Octamethylcyclotetrasiloxane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

1.5 µg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.15 µg/L

Assessment factor:

100

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

10 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

3 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.3 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:

0.84 mg/kg soil dw

Assessment factor:

50

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

41 mg/kg food

Assessment factor:

90

Additional information

The measured hydrolysis half-life of octamethylcyclotetrasiloxane (D4, CAS 556-67-2, EC No. 209-136-7) is 3.9 days at pH 7 and 25 °C. The measured water solubility of the substance in pure water is 0.056 mg/l at 23 °C. However, actual functional water solubility differs depending on test media and methods used to prepare test medium. The substance has a measured log K_ow of 6.98.

See Section 7.5 for further details of PNEC derivation. This derivation takes a precautionary worst-case approach to the interpretation of the aquatic studies available; the studies are reported accordingly. However, in section 7.6, a weight-of-evidence approach is applied in consideration of the self-classification for the environment to conclude that the effects seen are of low significance.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing, read-across is proposed to fulfil REACH Annex X requirements for sediment ecotoxicity for the registered substance from substances that have similar structure and physicochemical properties.

The registration substance (D4) and the substance used as surrogate for read-across are members of the Reconsile Siloxanes Category. Substances in this category tend to have low water solubility, high adsorption and partition coefficients and slow degradation in the sediment compartment. In the environment, the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

Read-across from decamethylcyclopentasiloxane (D5) to octamethylcyclotetrasiloxane (D4) is considered to be valid for sediment ecotoxicity.

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, with read-across proposed on a nearest-neighbour basis within the Category. The registered substance, octamethylcyclotetrasiloxane (D4, CAS 556-67-2) and the surrogate substance decamethylcyclopentasiloxane (D5, CAS 541-02-6) are cyclic siloxanes. D5 is a cyclic siloxane with five dimethylated silicon atoms linked by five oxygen atoms. D4 is a directly analogous structure with four silicon and four oxygen atoms. The substances have similar physicochemical properties: high molecular weight (370 and 296 respectively), low water solubility (both insoluble, at 0.017 and 0.056 mg/l respectively), high log K_ow (8.02 and 6.98 respectively) and high log K_oc (5.2 and 4.22 respectively).

Environmental toxicity data for siloxanes are consistent with a non-polar narcosis mechanism (Redman, 2012; Peter Fisk Associates 2017at). Given the similar properties and structural similarities, it is considered valid to read-across data from decamethylcyclopentasiloxane (D5) to octamethylcyclotetrasiloxane (D4). Additional information is given in Section 7.1.5 and in a supporting report (PFA 2017at) attached in Section 13 of the IUCLID 6 dossier.

 

Table7.0.1 Summary of ecotoxicological and physicochemical properties for the registered substance and the surrogate substance(aquatic and terrestrial toxicity)

CAS Number	556-67-2	541-02-6
Chemical Name	Octamethylcyclotetrasiloxane (D4)	Decamethylcyclopentasiloxane (D5)
Si hydrolysis product	Dimethylsilanediol	Dimethylsilanediol
Molecular weight (parent)	296.62	370.78
Molecular weight (hydrolysis product)	92.17	92.17
log Kow (parent)	6.98	8.02
log Kow (silanol hydrolysis product)	-0.38	-0.38
log Koc (parent)	4.22	5.2
Water solubility (parent)	0.056 mg/l	0.017 mg/l
Vapour pressure (parent)	132 Pa	33.2 Pa
Vapour pressure (hydrolysis product)	7 Pa	7 Pa
Hydrolysis t1/2 at pH 7 and 25°C	69-144 hours	1590 h
Short-term toxicity to fish (LC50)	96 h LC50 >22 μg/l;	>16 µg/l
Short-term toxicity to aquatic invertebrates (EC50)	>15 μg/l	>2.9 µg/l
Algal inhibition (ErC50 and NOEC)	ErC50 >22 µg/l and ErC10 ≥ 22 µg/l	ErC50: >12 μg/l; NOEC: ≥12 μg/l
Long-term toxicity to fish (NOEC)	≥4.4 μg/l	≥14 μg/l
Long-term toxicity to aquatic invertebrates (NOEC)	NOEC ≥15 µg/l	≥15 µg/l
Sediment toxicity (NOEC)	13 mg/kg dry weight, Lumbriculus variegatus 44 mg/kg dry weight, Chironomus riparius	70 mg/kg dwt,C. riparius ≥1272 mg/kg dwtL. variegatus 130 mg/kg dwt, H. azteca
Short-term terrestrial toxicity (L(EC)50)	n/a	(IC50) 209 mg/kg dwt,H. vulgare; >4054 mg/kg dwt, T. pratense
Long-term terrestrial toxicity (NOEC)	>100 mg/kg dw, soil microflora	≥4074 mg/kg dwt, E. andrei 377 mg/kg dwt, F. candida

 

Overview of toxicity to aquatic organisms

This discussion is a new consolidated review of the toxicity to aquatic organisms, and is a necessary step before PNECs and environmental classification are discussed in later sections. It has been prepared by the registrants in 2017 in response to ongoing discussions about the aquatic classification of D4.

