1,1,1,3,5,5,5-heptamethyltrisiloxane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

STP

Hazard assessment conclusion:

no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

8.9 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.89 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:

no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

13 mg/kg food

Assessment factor:

300

Additional information

The hydrolysis half-life of 1,1,1,3,5,5,5-heptamethyltrisiloxane (H-L3, CAS 1873-88-7) is 2.2 days at pH 7 and 25°C. The water solubility of the substance is low (0.02 mg/l) and the log K_ow is high (6.2). It is therefore likely that, under the conditions of the short-term aquatic toxicity tests with fish, invertebrates and algae that exposure will have been to low concentrations of the registered substance, rather than the hydrolysis products.

REACH guidance (ECHA 2016, R.16) states that “for substances where hydrolytic DT50 is less than 12 hours, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself”. ECHA Guidance Chapter R.7b (ECHA, 2017) states that where degradation rates fall between >1 hour and <72 hours, testing of parent and/or degradation product(s) should be considered on a case-by-case basis.

The substance will be exposed to the environment through wastewater treatment plant (WWTP) effluent only. The minimum residency time in the wastewater treatment plant is approximately 7 hours (although this is a conservative figure and wastewater treatment time may be hours to days longer) with an average temperature of 15°C (assumed to be at neutral pH). Due to the hydrolysis half-life of 2.2 days, it is expected that the receiving waters in the environment will predominantly be exposed to the parent substance.

Therefore, the environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of the parent substance, in accordance with REACH guidance.

READ-ACROSS JUSTIFICATION

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

The behaviour of H-L3 in the environment is dominated by the high partition coefficient (log K_ow 6.2) and adsorption coefficient properties (log K_oc 3.8).

The registered substance 1,1,1,3,5,5,5-heptamethyltrisiloxane (H-L3, CAS 1873-88-7) and the substances used as surrogates are part of the Reconsile Siloxanes Category. Substances in this group tend to have slow hydrolysis rates, low water solubility, high log K_ow, high adsorption coefficients and slow degradation in the sediment compartment.

Additional information is given in a supporting report (Siloxane Category Report, PFA 2017) attached in Section 13 of the IUCLID dossier. The Siloxane Category report (PFA, 2017) addresses the Assessment Elements of the RAAF and, in addition to describing the siloxanes category, forms the RAAF justification report.

 

In addition, in the following paragraphs the read-across approach for H-L3 is assessed for the surrogate substances taking into account structure, hydrolysis rate and physico-chemical properties.

Table: Summary of physico-chemical and ecotoxicological properties for the registered and surrogate substances.

CAS Number	1873-88-7		107-51-7		2487-90-3
Chemical Name	1,1,1,3,5,5,5-heptamethyltrisiloxane (H-L3)		Octamethyltrisiloxane (L3)		Trimethoxysilane
Si hydrolysis product	Trimethylsilanol	Methylsilanetriol	Trimethylsilanol	Dimethylsilanediol	Silicic acid
Molecular weight (parent)	222.51		236.54		122.2
Molecular weight (hydrolysis product)	90.2	94.14	90.2	92.17	96.1
log Kow(parent)	6.2		6.6		0.2
log Kow(silanol hydrolysis product)	1.2	-2.4	1.2	0.38	-2.9
Water sol (parent)	0.02 mg/l		0.034 mg/l		5.4E+04 mg/l
Water sol (silanol hydrolysis product)	996 mg/l	1000000 mg/l	996 mg/l	1000000 mg/l	7.6E+05 mg/l
Vapour pressure (parent)	approximately 850 Pa		530 Pa		113.7 hPa
Vapour pressure (hydrolysis product)	1900 Pa	0.05 Pa	1900 Pa	7 Pa (QSAR prediction)	5.2E-14 Pa
Hydrolysis t1/2at pH 7 and 25°C	2.2 days		329 hours (13.7 days)		?17 s at 2°C
Hydrolysis t1/2at pH 4 and 25°C	0.7 h		at pH 5 5.09 hours		?17 s at 2°C
Hydrolysis t1/2at pH 9 and 25°C	0.4 h		9.76 hours		?17 s at 2°C
Short-term toxicity to fish (LC50) *	>0.108 mg/l		>0.0194 mg/l		>100 mg/l
Short-term toxicity to aquatic invertebrates (EC50) *	>0.030 mg/l		>0.020 mg/l		>100 mg/l
Algal inhibition (ErC50and NOEC) *	EC50: ?0.019 mg/l; NOEC: ?0.019 mg/l		EC50: >0.0094 mg/l; NOEC: ?0.0094 mg/l		EC50: >100 mg/l;NOEC: <6.3 mg/l
Long-term toxicity to fish (NOEC) *	n/a		?0.027 mg/l		n/a
Long-term toxicity to aquatic invertebrates (NOEC) *	n/a		?0.015 mg/l		n/a
Sediment toxicity (NOEC) (normalised to 5% OC)	n/a		89 mg/kg dwt ,C. riparius; 61 mg/kg dwtL. Variegatus; 95 mg/kg dwt, H. azteca		n/a
Short-term terrestrial toxicity (L(E)C50)	n/a		n/a		n/a
Long-term terrestrial toxicity (NOEC)	n/a		n/a		n/a

* Aquatic tests carried out on the parent substance. Due to test methods/exposure regime/hydrolysis rates indication of whether the test organisms were exposed to parent substance or hydrolysis product is included in the end point summaries.

