Dichloro(diphenyl)silane

Endpoint summary

Additional information

The registered substance is part of a group of organosilicones, the toxicity of which is determined by a non-polar narcosis mechanism of toxicity, and as such log K_ow drives toxicity. Substances in this group include alkoxy- and chloro- silanes having secondary features that do not affect the toxicity of the substances. The side chain of the registered substance is an aryl. There is evidence that the phenyl (aryl) group’s toxicity is related to log K_ow alone as for other neutral organics and thus does not enhance toxicity. Additional information is given in a supporting report Application of Category - Analogue - QSAR for Reconsile (PFA, 2016b) attached in Section 13 of the IUCLID dossier.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex IX requirements for the registered substance from substances that have similar structure and physico-chemical 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 physico-chemical properties and the structure is altered.

In the following paragraphs the read-across approach fordichloro(diphenyl)silaneis assessed for each surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties.

·        Read-across from diphenylsilanediol to dichloro(diphenyl)silane:

Measured data were not available for dichloro(diphenyl)silane for aquatic toxicity. However, as the substance hydrolyses very rapidly (half-life of 10 seconds at pH 7) it is more environmentally relevant to consider the toxicity of the hydrolysis product, diphenylsilanediol (CAS 947-42-2) as organisms will predominately be exposed to the hydrolysis product.Therefore, the hazard assessment for the environment, including sediment and soil compartments, is based on the properties of the hydrolysis product,diphenylsilanediol, in accordance with REACH guidance.

Measured data were available for diphenylsilanediol. The lowest reported effect concentration was an EC₅₀of 9.0 mg/l for the effect on growth rate on the algae Pseudokirchnerella subcapitata. For fish an LC₅₀of 39 mg/l was reported and for invertebrates an EC₅₀of 24 mg/l. These studies were conducted according to OECD Guidelines 201 (Alga, Growth Inhibition Test), 203 (Fish, Acute toxicity test) and 202 (Daphnia sp. Acute Immobilisation Test), respectively and are considered reliable for assessment.

·        QSAR:

QSAR predictions for the hydrolysis product diphenylsilanediol confirm the low short-term toxicity of the substance to aquatic organisms. QSAR calculated LC₅₀of 31.6 mg/l for fish, 17.9 mg/l for Daphnia and an EC₅₀of 8.6 mg/l for algae.

·        Considerations on the non-silanol hydrolysis products:

Effects on aquatic organisms arising from exposure to hydrochloric acid are thought to result from a reduction in the pH of the ambient environment (arising from an increase in the H⁺concentration) to a level below their tolerable range. Aquatic ecosystems are characterized by their ambient conditions, including the pH, and resident organisms are adapted to these conditions. The pH of aquatic habitats can range from 6 in poorly-buffered ‘soft’ waters to 9 in well-buffered ‘hard’ waters. The tolerance of aquatic ecosystems to natural variations in pH is well understood and has been quantified and reported extensively in ecological publications and handbooks (e.g. OECD SIDS for CAS 7647-01-0, hydrochloric acid). It is not considered appropriate or useful to derive a single aquatic PNEC for hydrochloric acid because any effects will not be a consequence of true chemical toxicity and will be a function of, and dependent on, the buffering capacity of the environment.