Background

An overall WoE evaluation has been performed by the Industry to consider all relevant data including environmental behaviour of the substance, physical-chemical properties and ecotoxicological data and to address some of the uncertainties as well as the technical difficulties resulting from intrinsic physicochemical and partitioning properties of the substance when conducting guideline tests (GHS Revision 6, 2015, sections 4.1.2.5, A9.3.4.2, and A9.3.6.2.3).In addition, recently published expert reviews (Bridges and Solomon, 2016 and Fairbrother et al., 2016) have also been considered and provided as references in this analysis.

Detail

Discussion on Toxicity

GHS Guidance highlights that:

Section A9.3.4.2: apparent effects due to use in closed systems should be weighted accordingly

Section A9.3.4.2: “Substances, which are difficult to test, may yield apparent results that are more or less severe than the true toxicity. Expert judgment would also be needed for classification in these cases.”

Section 4.1.2.5: “There may be circumstances where the lowest toxicity value among taxa is not used for C&L where a WoE approach is used.”

Section A9.3.6.2.3:  “Classification should allow for use of reviews from national authorities and expert panels as long as the reviews are based on primary data.”

D4’s intrinsic properties lead to challenges in testing – Section A9.3.4.2: apparent effects due to use in closed systems should be weighted accordingly and Section A9.3.4.2: Substances, which are difficult to test, may yield apparent results that are more or less severe than the true toxicity. Expert judgment would also be needed for classification in these cases.

D4’s intrinsic properties lead to challenges in testing that must be taken into consideration when assessing the data set for classification and labelling.  D4 is a substance of very low water solubility (approximately 50 micrograms per litre), and its solubility in test media is likely to be lower than its solubility in pure water.  In addition, D4 is volatile with a high Henry’s law constant. These properties led to significant challenges in testing water column species and decisions based on testing under extremely unrealistic conditions should always be cautionary and expert judgement should be used for assessing classification and labelling.

While D4 has exhibited toxicity to water column organisms in the laboratory, toxicity has been demonstrated only in studies performed using closed, hermetically sealed dosing systems having no headspace, which essentially eliminates the predominant intrinsic physical/chemical property of volatilization. All toxicity studies performed using open non-sealed diluter systems that realistically allow equilibration with air, showed no effect at any concentration.

While D4 has exhibited toxicity to water column organisms in the laboratory, given D4’s very low water solubility the methods used to prepare and add stock solutions may have led to unrealistic test conditions. Test systems prepared using over statured stock solutions (prepared at ambient temperatures) can lead to a very high excess of the substance at the point of addition, and when added to a test system operating at the lower temperature of 12 degrees C, the solubility in the test media would remain uncertain.

There may be circumstances where the lowest toxicity value among taxa is not used for C&L where a WoE approach is used - Experts conclude the daphnia chronic NOEC value of 7.9 µg/L of D4 should not be considered the relevant NOEC.

Based on scientific expert analysis by Fairbrother et al., (2016) and Bridges and Solomon (2016), the 21-d Daphnia magna reproduction and survival NOEC value of 7.9 µg/L of D4 should not be considered the relevant NOEC on the following basis:

The study was conducted under closed system, with zero head-space conditions, i.e., the exposure apparatus was all glass, with no openings to the atmosphere to minimize volatilization and loss of D4 from aqueous solution.  This exposure arrangement is extreme and will not be replicated under natural field conditions.

In review of the statistical analysis of the reproduction data, a significant (p ≤ 0.05) difference was observed at the 7.9 µg/L treatment level in comparison with the control. However, this was not a population-relevant effect since the number of offspring per daphnid did not decrease from the control to the highest concentration (15 µg/L), but increased cumulatively from 111 (at control) to 167 (at 15 µg/L) offspring/daphnid. Therefore, the reproduction NOEC should be considered ≥ 15 µg/L.

