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EC number: 221-906-4
CAS number: 3277-26-7
Approach to the Terrestrial
Chemical Safety Assessment
(CAS 3277-26-7) hydrolyses very rapidly (t½ = 11.3 min
at pH 7 and 25°C)
to produce the intermediate hydrolysis products dimethylsilanol.
Dimethylsilanol hydrolyses further (t½ = 2.5 days at pH
7 and 25°C) to produce the ultimate hydrolysis
products, dimethylsilanediol (CAS 1066-42-8) and hydrogen.
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.
Exposure of soil is expected to be
via the partitioning of the substance to the wastewater treatment plant
(WWTP) sludge. Although the majority of the substance will partition to
water, the proportion of the substance that adsorbs to the WWTP sludge
will either be burnt or spread on soil, depending on the practices of
the WWTP. For sludge that is spread onto soil, the substance will have
undergone further hydrolysis by the time it reaches the soil, however
given the half-life of the second hydrolysis rate, it is possible that
residual intermediate hydrolysis product will be present. The
terrestrial chemical safety assessment is therefore based on the
properties of the intermediate and ultimate hydrolysis products.
In accordance with Column 2 of
REACH Annex IX, there is no need to further investigate the effects of
this substance in a long or short-term terrestrial toxicity to
invertebrates/higher plants study because, as indicated in guidance
R.7.11.6 (ECHA 2017), the chemical safety assessment indicates that the
risk is already adequately controlled and further testing is not
The substance is highly water
soluble, has low potential for bioaccumulation adsorption (based on log
Kow <3 (0.6) and Log Koc1.33,
derived using EUSES). Low toxicity was observed in short-term
aquatic tests for a structurally analogous substance, and there is no
reason to expect any specific mechanism of toxicity beyond narcosis.
Therefore, the occurrence of more severe toxic effects in the
terrestrial compartment that were not expressed in the aquatic studies
would be considered unlikely.
Dimethylsilanol is classed as
hazard category 1 for the terrestrial environment (Table R.7.11-2 of
ECHA guidance R7.c, 2017) based on the fact that there is no indication
for high adsorption (log Koc 1.33) or high persistence of the
substance in soil (the substance will hydrolyse to dimethylsilanediol)
and the substance displays low toxicity to aquatic organisms (EC/LC50
not <1 mg/l). In this situation, the approach for screening assessment
is to conduct a PEC/PNEC screen based on the equilibrium partitioning
The PNECscreen(EQPM) for
dimethylsilanol is derived from the short-term test results with
invertebrates and has a value of 0.068 mg/kg dwt. Based on the exposure
assessment of dimethylsilanol, the highest PEC/PNECscreen(EQPM) is 1.86E-4.
The RCR is therefore <1. Table R.7.11-2 of ECHA guidance R7.c, 2017,
concludes that if PEC/PNECscreen < 1: No toxicity testing for soil
organisms need to be done,
Overall it is concluded that
the risk characterisation conclusion is sufficiently conservative and
therefore further in vivo testing is not considered necessary.
Details on how the PNEC and
the risk characterisation ratio have been derived can be found in IUCLID
Section 6.0 and Chapters 9 and 10 of the Chemical Safety Report,
In accordance with Column 2
of REACH Annex IX, there is no need to further investigate the effects
of this substance in a long or short-term terrestrial toxicity to
study because, as indicated in guidance R.7.11.6 (ECHA 2016), the
quantitative chemical safety assessment (conducted according to Annex I
of REACH) indicates that the Risk Characterisation Ratio is below 1 and
therefore the risk is already adequately controlled and further testing
is not justifiable.
is classed as hazard category 3 for the terrestrial environment (Table
R.7.11-2 of ECHA guidance R7.c, 2017) based on potential for high
persistence (DT50 >180 days), lack of ready biodegradability
and low toxicity to aquatic organisms (EC/LC50 not <1 mg/l).
In this situation, a screening approach is applied: a confirmatory
long-term terrestrial test is required, in addition to the equilibrium
partitioning approach with an extra factor of ten in order to determine
whether further full tests are necessary.
A confirmatory test would be
conducted with the most sensitive organism group based on existing
aquatic data. For this substance, there were no effects observed in the
short-term data and the long-term toxicity study with aquatic
invertebrates. In this case it would be preferable to conduct a
confirmatory test with terrestrial invertebrates.
Only an indicative
PNECscreen(EQPM) for dimethylsilanediol can be derived from the
long-term aquatic test results because no effects were observed. The
PNEC derived in such a way has a value of ≥0.32 mg/kg dwt. For the
purpose of the screening assessment comparison only, an extra factor of
ten is applied (PECx10/PNECscreen(EQPM)). Based on the exposure
assessment of dimethylsilanediol, the highest PECx10/PNECscreen(EQPM) is
0.012x10/0.32 = RCR 0.38.
only an invertebrate study were required, the definitive terrestrial
risk characterisation would use a PNECsoil based on the lower of the
test results between invertebrates and the existing soil micro-organisms
study with an assessment factor of 50; if a plant study were also
conducted, the assessment factor would become 10.
To be more conservative than the
value of PNECscreen(EQPM), the terrestrial plants or invertebrates study
would have to exhibit a dose response with a NOEC/EC10 ≤16
mg/kg dw, and assuming an AF 50.
substance is highly water soluble, has low potential for bioaccumulation
and adsorption (based on log Kow <3 (-0.38)
and Log Koc 0.82, derived using EUSES) but is not
biodegradable. No toxicity
was observed in short- and long-term aquatic tests at the highest
concentrations tested, and there is no reason to expect any specific
mechanism of toxicity beyond narcosis. Therefore, the occurrence of more
severe toxic effects in the terrestrial compartment that were not
expressed in the aquatic studies would be considered unlikely.
In the case of dimethylsilanediol,
the registrants therefore consider that a long-term terrestrial study is
unlikely to affect the outcomes of the chemical safety assessment. As
such the registrants propose that further testing (including the
confirmatory study) is not necessary.
Details on how the PNEC and the
risk characterisation ratio have been derived can be found in IUCLID
Section 6.0 and Chapters 9 and 10 of the Chemical Safety Report,
In addition, a soil
micro-organism toxicity study, equivalent to OECD 216 and 217, is
available with dimethylsilanediol where no effects were reported at the
highest concentrations tested (up to 300 ppm, equivalent to 300 mg/kg
Finally, it should be noted that a
plant toxicity study with dimethylsilanediol is available indicating
effects to wheat plants exposed to aqueous solutions of the substance
(Cote-Beaulieu et al. 2009). The study assessed the effects of
exposure to methyl silicic acids on growth and development of wheat
seedlings and on susceptibility of the seedling to infection by powdery
mildew. The methods used were not in accordance with any standard
guideline and the presence of silica was observed also in the control of
the peat-based studies. It is difficult to assess the significance of
the results for the standard REACH endpoints. Therefore, the study is
considered reliability 4 and the data are not considered to be relevant
for the risk assessment.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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