Octamethyltrisiloxane

PBT assessment

PBT assessment: overall result

Reference

Name:

octamethyltrisiloxane

Type of composition:

boundary composition of the substance

State / form:

liquid

Reference substance:

octamethyltrisiloxane

PBT status:

the substance is not PBT / vPvB

Justification:

The substance does not meet the definitive criteria for persistence in the aquatic or soil compartments; it is not P or vP. The criteria for persistence (vP) in the sediment compartment are met.

The substance does not meet the definitive criteria for bioaccumulation based on weight of evidence determination using expert judgement; it is not B or vB

The substance does not meet the definitive criteria for toxicity; it is not T.

Registrants comment on aquatic toxicity

L3 is non-toxic to aquatic organisms.At the highest concentrations that could be tested in water (limited by low water solubility), there were no observable effects in both short- and long-term studies with fish, daphnids, or algae.

Based on this ecotoxicity data it can be concluded that the criteria for T are not met.

As an alternative approach to the aquatic ‘T’, the possible back-calculation from a sediment NOEC value to an aquatic NOEC value using the equilibrium partitioning (EqP) calculation is set out in REACH Guidance R.11 (PBT/vPvB Assessment).The equilibrium partitioning theory is intended to be used to calculate an initial PNEC_{sediment/soil}from a pelagic PNEC_waterwhen no sediment/soil studies are available. The R11 guidance allows for the use of equilibrium partitioning (EqP) theory to back-calculate a NOEC value of a sediment test to a pelagic NOEC value which can then be compared with theAnnex XIII aquaticT criterion. The method converts the sediment NOEC to a dissolved water concentration (or aquatic NOEC).

The ‘T’ criteria defined in REACH Regulation Annex XIII and the REACH PBT guidance (R11) do not cover sediment or soil. The registrant considers that this gap in regulatory guidance should not be filled using the equilibrium partitioning calculation, which is not validated in this respect. The REACH Guidance R.11 suggests that this method should only be considered if it is technically not feasible to perform a test via the water phase (e.g. for substances with Log K_ow> 6, where the substance partitions out of solution). However, direct studies on pelagic organisms are available for L3 and these should be given more weight.

In addition, use of EqP theory to back-calculate a NOEC value of a sediment test to a pelagic NOEC value is based by implication on the assumption that the sediment toxicity is mediated through the pore water. However, the benthic studies include exposure via direct contact and ingestion of solid phase as well as via pore water. Use of EqP methods necessitates that the method is shown to be valid for the particular substance. The absence of pelagic effects for L3 means that use of EqP for any purpose has not been validated.

Selck and Forbes (2018) have set out a robust rationale explaining why the assessment of risks to the benthic compartment should be based on data for that compartment, and not the pelagic compartment. The corollary is clear, that risks to the pelagic compartment should not be based on data from the benthic compartment.

Therefore, the registrant considers the use of EqP theory to back-calculate a NOEC value of a sediment test to a pelagic NOEC value to not be valid.