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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

Bioaccumulation: aquatic / sediment: BCFss 12400 l/kg (0.26 µg/l). BCFk 13400 l/kg (0.26 µg/l), read-across from a structurally-related substance. A BCF value of 13400 is used in the exposure assessment as a worst case.
Depuration rate constant from BCF study: 0.183 d-1 (0.26 µg/l).

Key value for chemical safety assessment

BCF (aquatic species):
13 400 L/kg ww

Additional information

There are no bioaccumulation data available for 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4-D4), therefore good quality data for the structurally-related substance, D4 (CAS 556-67-2), have been read across.

There are no bioaccumulation data available for 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4-D4), therefore good quality data for the structurally-related substance, D4 (CAS 556-67-2), have been read across.

The registration substance has an average purity of >80% Vi4-D4, with <15% 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (Vi5-D5; CAS 17704-22-2; Impurity 1) and <10% 2,4,6-trimethyl-2,4,6-trivinylcyclotrisiloxane (Vi3-D3; CAS 3901-77-7; Impurity 2) present as impurities. Read-across studies are in place as supporting studies, to consider the properties of the impurities. Data for Vi5-D5 are read-across from decamethylcyclopentasiloxane D5 (CAS 541-02-6). Bioaccumulation data are not relevant for Vi3-D3, as it hydrolysis rapidly; a data waiver is in place.

2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4-D4) and octamethycyclotetrasiloxane (D4), and 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (Vi5-D5) and decamethylcyclopentasiloxane (D5), are within the Reconsile Siloxane Category which have similar properties with regard to bioaccumulation.This Category consists of linear/branched and cyclic siloxanes which have a low functionality and a hydrolysis half-life at pH 7 and 25°C >1 hour and log Kow>4. The Category hypothesis is that the bioaccumulation of a substance in fish (aquatic bioconcentration) is dependent on the octanol-water partition coefficient and chemical structure. In the context of the RAAF, Scenario 4 is applied.

Partitioning between the lipid-rich fish tissues and water may be considered to be analogous to partitioning between octanol and water.A review of the data available for substances in this analogue group indicates that BCF is dependent on log Kowas well as on chemical structure. 

The predicted log Kowvalue of Vi4-D4 and the measured value for D4 are6.47 and 6.49 respectively.D4 is a cyclic siloxane chain with four silicon atoms, connected by four oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are fully substituted with methyl groups. Similarly, the submission substance is a cyclic siloxane chain with four silicon atoms, connected by four oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are substituted with one methyl group and one vinyl group. The structural relationship between Vi5-D5 and D5 is analogous to Vi4-D4 to D4, however the cyclic chain consists of five silicon atoms connected by five oxygen atoms.A comparison of the key physicochemical properties is presented in the table below. Both substances have negligible biodegradability and similar hydrolysis rates.

Table: Key physicochemical properties of Vi4-D4 and Vi5-D5 and surrogate substance D4 and D5

CAS Number





Chemical Name



2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (Impurity 1 (Vi5-D5))

Decamethylcyclopentasiloxane (D5)

Ultimate Si hydrolysis product





Molecular weight (parent)





Molecular weight (hydrolysis product)





log Kow(parent)





Water sol (parent)

0.0073 – 0.0088 mg/l at 23°C

0.056 mg/l

9.1E-06 mg/l

0.017 mg/l

Vapour pressure (parent)

93.5 Pa

132 Pa

0.6 Pa

33 Pa

Hydrolysis t1/2at pH 7 and 25°C

approximately 63 hours

69-144 hours

1600 hours

1590 hours


It is therefore considered valid to read-across the results for D4 to fill the data gap for the registered substance.

Additional information is given in a supporting report (PFA, 2017) attached in Section 13 of the IUCLID 6 dossier.

The BCF values determined for D4 were: steady-state BCF 12400 l/kg and kinetic BCF 13400 l/kg. The BCF values determined for D5 were: steady-state BCF 7060 l/kg and kinetic BCF 13000 l/kg.

Fish bioconcentration (BCF) studies are most validly applied to substances with log Kowvalues between 1.5 and 6. Practical experience suggests that if the aqueous solubility of the substance is low (i.e. below ~0.01 to 0.1 mg/l) (REACH Guidance R.11; ECHA, 2014), fish bioconcentration studies might not provide a reliable BCF value because it is very difficult to maintain exposure concentrations. Dietary bioaccumulation (BMF) tests are practically much easier to conduct for poorly water-soluble substances, because a higher and more constant exposure to the substance can be administered via the diet than via water. In addition, potential bioaccumulation for such substances may be expected to be predominantly from uptake via feed, as substances with low water solubility and high Kocwill usually partition from water to organic matter.

