Registration Dossier

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
no hazard identified

Marine water

Hazard assessment conclusion:
no hazard identified

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
1 mg/L
Assessment factor:
100
Extrapolation method:
assessment factor

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:
1.7 mg/kg food
Assessment factor:
300

Additional information

Category read-across discussion

The hydrolysis half-life of decamethyltetrasiloxane (L4) is 30.3 d (728 h) at pH 7 and 25°C (measured data). The water solubility of the substance is low (6.7 μg/l) and the log Kow is high (8.21). It is therefore likely that, under the flow-through exposure conditions of the fish and invertebrate tests that the test organisms will have been predominantly exposed to very low concentrations of the registered substance.

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. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore, the hydrolysis rate of the substance is particularly important since after hydrolysis occurs the resulting product has different physicochemical properties and structure.

In aqueous media, decamethyltetrasiloxane hydrolyses very slowly in water (half-life 728 hours at pH 7 and 25°C).

The registration substance, decamethyltetrasiloxane (L4, CAS 141-62-8) 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 rates in the sediment compartment. For substances with a log Kow of 6 and above no effects are seen with aquatic organisms due to the substance low water solubility limiting the effects seen. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

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

In the following paragraphs the category read-across approach for decamethyltetrasiloxane is outlined, taking into account structure and physicochemical properties.

Sediment toxicity: Read-across from L3 to L4

The registration substance, L4, and the surrogate substance octamethyltrisiloxane (L3, CAS 107-51-7) are linear siloxanes four silicon atoms and three silicon atoms respectively, alternated by oxygen atoms. In both L3 and L4, the Si atoms are fully methyl substituted. L3 and L4 possess similar physicochemical properties.  A comparison of the key physicochemical properties is presented in table 7.0.1 below. Both substances have negligible biodegradability and similar moderate hydrolysis rates.

No effects at the limit of solubility have been reported in short-term and long-term studies in other trophic levels conducted with both substances.

Given the similar properties and structural similarities, it is considered valid to read across sediment toxicity data from L3 to L4.

Table 1: Summary of ecotoxicological and physicochemical properties for the registered substance and the sediment and terrestrial toxicity data surrogate substances.

CAS Number

141-62-8

107-51-7

Chemical Name

Decamethyltetrasiloxane (L4)

Octamethyltrisiloxane (L3)

Si hydrolysis product

Trimethylsilanol and dimethylsilanediol

 

Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to trimethylsilanol. For properties of dimethylsilanediol, see data reported for D5 hydrolysis product in next column

Trimethylsilanol; dimethylsilanediol

 

Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to trimethylsilanol. For properties of dimethylsilanediol, see data reported for D5 hydrolysis product in previous column

Molecular weight (parent)

310.69

236.54

Molecular weight (hydrolysis product)

90.2 (trimethylsilanol)

92.2(dimethylsilanediol)

log Kow(parent)

8.2

6.6

log Kow(silanol hydrolysis product)

1.2 (trimethylsilanol)

-0.41(dimethylsilanediol)

Log Koc(parent)

5.16

4.34

Water sol (parent)

0.00674 mg/l

3.4E-02 mg/l

Water sol (silanol hydrolysis product))

995 mg/l (trimethylsilanol)

1.0E+06 mg/l (dimethylsilanediol)

Vapour pressure (parent)

73 Pa

530 Pa

Vapour pressure (hydrolysis product)

1900 Pa (trimethylsilanol)

7 Pa (dimethylsilanediol)

Hydrolysis t1/2at pH 7 and 25°C

728 h

329 h

Short-term toxicity to fish (LC50)

>6.3 μg/l

>19.4 μg/l

Short-term toxicity to aquatic invertebrates (EC50)

n/a

>20 μg/l

Algal inhibition (ErC50and NOEC)

ErC50: >2.2 μg/l; NOEC: ≥2.2 μg/l

EC50: >9.4 μg/l; NOEC: ≥9.4 μg/l

Long-term toxicity to fish (NOEC)

≥7.9 μg/l

≥27 μg/l

Long-term toxicity to aquatic invertebrates (NOEC)

≥4.9 μg/l

≥15 μg/l

Sediment toxicity (NOEC)

(normalised to 5% OC)

92 mg/kg dwtH. azteca

89 mg/kg dwt ,C. riparius
61 mg/kg dwtL. variegatus
95 mg/kg dwt, H. azteca

Short-term terrestrial toxicity (L(EC)50)

n/a

n/a

Long-term terrestrial toxicity (NOEC)

n/a

n/a

 

 

Conclusion on classification

The substance has reliable short-term E(L)C50 values of >6.3 μg/l in fish and >2.2 μg/l in algae. It has reliable NOEC 2.2 μg/l in algae and reliable long-term NOEC of 7.9 μg/l in Daphnia and 6.3 μg/l in fish.

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 slowly 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.