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Key value for chemical safety assessment

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Introduction

There are no in vitro or in vivo data available on the toxicokinetics of MS-Silane. The following summary has therefore been prepared based on validated predictions of the physicochemical properties of the substance itself and its hydrolysis products.

 

MS-Silane is an organosilicon UVCB (substance of unknown or variable composition) and is a moisture-sensitive, volatile liquid that hydrolyses rapidly in contact with water, generating hydrogen chloride and various silanes. Human exposure can occur via the inhalation or dermal routes. Relevant inhalation exposure would be to the hydrolysis products (hydrolysis would occur rapidly when inhaled, even if a mixture of parent and hydrolysis products were present in air). The substance would also hydrolyse rapidly in contact with moist skin. The resulting hydrogen chloride hydrolysis product would be severely irritating or corrosive.

 

Absorption

 

Oral

Significant oral exposure is not expected for this corrosive substance.

 

Dermal

There are no data available on the dermal absorption of MS-Silane. Limited data, however, are available for the hydrolysis products of two of the constituents of MS-Silane: Dichloro(dimethyl)silane (CAS 75-78-5; 20 – 35% concentration in MS-Silane) and chlorotrimethylsilane (CAS 75-77-5; 0 – 5% concentration in MS-Silane). Summaries of key findings from these studies follow.

 

Dimethylsilanediol [hydrolysis product of Dichloro(dimethyl)silane]:

A reliable in vitro dermal absorption study for dimethylsilanediol (DMSD, CAS 1066-42-8) using human skin showed a dermal absorption for the hydrolysis product, DMSD, of 11.4% following a 24 hour exposure period. It also showed that approximately 60% of the applied dose vaporized from the application site. This result is supported by the physicochemical properties of DMSD. The very high water solubility (1E+06 mg/l) and low predicted log Kow (-0.4) of the hydrolysis product suggest that it is too hydrophilic to cross the lipid rich stratum corneum. Since dichloro(dimethyl)silane is corrosive to the skin, damage to the skin might increase penetration of the hydrolysis product. There are no reliable studies to check for signs of dermal toxicity. Skin irritation/corrosion studies did not report any signs of systemic toxicity.

 

Trimethylsilanol [hydrolysis product of Chlorotrimethylsilane]:

In a well conducted guideline study conducted to OECD 428 and GLP (reliability score 1), the total percentage of the dose of the hydrolysis product, 14C-Trimethylsilanol (TMS; CAS 1066-40-6) absorbed was estimated to be <0.08% of the applied dose. Almost all (99.9%) of the recovered 14C-TMS volatilized from the skin surface and was captured in the charcoal baskets placed above the exposure sites. The majority of the absorbed dose penetrated through the skin to the receptor fluid. Therefore once hydrolysis has occurred, absorption of trimethylsilanol is poor. Since the other hydrolysis product, HCl is corrosive to the skin; damage to the skin might increase penetration. Deaths, and lung and liver pathology in the key acute dermal study provide evidence that chlorotrimethylsilane is absorbed in vivo following exposure to high doses (in the presence of severe dermal irritation).

 

The molecular weights of the constituents in the parent and hydrolysis products favor absorption across the skin. However, the very high water solubility and low predicted log Kow of the hydrolysis products suggest that they are too hydrophilic to cross the lipid rich stratum corneum. Therefore dermal uptake is likely to be low for MS-Silane. Since MS-Silane is corrosive to the skin, damage to the skin might increase penetration. Available dermal studies for various MS-Silane constituents did not show evidence of systemic availability, as effects (such as those on body weights) are generally thought to be secondary to corrosion of the skin; likewise, skin irritation/corrosion studies did not report any signs of systemic toxicity.

 

Inhalation

The partition coefficient values for the various hydrolysis products of the MS-Silane constituents indicate that MS-Silane is likely to be absorbed directly across the respiratory tract epithelium by passive diffusion. However, the high water solubility might lead to some of the hydrolysis product being retained in the mucous of the lungs. As with dermal exposure, damage to membranes caused by the corrosive nature of MS-Silane could potentially enhance the uptake. Available acute inhalation studies for some of the MS-Silane constituents showed local signs but no definite systemic effects.

 

Distribution

All absorbed material is likely to be in the form of the hydrolysis products (i.e., HCl and various silanes). Many of the silanes resulting from the hydrolysis of MS-Silane are small molecules, and are likely to be widely distributed, but their hydrophilic nature will limit their diffusion across membranes (including the blood-brain and blood-testes barriers) and their accumulation in fatty tissues. Hydrogen and chloride ions will enter the body’s natural homeostatic processes.

 

Metabolism

There are no data regarding the metabolism of MS-Silane. MS-Silane is rapidly hydrolysed to various silanes and hydrogen chloride in the presence of moisture. Most if not all of this will have occurred before absorption into the body. Any Silicic acid that may be formed is not metabolised, but forms a precipitate. Genetic toxicity tests in vitro of various MS-Silane constituents showed no observable differences in effects with and without metabolic activation.

 

Excretion

The low molecular weight and high water solubility of the various silanes formed during the hydrolysis of MS-Silane suggest that MS-Silane is likely to be rapidly eliminated via the kidneys in urine. Any precipitated silica will be eliminated in faeces. There is therefore no evidence to suggest that this substance will accumulate in the body.