However, there are limitations with laboratory studies such as BCF and BMF studies with highly lipophilic and adsorbing substances. Such studies assess the partitioning from water or food to an organism within a certain timescale. The studies aim to achieve steady-state conditions, although for highly lipophilic and adsorbing substances such steady-state conditions are difficult to achieve. In addition, the nature of BCF and BMF values as ratio values, means that they are dependent on the concentration in the exposure media (water, food), which adds to uncertainty in the values obtained.

For highly lipophilic and adsorbing substances, both routes of uptake are likely to be significant in a BCF study, because the substance can be absorbed by food from the water. 

Dual uptake routes can also occur in a BMF study, with exposure occurring via water due to desorption from food, and potential egestion of substance in the faeces and subsequent desorption to the water phase. Although such concentrations in water are likely to be low, they may result in significant uptake via water for highly lipophilic substances.

Gosset al. (2013) put forward the use of elimination half-life as a metric for the bioaccumulation potential of chemicals. Using the commonly accepted BMF and TMF threshold of 1, the authors derive a threshold value for keliminationof >0.01 d-1(half-life 70 d) as indicative of a substance that does not bioaccumulate.

Depuration rates from BCF and BMF studies, being independent of exposure concentration and route of exposure, are considered to be a more reliable metric to assess bioaccumulation potential than the ratio BCF and BMF values obtained from such studies.

The depuration rate constant of 0.183 d-1obtained from the BCF study with the read-across substance D4 is considered to be valid and to carry most weight for bioaccumulation assessment. This rate is indicative of a substance which does not bioaccumulate.

Burkhardet al., 2012 has described fugacity ratios as a method to compare laboratory and field measured bioaccumulation endpoints. By converting data such as BCF and BSAF (biota-sediment accumulation factor) to dimensionless fugacity ratios, differences in numerical scales and unit are eliminated.

Fugacity is an equilibrium criterion and can be used to assess the relative thermodynamic status (chemical activity or chemical potential) of a system comprised of multiple phases or compartments (Burkhard et al., 2012). At thermodynamic equilibrium, the chemical fugacities in the different phases are equal. A fugacity ratio between an organism and a reference phase (e.g. water) that is greater than 1, indicates that the chemical in the organism is at a higher fugacity (or chemical activity) than the reference phase.

The fugacity of a chemical in a specific medium can be calculated from the measured chemical concentration by the following equation:

f = C/Z

Where f is the fugacity (Pa), C is concentration (mol/m3) and Z is the fugacity capacity (mol(m3.Pa)).

The relevant equation for calculating the biota-water fugacity ratio (Fbiota-water) is:

Fbiota-water = BCFWD/LW /  Klw x ρlB

Where BCFWD/LWis ratio of the steady-state lipid-normalised chemical concentration in biota (µg-chemical/kg-lipid) to freely dissolved chemical concentration in water (µg-dissolved chemical/l-water), Klw is the lipid-water partition coefficient and ρlis the density of lipid and ρBis the density of biota.

It can be assumed that n-octanol and lipid are equivalent with respect to their capacity to store organic chemicals, i.e. Klw= Kow. For some substances with specific interactions with the organic phase, this assumption is not sufficiently accurate. Measurement of Klwvalues for siloxane substances is in progress. Initial laboratory work with olive oil as lipid substitute indicates that the assumption that Klw= Kowis appropriate (Reference: Dow Corning Corporation, personal communication). However, the calculated fugacity ratios presented here should be used with caution at this stage.

The table below presents fugacity ratios calculated from the BCF data for D4, using Kowfor Klw.

Table: Calculated biota-water fugacity ratios for read-across substance D4


Exposure concentration




0.26 µg/l




0.26 µg/l



*Using log Kow6.49


The fugacity-based BCFs directly reflect the thermodynamic equilibrium status of the chemical between the two media included in the ratio calculations. The fugacity ratios calculated are all below 1, indicating that the chemical in the organism tends to be at a lower fugacity (or chemical activity) than in the water. It should be noted however, that the BCF study may not have reached true steady-state in the timescale of the laboratory studies. The fugacity ratio indicates that uptake may be less than expected on thermodynamic grounds, suggesting that elimination is faster than might be expected on grounds of lipophilicity alone.