Dimethoxydimethylsilane

Toxicity to microorganisms

Link to relevant study record(s)

Reference

Endpoint:

activated sludge respiration inhibition testing

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

Please refer to the analogue approach justification provided in section 13.

Reason / purpose for cross-reference:

read-across source

Duration:

3 h

Dose descriptor:

NOEC

Effect conc.:

>= 1 000 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks on result:

other: test concentration series with pH adjustment

Duration:

3 h

Dose descriptor:

EC50

Effect conc.:

> 1 000 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks on result:

other: test concentration series with pH adjustment

Duration:

3 h

Dose descriptor:

NOEC

Effect conc.:

320 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks on result:

other: test concentration series without pH adjustment

Duration:

3 h

Dose descriptor:

LOEC

Effect conc.:

1 000 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks on result:

other: test concentration series without pH adjustment

Duration:

3 h

Dose descriptor:

EC50

Effect conc.:

436 mg/L

95% CI:

>= 235 - <= 776

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks on result:

other: test concentration series without pH adjustment

Conclusions:

For the toxicity toward activated sludge a NOEC (3 h) of ≥ 1000 mg/L was determined in a reliable guideline study with GLP compliance, equivalent to a NOEC (3 h) of ≥ 714 mg/L when expressed in terms of the silanol hydrolysis product, dimethysilanediol (DMSD, CAS No. 1066-42-8) in common with the registered substance.

Description of key information

NOEC (3 h) ≥ 1000 mg/L (nominal, OECD 209, activated sludge, CAS No. 75-78-5), equivalent to ≥ 714 mg/L when expressed in terms of the silanol hydrolysis product DMSD

Key value for chemical safety assessment

EC10 or NOEC for microorganisms:

714 mg/L

Additional information

Dimethoxy(dimethyl)silane (CAS No. 1112-39-6) hydrolyses rapidly in contact with water (DT50 < 0.6 h), to the silanol hydrolysis product dimethylsilanediol (DMSD, CAS No. 1066-42-8) and the alcohol hydrolysis product methanol (CAS No. 67-56-1). The ECHA guidance R.16 states that “for substances where hydrolytic DT50 is less than 12 h, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself” (ECHA, 2016). The ECHA guidance R.16 also suggests that in case the hydrolysis half-life is less than 12 h, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity. Methanol is well described in the public domain literature. It has a low environmental hazard profile and is therefore not considered contributory to the overall aquatic toxicity of the registered substance (OECD SIDS, 2004). Thus, the ecotoxicity assessment was based on the silanol hydrolysis product dimethylsilanediol (DMSD, CAS No. 1066-42-8).
For the toxicity to microorganisms, a reliable ASRI study is available for the analogue substance, dichloro(dimethyl)silane (CAS 75-78-5, EC 200-901-0). The hydrolysis half-life of dichloro(dimethyl)silane (<<1 min at pH 7 and 25°C) indicates that, under conditions relevant to ecotoxicity assessment, it will hydrolyse to the silanol hydrolysis product dimethylsilanediol (DMSD, CAS No. 1066-42-8) in common with registered (target) substance dimethoxy(dimethyl)silane. The other hydrolysis product is hydrogen chloride. Effects of hydrogen chloride on aquatic organisms are limited to those that result from changes to pH in unbuffered media and are considered negligible. Details on the read-across can be found in the analogue justification attached in IUCLID Section 13.

In the study conducted with dichloro(dimethyl)silane (CAS No. 75-78-5), a NOEC (3 h) value of ≥1000 mg/L (nominal concentration) has been determined for the effects of the test substance on the respiration rate of microorganisms (Charles River Laboratories, 2022). The test was conducted according to OECD TG 209, and in compliance with GLP. In the preliminary tests (a range-finding test and a limit test at 1000 mg/L), the pH decreased with increasing test concentration, therefore two test concentration series were conducted in the definitive test: with and without pH adjustment. As statistically significant effects were observed in the highest test concentration without pH adjustment and the pH decreased with increasing test substance concentration, the effects were attributed to the reduction in pH and not to true toxicity. Therefore, the test concentration series with pH adjustment were selected as key and the NOEC derived from this series was concluded as ≥1000 mg/L. An EC50 (3 h) value of >1000 mg/L was also determined for the test concentration series with pH adjustment.
The results may be expressed in terms of the concentration of the hydrolysis product, dimethylsilanediol (DMSD, CAS No. 1066-42-8), resulting in a NOEC (3 h) ≥ 714 mg/L and EC50 (3 h) > 714 mg/L, obtained by application of a molecular weight correction factor of 0.714 (MW of silanol/Mw of parent = 92.17 / 129.06).

Reference
OECD SIDS, 2004. Methanol - SIDS Initial Assessment Report For SIAM 19, Berlin, Germany: UNEP Publications.