified