Hexamethyldisiloxane

Bioaccumulation: aquatic / sediment

Description of key information

Bioaccumulation: aquatic / sediment (key study): BCF: 1290 - 2410 l/kg (40 µg/l); 776 - 1660 l/kg (4 µg/l) (national method equivalent to OECD 305 C). Empirical average BCF: 1684 l/kg (40 µg/l); 1181 l/kg (4 µg/l) (calculated by the registrant from the experimental data). Lipid-normalised empirical average BCF: 1958 l/kg (40 µg/l) and 1374 l/kg (4 µg/l) (calculated by the registrant from the experimental data). Growth-corrected kinetic BCF: 1695 l/kg (40 µg/l); 1420 l/kg (4 µg/l) (calculated by registrant from the experimental data using first-order fish uptake/depuration model). Growth-corrected lipid-normalised kinetic BCF: 1971 l/kg (40 µg/l) and 1652 l/kg (4 µg/l) (calculated by registrant from the experimental data using first-order fish uptake/depuration model). In exposure modelling the growth-corrected lipid-normalised kinetic BCF value of 1971 l/kg (40 µg/l) will be used as a worst case.

BMF 0.38 (lipid-normalised steady-state); BMF 0.86 (lipid-normalised, growth-corrected kinetic), read-across from L3. A BMF value of 0.86 is used in the exposure assessment as a worst case.

Depuration rate constants from BCF study: growth-corrected depuration rate constants of 0.059 to 0.59 day^-1 (calculated by the registrant from the experimental data using a first-order fish uptake/depuration model)

Key value for chemical safety assessment

BCF (aquatic species):

1 971 L/kg ww

BMF in fish (dimensionless):

0.86

Additional information

BCF values of 1290 - 2410 l/kg (40 ug/l); 776 - 1660 l/kg (4 ug/l) have been determined with carp in separate exposures at two concentrations in a reliable study conducted according to an appropriate test protocol (Japanese Industrial Standard test method, essentially the same as OECD 305C), with acceptable restrictions. The restrictions were that there was no depuration and there was significant variability in concentration in fish between replicates at later sampling points.

The study reports the range of BCF values at each exposure level; no steady state BCF (BCF_ss) values are reported.

Empirical average BCF values at each exposure level of 1670 l/kg (40 µg/l) and 1080 l/kg (4 µg/l) have been calculated by the registrant from the experimental data. Lipid-normalised empirical average BCF values (normalised to default fish lipid content of 5% wwt) are 1940 l/kg (40 µg/l) and 1260 l/kg (4 µg/l).

Application of a first-order fish uptake/depuration model to the experimental data, by the registrant, produced an acceptable fit at both the low- and high-level HMDS exposures, with uptake rate constants ranging from 84 to 1000 l kg^-1day^-1and growth-corrected depuration rate constants ranging from 0.059 to 0.59 day^-1. These values produce steady-state kinetic BCF (BCFk) values of 1420 and 1695 l/kg for the low- and high-level HMDS exposures, respectively; these values are consistent with the empirical BCF values, indicating that experimental steady-state concentrations of HMDS were achieved in the carp. Lipid-normalised (5% lipid) BCFk values are 1971 l/kg (40 µg/l) and 1652 l/kg (4 µg/l). For further information refer to Dow Corning Corporation, 2014.

A fish feeding study is available with the structural analogue, octamethyltrisiloxane (L3. CAS 107-51-7). Both the registration substance and surrogate substance are methylated linear siloxanes; the registration substance HMDS consists of two silicon atoms linked by an oxygen atom, whilst the surrogate substance L3 consists of three silicon atoms each linked by an oxygen atom. L3 has a lower water solubility (0.034 mg/l at 23°C) and higher log K_ow(6.6 at 25.3°C) and is therefore considered to have a greater bioaccumulation potential. It is therefore considered valid to read-across the result of the BMF study with L3 as a worst-case.A lipid-adjusted steady-state BMF value of 0.38 and lipid-adjusted, growth corrected kinetic BMF value of 0.86 were determined in a reliable study conducted in compliance with GLP. The food in this study was very highly dosed (500 µg/g of¹⁴C-L3 nominal; 436 µg/g mean measured), which may limit the applicability of the values obtained.

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.

Goss et 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 k_eliminationof >0.01 d^-1(half-life 70d) 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 0.0378d^-1from the BMF study for L3, may not be valid due to the very high loading of the food in this study potentially overloading metabolic/elimination pathways. This depuration rate is therefore not taken into account in the assessment of bioaccumulation.

The growth-corrected depuration rate constants of 0.059 to 0.59 day^-1obtained from the first-order fish uptake/depuration modelling (Dow Corning Corporation, 2014) of the BCF study are considered to be valid and to carry most weight for bioaccumulation assessment. These rates are indicative of a substance which does not bioaccumulate.

Burkhard et 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 (F_biota-water) is:

F_biota-water= BCF_WD/LW / K_lw x ρ_l/ρ_B

where BCF_WD/LW is 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), K_lw is the lipid-water partition coefficient, ρ_l is the density of lipid and ρ_B is the density of biota (Burkhard et al. 2012).

A study to determine storage lipid-air partition coefficients of cVMS has been carried out (Dow Corning Corporation, 2015c). The conclusion from that study is that partitioning of cVMS compounds between storage lipids and air or water is reasonably similar, but not identical, to octanol. K_{storage lipid-air}values for cVMS were systematically lower than K_octanol-airby 0.2 to 0.4 log units depending on temperature. K_{octanol-water}values may be expected to be similar.

The carp had a measured lipid content of 4.3% and the estimated freely-dissolved fraction of HMDS in water is 100%, resulting in a lipid-adjusted, freely-dissolved BCF (BCF_LW/fd ) values of 3.30 E+04 and 3.94 E+04 l/kg-lipid, for the low- and high-level exposures, respectively. These values, expressed as a biota/water fugacity ratio, F_biota/water, produces values for the low- and high-level HMDS exposures of 0.29 and 0.34, respectively. Fugacity ratio values less than 1.0 are indicative of materials that do not pose a bioaccumulation/biomagnifications risk to aquatic organisms.

BCF values of 1290 - 2410 l/kg (40 µg/l) and 776 - 1660 l/kg (4 µg/l) have been determined with carp in separate exposures at two concentrations in a reliable study with the registration substance.  Growth-corrected lipid-normalised kinetic BCF values of 1971 l/kg (40 µg/l) and 1652 l/kg (4 µg/l) were calculated by the registrant from the experimental data using a first-order fish uptake/depuration model. In exposure modelling the growth-corrected lipid-normalised BCF_kvalue of 1971 l/kg (40 µg/l) will be used as a worst case. The BCF_kvalue of 1971 l/kg indicates that a conclusion of ‘not B’ is applicable to this substance. This conclusion is supported by the calculated depuration rate constants in this study being >0.01 d-1 and the calculated fugacity ratios being <1.0. The BCF results are support by a BMF <1 read-across from the structural analogue, L3.

 

For further information refer to Dow Corning Corporation, 2014. DOW CORNING TECHNICAL REPORT Kinetic Modeling Interpretation of Hexamethyldisiloxane (CAS RN 107-46-0) Fish Bioconcentration Data with the Common carp (Cyprinus carpio)). 2014. Attached to the EPSR.

References:

Dow Corning Corporation (2015c) Non-regulated study: Determination of storage lipid-to-air partition coefficients and their temperature dependence for Octamethylcyclotetrasiloxane (D4; CAS 556-67-2), Decamethylcyclopentasiloxane (D5; CAS 541-02-6) and Dodecamethylcyclohexasiloxane (D6; CAS 540-97-6). DOW CORNING CORPORATION HEALTH AND ENVIRONMENTAL SCIENCES (HES) TECHNICAL REPORT. HES Study No.: 17240-108. Report date: May 20, 2015.

Goss, K-U., Brown, T. N. and Endo, S. (2013). Elimination half-life as a metric for the bioaccumulation potential of chemicals in aquatic and terrestrial food chains.Environmental Toxicology and Chemistry32, 1663-1671.

Burkhard, L. P., Arnot, J. A., Embry, M. R., Farley, K. J., Hoke, R. A., Kitano, M., Leslie, H. A., Lotufo, G. R., Parkerton, T. F., Sappington, K. G., Tomy, G. T. and Woodburn, K. B. (2012). Comparing Laboratory and Field Measured Bioaccumulation Endpoints. Integrated Environmental Assessment and Management 8, 17-31.