A significant difference (p ≤ 0.05) was also observed in the survival rate at the 7.9 µg/L treatment level in comparison with the control. The survival rate of parent daphnia changed from 87% to 77% at 7.9 µg/L and 15 µg/L dose levels respectively in comparison with the controls with a survival rate of 93%. However, the survival rate of 77% in the high dose group is the arithmetic mean of just 2 replicates, where in fact only in 1 replicate a survival rate below 80% was observed (replicate 1: 67%; replicate 2: 87%). In addition, the allowable survival rate for controls is 80% so this is considered only a slight reduction in survival, and ultimately the substance did not affect reproduction or neonate size.

Furthermore, Fairbrother et al., (2016), has noted that the “functional water solubility is defined as the maximum achievable solubility under the specific conditions and dilution water quality for a particular study”.  The water solubility of D4 in the 21-d Daphnia magna study is notably lower (15 μg/L) than the actual water solubility of 56 µg/L.

Bridges and Solomon (2016), using a cut-off for environmental concentrations (based on the upper 99.9th centile of the maximum values reported in receiving waters) reported that in the 21-d chronic daphnia study the NOEC was 750 times greater than the water concentration cut-off of 0.02 μg/L and the response was judged to not be adverse (Bridges & Solomon, 2016).

Therefore, based on the above, registrants agree that the overall chronic daphnia D4 NOEC in this study should be considered ≥15 µg/L.

Classification should allow for use of reviews from national authorities and expert panels as long as the reviews are based on primary data – Experts conclude D4 is low risk for Aquatic organisms.

A Weight of Evidence (‘WoE’) analysis of laboratory testing of toxicity to aquatic organism from D4 (Bridges and Solomon, 2016) concluded that: “the overall WoE analysis shows that there is moderate to strong evidence of no adverse effects from concentrations of D4 as measured or expected to be in the environment”.

Mackay et al. (2015) also concluded that super-hydrophobic substances such as D4 often fail to reach a toxic endpoint at their functional solubility limit. As noted by the authors, this is due to the fact that such materials generally act via a narcotic mode of action (MOA), and D4 is unlikely to achieve a body burden at the narcotic MOA level due to limited water concentrations, poor bioavailability, and insufficient time of exposure. In addition, Mackay et al., (2015) have estimated the critical body burden for toxicity as 3 mmol/kg, for a narcotic mode of action.

The estimated time (days) to achieve a critical body residue of 3.0 mmol/kg (888 mg/kg) for D4 is calculated to be 72 to >1000 days for dose levels of reported functional water solubility of 22 down to 4.4 µg/L (See attached Table).

Fairbrother et al., (2016) also indicate that “It is not surprising that D4 has no toxicity or a low level of toxicity in most aquatic species. Like most hydrophobic chemicals, D4 acts via a narcosis mode of action, which requires the accumulation of the chemical in the tissues to achieve a critical (toxic) body burden.” They conclude that “the concepts of narcosis mode of action and chemical activity explain the apparent lack of toxicity of D4 to water column species under environmentally realistic conditions.”

Thus, given the mode of action (narcosis mode of action) and that estimated time (days) to achieve a critical body residue of 3.0 mmol/kg (888 mg/kg) for D4 is calculated to be 72 to >1000 days for the concentration levels used in the 14-day study or up to the functional water solubility of 22 µg/L, the occurrence of mortality in adult rainbow trout after only 14-d exposure is surprising and mortality would not have been expected even if higher concentrations were used in the long-term (93-day) (SEHSC 1991) rainbow trout early life stage toxicity study.

Furthermore, the concentration of D4 in fish following aqueous exposure over a 100-d period has been modelled, assuming a standard kinetic model cfish = (kr/k2)*cw*[1-e(-k2t)]. The uptake (kr¬) and depuration (k2) rates were taken from the D4 BCF study in minnow.

The estimated cfish was then compared with the Critical Body Burden (CBB) of 3 mmol/kg (888 mg/kg), assumed by Mackay et al. (2015) for a non-polar narcosis mode of action. Redman et al. (2012) have concluded that cyclic siloxane compounds, including D4, act through a narcotic mode of action.

Aqueous exposure to 4.4 µg/L, 6.9 µg/L, 12 µg/L, and 22 µg/L D4 in fresh water, as in the prolonged 14-d acute toxicity study with trout, indicates that the steady state cfish which would be reached after approximately 60 days is far below the CBB (See attached Fig. 1). In contrast, exposure at the limit of solubility for D4 in purified water (56 µg/L) indicates that the CBB would be exceeded after approximately 30 days. However, neither 56 µg/L was achieved in the study nor did the study persist for 30 days or longer. Therefore, lethality is not expected in a 14-d fish toxicity test.