·        Read-across from L3 to H-L3:

The registration substance, 1,1,1,3,5,5,5-heptamethyltrisiloxane (H-L3) is a siloxane of three silicon atoms linked by oxygen atoms. The structurally analogous substance proposed for read-across is octamethyltrisiloxane (L3, CAS 107-51-7), which is also a linear siloxane made up of a chain of 3 silicon atoms linked by oxygen. In both substances, methyl groups form the silicon side groups, however the difference is that in H-L3 the middle silicone has an Si-H bond in place of one of the methyl groups.

Significant reaction of the Si-H bond, present in the registration substance but not in L3, would be expected to occur during long-term aquatic toxicity tests (see section on hydrolysis for more information). To exclude the possibility that by-products of this Si-H reaction might bring unpredicted toxicity to aquatic organisms, data have been read-across from another substance that contains a Si-H bond (see “Read-across from trimethoxysilane to H-L3” below).

H-L3 and L3 are part of the Reconsile Siloxanes Category. They have low water solubility (0.02 and 0.034 mg/l respectively), high log K_ow (6.2 and 6.6 respectively) and slow hydrolysis rate (2.2 days at pH 7 and 20-25°C and 13.7 days at pH 7 and 25°C respectively) indicating that organisms can only be exposed to very low concentrations of the substance in solution. In the environment, the substances will adsorb to particulate matter and will partition to soil and sediment compartments. Both substances also are not readily biodegradable and have high adsorption to sediment potential.

No effects at the limit of solubility have been reported in short-term and long-term studies in other trophic levels conducted with the surrogate substance, L3. L3 is used to read-across to long-term toxicity to fish and invertebrates endpoints, for which a 90-d NOEC of ≥0.027 mg/l and a 21-d NOEC value of ≥0.015 mg/l, respectively, have been determined. L3 is also used to read-across to sediment toxicity endpoints. Both the registration and read-across substances are expected to act via a non-polar narcotic mechanism of toxicity; neither substance has functional groups that affect the toxicity. Given the similar properties and structural similarities, it is considered valid to read-across data.

·        Read-across from trimethoxysilane to H-L3:

Trimethoxysilane (CAS 2487-90-3) has been included as read-across in the dataset for H-L3 because, in addition to the Si-OMe group, it has an Si-H bond which will undergo the same hydrolysis reaction as the registration substance. The Si-OMe hydrolyses rapidly (hydrolysis half-life of ≤17 seconds at pH 4, 7 and 9, 2˚C). The Si-H bond is expected to react in water, however the rate of this reaction is uncertain but it is expected to react over the course of the short-term tests. Read-across is used to fulfil long-term aquatic toxicity endpoints from a substance that does not contain an Si-H bond. The Si-H bond is expected to react during the course of long-term aquatic toxicity tests. To exclude the possibility that this functional group might cause toxicity to aquatic organisms, data for trimethoxysilane have also been included in the dataset for H-L3.

The short-term toxicity data for this substance report L(E)C₅₀ values in excess of 100 mg/l. The data indicate that the Si-H bond further reactions are not likely to affect the toxicity.

Trimethoxysilane is used to read-across to short-term toxicity to fish, aquatic invertebrates and algae endpoints. E(L)C₅₀ values of >100 mg/l for each species have been determined. Read-across from trimethoxysilane is not used to fulfil any data-gap, but is used to demonstrate that presence of the Si-H bond does not cause toxic effects.

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ values of >0.108 mg/l in fish, >0.030 mg/l in invertebrates and >0.019 mg/l in algae. It has reliable NOEC ≥0.019 mg/l in algae, and long-term data (read across from a structural analogue) give NOEC ≥0.027 mg/l in fish and NOEC ≥0.015 mg/l in invertebrates.

The available short- and long-term aquatic toxicity data indicate that there are no effects on aquatic organisms at the limit of solubility of the substance in water.

The substance hydrolyses at a moderate rate in water and is not readily biodegradable.

These data are consistent with the following classification under Regulation (EC) No 1272/2008 (as amended) (CLP):

Acute toxicity: Not classified.

Chronic toxicity: Not classified.