Dichlorocyclohexylmethylsilane

Short-term toxicity to aquatic invertebrates

Link to relevant study record(s)

Reference

Endpoint:

short-term toxicity to aquatic invertebrates

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1995-10-10 to 1995-10-12

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

EU Method C.2 (Acute Toxicity for Daphnia)

Deviations:

no

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 202 (Daphnia sp. Acute Immobilisation Test)

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Analytical monitoring:

yes

Details on sampling:

- Concentrations: control, 1, 3.2, 10, 32 and 100 mg/l, both replicates.

- Sampling method: Samples were taken at 0 h and 48 h and extracted immediately upon preparation.

Vehicle:

no

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION

- Method: 200 mg was dispersed in reconstituted water with the aid of ultrasonic disruption and the volume adjusted to 2 L to give a 100 mg/l test concentration from which dilutions were made to obtain nominal test concentrations.

- Evidence of undissolved material (e.g. precipitate, surface film, etc): The reports states some degradation product adsorbed to the glass vessels used in the study.

Test organisms (species):

Daphnia magna

Details on test organisms:

TEST ORGANISM

- Common name: water flea

- Source: strain supplied from Insitut National de Recherche Chimique Appliquee (IRCHA), France. Cutltured in lab.

- Age at study initiation (mean and range, SD): 1st instar

- Method of breeding: Adult daphnia were maintained in propylene vessels containing approx. 2L water at 21degC. Lighting cycle: 16 h light, 8 h dark. Fed algae Chlorella spp. predominantly, daily.

- Feeding during test: no


ACCLIMATION

- Acclimation period: 24 h

- Acclimation conditions (same as test or not): same as test

- Health during acclimation (any mortality observed): not reported

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

48 h

Test temperature:

21 degC

pH:

7.7

Dissolved oxygen:

7.9-8.4

Nominal and measured concentrations:

Nominal concentrations: 0, 1, 1.8, 3.2, 5.6, 10, 18, 32, 56 and 100 mg/l

Details on test conditions:

TEST SYSTEM

- Test vessel: jar

- Type (delete if not applicable): closed

- Material, size, headspace, fill volume: glass, 250 ml vessel filled with 200 ml test solution

- Aeration: no

- Renewal rate of test solution (frequency/flow rate): static test

- No. of organisms per vessel: 10

- No. of vessels per concentration (replicates): 2

- No. of vessels per control (replicates): 2


TEST MEDIUM / WATER PARAMETERS

- Source/preparation of dilution water: reconstituted water

- Alkalinity: 270 mg/l as CaCO3

- Conductivity: <5 uS/cm

- Culture medium different from test medium: no

- Intervals of water quality measurement: water temperature was recorded daily. pH and DO were recorded at the start and termination of the study.

OTHER TEST CONDITIONS
- Adjustment of pH: not reported
- Photoperiod: 16 h light, 8 h dark
- Light intensity: not reported

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) : any immobilisation or adverse reactions to exposure were measured at 24 h and 48 h.

TEST CONCENTRATIONS
- Spacing factor for test concentrations: 1.8-2

- Range finding study
- Test concentrations: 0.1, 1, 10 and 100 mg/l
- Results used to determine the conditions for the definitive study: immobilisation only observed at 100 mg/l.

Reference substance (positive control):

no

Duration:

24 h

Dose descriptor:

EC50

Effect conc.:

31 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

mobility

Remarks on result:

other: (27-35)

Duration:

48 h

Dose descriptor:

EC50

Effect conc.:

24 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

mobility

Remarks on result:

other: (21-29)

Duration:

48 h

Dose descriptor:

NOEC

Effect conc.:

10 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

mobility

Duration:

24 h

Dose descriptor:

EC50

Effect conc.:

8 mg/L

Nominal / measured:

meas. (not specified)

Conc. based on:

test mat.

Basis for effect:

mobility

Remarks on result:

other: (7.1-9.0) measured arithmetic average at the end of the study

Duration:

48 h

Dose descriptor:

EC50

Effect conc.:

6.9 mg/L

Nominal / measured:

meas. (not specified)

Conc. based on:

test mat.

