Registration Dossier

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
PNEC aqua (freshwater)
PNEC value:
4.23 mg/L
Assessment factor:
500
Extrapolation method:
assessment factor
PNEC freshwater (intermittent releases):
42.3 mg/L

Marine water

Hazard assessment conclusion:
PNEC aqua (marine water)
PNEC value:
0.423 mg/L
Assessment factor:
5 000
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:
16 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:
PNEC sediment (marine water)
PNEC value:
1.6 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:
no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:
PNEC soil
PNEC value:
0.62 mg/kg soil dw
Extrapolation method:
equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
no potential for bioaccumulation

Additional information

The application of QSARs in ecotoxicology is well established. QSARs are at their strongest when they have been developed using an extensive and reliable training data set for a well defined type of chemical structure and mode of action (e.g. non-specific narcosis).

Dimethoxymethylsilane and its hydrolysis product methylsilanetriol are part of a category of organosilicon substances containing only alkyl, aryl, alkoxy or hydroxy groups attached to the Si atom when present in aqueous solution. Secondary features may be present in the alkyl chain (e.g. halogen, nitrile, unsaturated bonds) but do not affect the toxicity of the substances. Short-term aquatic toxicity test data are available for at least one trophic level (data for fish, invertebrates and algae are available) for approximately 40 such substances.

QSAR methods for predicting the short- and long-term toxicity of the category to aquatic organisms have been explored. The first step was to take hydrolysis into account as many members of the category are highly-reactive chlorosilanes or alkoxysilanes. In line with technical guidance, it was considered that the parent substance dominates the acute responses if the half-life is more than 12 hours at pH 7. Otherwise, effect concentrations and physicochemical properties are expressed in terms of the hydrolysis products, which are silanols.

The USEPA model ECOSAR (EPA, 2010) is well-validated for ‘neutral organics’, i.e. those which act by a general narcotic mechanism, the potency of which is usually related to log Kow. The scope of the model is short-term and long-term effects for the three standard trophic levels i.e. fish, invertebrates and algae.

The ECOSAR ‘neutral organics’ QSARs for acute data were applied to the category members and the effect concentrations (EC50or LC50values) calculated using log Kowand molar mass as input variables.

The dataset used in the development of the ECOSAR QSARs contained only a small number of organosilicon compounds; therefore, further validation was attempted. The trends were as expected; however, the approach was considered to be sub-optimal because many of the data points are greater than limit values. These values need to be excluded from the QSAR validation and these results in much of the available information should be neglected.

As an alternative approach, data were considered in terms of the range of the E(L)C50, in accordance with normal classification bands:

  1. E(L)C50< 1 mg/l;
  2. E(L)C50in the range > 1 mg/l to 10 mg/l;
  3. E(L)C50in the range > 10 mg/l to 100 mg/l;
  4. E(L)C50> 100 mg/l.

 

The resulting data showed that in broad terms, ECOSAR predicted correctly for most substances for each trophic level. However, performance (in terms of agreement between measured and predicted effect band) was improved significantly by multiplying each predicted value (expressed in mg/l) by a factor of 0.2. This is equivalent to saying that the organosilicon substances are slightly more toxic than the general ECOSAR ‘neutral organics’ regression lines, although still well within the range of each model when the method errors are considered. The factor of 0.2 is applicable to fish,Daphniaand algae, across the whole range of log Kowvalues.

Organosilicon chemistry differs from hydrocarbon chemistry in terms of the nature and type of bonds formed by Si to O. However, that does not exclude modelling concepts such as Kowfrom being useful. The O atom attached to Si has hydrogen-bond basicity, making it slightly more electrophilic than its hydrocarbon equivalent. Therefore it is not surprising that the ECOSAR equations had to be modified slightly in order to achieve a better match between observed and predicted data. No mechanistic explanation for the factor of 0.2 is immediately available. However, it is considered that the data are consistent with a simple polar narcosis interpretation.

It is concluded that the acute toxicity of the substances in the sub-category can be predicted using slightly modified versions of the ECOSAR algorithms.

For the registered substance hydrolysis product, methylsilanetriol, the inputs to and outputs from these calculations are as follows:

Log Kow: -2.4

MW: 94.14

Fish, 96-h LC50: 100000 mg/l

Daphnid, 48-h LC50: 30 000 mg/l

Algae, 96-h EC50: 2115 mg/l

These values are within the ranges applicable to the validation set (log Kowis -3 to 9, MW is 32-500 g/mol and effect levels are 10-5to 105mg/l) and methylsilanetriol is within the structural definition of the category. Therefore, it is considered appropriate to use the predictive method for this substance.

Further details of the validation of the predictive method are given in PFA, 2010. Long-term effects and algal QSAR predictions are considered to need further validation.

The standard approach to PNEC calculation, when short-term data are available for three trophic levels, is to apply an assessment factor of 1000 to the lowest lethal or effect concentration (E(L)C50). However, this assessment factor may be reduced by up to a factor of 10 if this is justified by the available data, for example:

  • Evidence from structurally similar compounds which may demonstrate that a higher or lower factor may be appropriate.
  • Knowledge of the mode of action as some substances by virtue of their structure may be known to act in a non-specific manner. A lower factor may therefore be considered. Equally, a known specific mode of action may lead to a higher factor (ECHA 2009).

  Calculation of Predicted No Effect Concentration (PNEC)

 

Dimethoxymethylsilaneand its silanol hydrolysis product, methylsilanetriol, are part of a category of organosilicon substances containing only alkyl, halogen or alkoxy groups attached to the Si atom. The category (containing approximately 40 substances with reliable data on toxicity to aquatic organisms) has a low hazard profile and data are consistent with a simple polar narcosis mechanism, the potency of which is usually related to log Kow. It is considered this justifies a reduction of the assessment factor from the normal value of 1000. A value of 500 is used to reflect the increased confidence in the individual values due to the low variability across the category.. The QSAR data are estimates and the PNECs based on these values are indicative only.

Conclusion on classification

Dimethoxymethylsilane (CAS 16881 -77-9) hydrolyses rapidly (half-life of 0.3 h at pH 7 and 20 °C) and is not readily biodegradable. Due to the rapid rate of hydrolysis, assessment was based on the hydrolysis product as under conditions relevant to ecotoxicity testing organisms will predominantly be exposed to the hydrolysis product. According to the Regulation (EC) No.1272/2008 (CLP) dimethoxymethylsilane is not classified as hazardous to the environment, since no toxicity towards aquatic organisms was determined.