Hydroxytrimethylsilane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.002 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

0.003 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

10 mg/L

Assessment factor:

10

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:

PNEC soil

PNEC value:

0.083 mg/kg soil dw

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

67 mg/kg food

Assessment factor:

300

Additional information

The hydrolysis half-life of hexamethyldisiloxane (HMDS) (107-46-0, L2) is 116 hours (5 days) at pH 7 and 25˚C. The water solubility of the substance is low (0.93 mg/L) and the log K_ow is high (5.1). It is therefore likely that, under the flow-through exposure conditions of the aquatic toxicity tests reported in the following sections, the test organisms will have been predominately exposed to very low concentrations of the registered substance.

READ-ACROSS JUSTIFICATION

In order to reduce the need for testing, read-across is proposed to fulfil REACH Annex VII-X 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, HMDS hydrolyses in water with a half-life 116 hours at pH 7 and 25°C.

HMDS and the substances used as surrogates for read-across are part of the Reconsile Siloxane Category of compounds. Substances in this group tend to have low water solubility, high adsorption and partition coefficients and slow degradation in the sediment compartment. For substances with a log Kow of 8 and above, no long-term toxicity effects are seen with aquatic organisms due to the low water solubility limiting the amount of substance that can be taken up by organisms. In the environment, the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

In the following paragraphs the read-across approach for HMDS is assessed for the surrogate substances taking into account structure, hydrolysis rate and physicochemical properties.

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

Read-across from octamethyltrisiloxane (CAS 107-51-7, L3) to hexamethyldisiloxane (CAS 107-46-0, L2).

There are no long-term fish toxicity data available for hexamethyldisiloxane, therefore data from the structurally-related substance octamethyltrisiloxane (CAS 107-51-7), have been read across. HMDS and L3 are within the Reconsile Siloxane Category of compounds, and have high log K_ow (5.1 and 6.6), high log K_oc (3.0 and 4.3) and low water solubility (0.93 mg/l and 0.034 mg/l). In addition, the substances have a slow hydrolysis rate relative to the time-scale of ecotoxicity testing (t_1/2 = 116 h and 329 h at pH 7 and 25°C). Environmental toxicity data for siloxanes are consistent with a non-polar narcosis mechanism (Redman 2012, Peter Fisk Associates 2013bc). Given the similar properties, structural similarities, and expected mode of action, it is considered valid to read-across the results for octamethyltrisiloxane to fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2017i) attached in Section 13.

Conclusion on classification

The substance is classified as follows:

According to Regulation (EC) No 1272/2008:

Aquatic Acute 1 (Hazard statement: H400: Very toxic to aquatic life.). An M-factor of 1 applies.

Aquatic Chronic 2 (Hazard statement: H411: Toxic to aquatic life with long lasting effects)

According to Directive 67/548/EEC:

N; R50 Dangerous for the environment; Very toxic to aquatic organisms.

Reliable short-term toxicity tests results are available for the effects of the test substance on freshwater fish (Oncorhynchus mykiss), and algae (Selenastrum capricornutum (new name: Pseudokirchneriella subcapitata)). A 96-hour LC₅₀ value of 0.46 mg/l (expressed as measured concentration) or 3.02 mg/l (nominal concentration) has been determined for effects on mortality of O. mykiss. A 70-hour E_rC₅₀ value of >0.55 mg/L and ErC₁₀ of 0.09 mg/L have been determined for the effects of the substance on growth rate of S. capricornutum.

In longer-term tests a 90-day NOEC of ≥0.027 mg/l in fish is conservatively read across from a structural analogue and a chronic NOEC of ca. 0.02 mg/l has been estimated for the effects of the substance in fish. A 21-day EC₅₀ value of 0.30 mg/L and NOEC of 0.08 mg/L have also been determined for the effects of the substance on reproduction of D. magna.

The substance is hydrolytically unstable with a half-life of 120 h at pH 7 and 25°C, meaning that for classification and labelling purposes, it is considered to be a rapidly degradable substance. The hydrolysis product, trimethylsilanol, has a log K_ow of 1.19.