See attached Figure 1. Modelled concentrations in fish assuming a standard kinetic model c_fish= (k_r/k₂)*c_w*[1-e^(-k2t)] and using the re-calculated rate constants from Smit et al.(2012). Red dashed line shows the CBB of 3 mmol/kg (888 mg/kg D4) for a non-polar narcosis mode of action.

These data reinforce the outlier status of the 14-day sub chronic fish study and support that effects are not likely to occur even if higher concentrations were used in the long-term (93-day) rainbow trout early life stage toxicity study (SEHSC 1991).

As noted in the Classification and Labelling dossier there is one toxicity study with algae reported (SEHSC, 1990). As it is a limit test, the validity of the study for use in chronic classification is questioned. In addition, growth in controls was reduced similar to that of the treated flask (this was a limit test so there was only one treatment level at the functional solubility of 22 ug/L). Cell density essentially remained unchanged in all flasks suggesting that D4 is not acutely toxic to the algae. The NOEC for algae (SEHSC, 1990) < 22 µg/L was based on yield/biomass, whereas, in accordance with CLP guidance (ECHA, 2015), it is preferred to base it on growth rate. The OECD TG 201 clearly indicates that the growth rate is preferred and that ErC₁₀ or ErC₂₀ is more scientifically founded than NOEC by saying that “the use of average specific growth rate for estimating toxicity is scientifically preferred” (para 47). Taking this into account it is more appropriate to use ErC₁₀ > 22 µg/L. As raw data have been reported in the study report, Reconsile has conducted a re-analysis revealing an inhibition of the average specific growth rate in the treatment group by less than 7 % after 72 h and 96 h, respectively, when compared to the control. Therefore, the ErC₁₀ > 22 µg/L, which is the maximum water solubility level in the test medium.

It is important to note that although the summarised results above are considered reliable, there are some uncertainties associated with testing a substance such as D4 that must be considered. D4 is a substance of very low solubility in water (56 µg/L), and its solubility in test media is likely to be lower than its solubility in pure water. In addition, D4 is volatile with a high Henry’s law constant. Often in order to meet the requirements of the testing guidelines extra measures must be used to maintain D4 in the test system. These include using closed, sealed systems that have no headspace, test systems prepared using saturated stock solutions (prepared at ambient temperatures) which can lead to an excess of the substance at the point of addition and, when added to a test system operating at the lower temperature of 12 °C the solubility in the test media is uncertain, or solvent addition. Analytical measurement of test solutions is often carried out using GC-MS a method that does not necessarily distinguish between dissolved and dispersed test material. As D4 is a clear liquid, undissolved test material may not be obvious therefore often the solubility in the test media is uncertain.

GHS Guidance clearly highlights the need for WOE and expert judgement when assessing a substance that is difficult to test.

Conclusion on classification

The substance has an EU harmonised classification as follows:

Chronic Cat.1 (aquatic), H410: Very toxic to aquatic life with long lasting effects; M-factor = 10 in Annex VI of CLP Regulation (EC) No 1272/2008.

In January 2017, the European Chemicals Agency (ECHA) published a proposal to reclassify D4 from Aquatic Chronic Category 4 (H413) to Aquatic Chronic Category 1 (H410) under the Regulation on the classification, labelling and packaging of substances and mixtures (CLP).

The RAC opinion adopted on 9 March 2018, concluded that following the criteria for long-term (chronic) hazard, D4 warrants classification as Aquatic Chronic 1 (H410) with an M-factor of 10 (not rapidly degradable and chronic toxicity in range of 0.01 < NOEC ≤ 0.001). 

The revision to the harmonised classification was made in the 15th adaptation to technical progress (ATP) amendment to Annex VI of CLP Regulation (EC) No 1272/2008.

The dossier has been updated with the revised EU harmonised classification, however, in the view of the registrants, based on a weight of evidence (WoE) approach and following expert judgment and review, D4 should not be classified on the basis of aquatic toxicological effects.A WoE approach is recommended to address some of the uncertainties as well as the technical difficulties resulting from intrinsic physicochemical and partitioning properties of the substance when conducting guideline tests (GHS Revision 9, 2021, sections 4.1.2.5, A9.3.4.2, A9.3.6.2.3). Therefore, further scientific analysis of all the available data as well as recent expert reviews is attached.

(Attachment: <556-67-2 D4 Section 7.7 Environmental classification.pdf>)