Basis for effect:

mobility

Remarks on result:

other: (6.0-7.9) measured arithmetic average at the end of the study

Duration:

48 h

Dose descriptor:

EC50

Effect conc.:

13 mg/L

Nominal / measured:

meas. (geom. mean)

Conc. based on:

test mat.

Basis for effect:

mobility

Details on results:

- Behavioural abnormalities: none observed

- Mortality of control: 0%

- Any observations (e.g. precipitation) that might cause a difference between measured and nominal values: the substance is reported to have adhered to the glassware, but it is unclear if this was observed during the range finding test only or also during the definitive test.

Reported statistics and error estimates:

EC50 values were calculated by moving average method of Thompson (1947)

Table 1. Analytical analysis results.

Nominal concentration	Concentration found (mg/l)
	0 h	% nominal	48 h	% nominal
Control	<LOQ	-	<LOQ	-
1.0	0.973	97	0.288	29
1.8	-	-	0.487	27
3.2	2.91	91	0.830	26
5.6	-	-	1.64	29
10	10.3	103	3.21	32
18	-	-	4.84	27
32	31.0	97	9.78	31
56	-	-	14.3	26
100	200	200*	32.5	32

Analysis of the test preparations at 0 h showed the measured concentrations to be near nominal, with the exception of the 100 mg/l test group which gave a measured value of 200% of nominal. * This was considered to be due to analytical variation given that the 32 mg/l test sample was serially diluted from the 100 mg/l sample, was near to the nominal concentration. Additionally the 48 h value for this concentration was in line with the rest of the concentrations. Analysis of the test preparations at 48 h showed measured concentrations ranging from 26 -32% nominal. This is due to the substance being hydrolytically unstable.

Table 2. Mortality during the 48 h.

Nominal concentration (mg/l)	Percentage mortality
	24 h	48 h
Control	0	0
1.0	0	0
1.8	0	0
3.2	0	0
5.6	0	0
10	0	0
18	0	25
32	55	70
56	100	100
100	100	100

Average of two replicates containing 10 daphnids in each.

Geometric means have been calculated by the reviewer based on the initial and 48-h old measured concentrations:

Nominal concentration (mg/l)	Geometric mean (mg/l)	Factor
Control	<LOQ	-
1.0	0.53	1.88
1.8	-	-
3.2	1.55	2.06
5.6	-	-
10	5.75	1.74
18	-	-
32	17.4	1.84
56	-	-
100*	-	-

* This sample was 200% at the start of the test so the result is not taken into account to calculate the geometric mean. The variation was considered to be due to analytical variation given that the 32 mg/l test sample was serially diluted from the 100 mg/l sample, was near to the nominal concentration.

The EC50 geometric mean is calculated by dividing the EC50 value of 24 mg/l times a factor of 1.8 = 13 mg/l

It is unclear whether a significant part of the 50% of substance seen by the GC-MS is due to poor extraction efficiencies and so a good measure of the concentration in solution. The hexane would extract the non-polar parent and not the polar hydrolysis product.

Validity criteria fulfilled:

yes

Conclusions:

A 48 h EC50 nominal value of 24 mg/L, corresponding to a geometric mean value of 13 mg/L parent substance, has been determined for the effects of the test substance on immobility of Daphnia magna. The test substance hydrolyses in water and it is likely that the test organisms were predominantly exposed to the hydrolysis products of the substance because the study was conducted under a static regime. It is unclear if during the definitive test the substance adhered to the glass of the test vessels. The majority of the immobilised organisms were observed in the first 24 hours of the study where the organisms would have been exposed predominantly to the parent substance. Therefore the observed effects have been attributed to the parent substance and not the hydrolysis product.

Description of key information

Short-term toxicity to invertebrates: 48 hour EC₅₀ 24 and 13 mg/l (nominal and geometric mean measured) (equivalent to OECD TG 202), read-across from an analogous/structurally related substance, cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) in terms of the substance as tested.

