Registration Dossier

Administrative data

Link to relevant study record(s)

Description of key information

Key value for chemical safety assessment

Additional information

Introduction

There are no in vivo data on the toxicokinetics of hexamethylcyclotrisiloxane (D3). The majority of the following summary has therefore been prepared based on validated predictions of the physicochemical properties of the substance itself and its hydrolysis products. D3 is a volatile liquid with low water solubility and fairly high log Kow. It hydrolyses rapidly, initially to hexamethyltrisiloxanediol (3663-50-1) and finally to dimethylsilanediol (DMSD, CAS 1066-42-8). Exposure may occur via the inhalation or dermal routes. Exposure may be to either the parent substance or the hydrolysis products.

 

Absorption

Oral

Oral exposure is not expected for this industrial substance.

Dermal

D3 is expected to hydrolyse on moist skin with a half life of approximately 0.5 hours. Thus over the duration of a working day exposure to the parent and hydrolysis products is expected. While the log Kow (3.9) of D3 is favourable for dermal absorption, its water solubility (1.6 mg/l) suggests that there will be limited partition from the stratum corneum into the epidermis, and therefore only low dermal absorption. D3 is also likely to evaporate from the skin, thus reducing absorption further.

The intermediate hydrolysis product, hexamethyltrisiloxanediol, has estimated water solubility (941 mg/l) and log Kow(1.8) values which suggest that it will be moderately absorbed through the skin.

The very high water solubility (1E+06 mg/l) and low predicted log Kow(-0.4) of the hydrolysis product, DMSD, suggest that it is too hydrophilic to cross the lipid rich stratum corneum. Therefore, dermal uptake is likely to be low.

There are no reliable studies to check for signs of dermal toxicity, and skin irritation/corrosion studies did not report any signs of systemic toxicity.

 

Inhalation

Exposure following inhalation of D3 is likely to be to D3 and its hydrolysis products. The log Kowof D3 is favourable for absorption directly across the respiratory tract epithelium by passive diffusion.

The intermediate hydrolysis product, hexamethyltrisiloxanediol, has estimated water solubility and log Kowvalues which suggest that it will be moderately absorbed via the lungs.

The partition coefficient value for the hydrolysis product, DMSD, indicates that it is likely to be absorbed directly across the respiratory tract epithelium by passive diffusion. However, the high water solubility might lead to some of this hydrolysis product being retained in the mucous of the lungs.

 

Distribution

D3 is a lipophilic molecule and is likely to be widely distributed into cells and the intracellular concentration might be higher than extracellular concentration particularly in fatty tissues. However, D3 will be hydrolysed relatively rapidly once in the blood, so in reality distribution and retention of D3 in fatty tissues is likely to be negligible.

DMSD is a small molecule, and is likely to be widely distributed, but its hydrophilic nature will limit its diffusion across membranes (including the blood-brain and blood-testes barriers) and its accumulation in fatty tissues.

 

Metabolism

D3 hydrolyses in the presence of moisture. The half life of this hydrolysis reaction depends on pH. There are no data regarding the metabolism of D3 or DMSD. Genetic toxicity tests in vitro showed no observable differences in effects with and without metabolic activation.

 

Excretion

Since all D3 is likely to be hydrolysed to DMSD once absorbed into the body, it is DMSD that should be considered for excretion. The low molecular weight and high water solubility of DMSD suggest that it is likely to be rapidly eliminated via the kidneys in urine. There is therefore no evidence to suggest that this substance will accumulate in the body.