Measured analysis indicated loss of parent substance, therefore the observations in this study are attributed to the exposure of test organisms to a mixture of the parent and the hydrolysis products in the test system. The results expressed in terms of the silanol hydrolysis product cyclohexyl(methyl)silanediol are EC₅₀ 20 mg/l.

Key value for chemical safety assessment

Fresh water invertebrates

Fresh water invertebrates

Effect concentration:

20 mg/L

Additional information

There are no reliable invertebrate toxicity data available for dichloro(cyclohexyl)methylsilane (CAS 5578-42-7), therefore good quality data for an appropriate structural analogue, cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6), have been read across. Both substances share the same silanol hydrolysis product, cyclohexyl(methyl)silanediol. In the test with cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) the observations are attributed to the exposure of test organisms to a mixture of the parent substance and the silanol hydrolysis product in the test system. There is no basis to expect that the non-silanol hydrolysis product, methanol, significantly influenced the results of the test. The toxicity of methanol is discussed further in the ecotoxicological information overview endpoint summary (additional information).

48 hour EC₅₀ values of 24 and 13 mg/l (nominal and geometric mean measured respectively) have been determined for the effects of cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6), on mobility of Daphnia magna. The nominal test concentrations are supported by gas chromatography analysis of the parent test substance.

In view of the test media preparation method and exposure regime it is likely that the test organisms were exposed to a mixture of the parent substance and hydrolysis products of the tested substance. Because the relevant assessment entity is the silanol hydrolysis product cyclohexyl(methyl)silanediol, the nominal test substance concentration is used for chemical safety assessment with a molecular weight correction to reflect the concentration of cyclohexyl(methyl)silanediol:

(MW of silanol = 160.29 / MW of parent = 188.34) * [CONCENTRATION OF PARENT = 24] = EC₅₀ 20 mg/l.

This read-across is considered to be a worst-case scenario because the parent test substance, cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6), hydrolyses much more slowly than the registration substance, therefore organisms will have been exposed to the parent substance during the read-across tests, but would be unlikely to be exposed to parent substance if the registration substance were tested. Due to the log K_ow of cyclohexyl(dimethoxy)methylsilane being 4.3, it is likely that the parent substance is more toxic than the silanol hydrolysis product. In addition, the majority of the immobilised daphnids were observed in the first 24 hours of the study where the organisms would have been exposed predominantly to the parent substance. Therefore the observed effects may attributed to the parent substance rather than the hydrolysis products. The read-across is therefore very conservative.

Discussion of a supporting study:

A 48 hour EL₅₀ value of 26 mg/l and a NOELR of <2.15 mg/l have been determined for the effects of the registered substance on mobility of Daphnia magna based on nominal test substance loading rate. Corresponding 48 hour EC₅₀ of approximately equivalent to 5-13 mg/l (based on recoveries at the 21, 46 and 100 mg/l concentrations) and NOEC of <2.15 mg/l have been estimated by the reviewer based on measured DOC (Dissolved Organic Carbon) concentration back-calculated to equivalent test substance concentration. It is likely that the test organisms were exposed solely to the hydrolysis products of the test substance.

The stock solutions in this study was prepared at a concentration of 100 and 215 mg/l and shaken for 48 hours. At this concentration and during this period the silanol hydrolysis product (which is formed in significant quantities over the time-scale of the media preparation) can undergo condensation reactions to give dimers, trimers and tetramers. The tetramers will then have cyclised and be of very low water solubility. The stock solution was filtered prior to dilutions being made and the extensive formation of insoluble cyclic tetramers during the test media preparation, calculated by the reviewer, explains the difference between nominal and measured concentrations.

The DOC analysis is not substance specific, and therefore it is not possible to determine what form of the substance the organisms were exposed to. Formation of tetramers is unlikely to occur in the environment due to the low PECs (see Section 9). In addition, the measured concentrations around the EC₅₀ are below the limit of quantitation. Given the uncertainty of the results and extrapolation of the EC₅₀ concentrations, it is not considered to be appropriate to base the PNEC and the hazard assessment on this study.