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The surface of silicon is composed of a thin oxidized silicon layer resembling the surface of amorphous silicon dioxide. Both silicon and amorphous silica release silicon from particles in the form of monosilicic acid. The available in vitro data on the dissolution kinetics of silicon in different artificial biological fluids show that the dissolution pattern of silicon from silicon particles in synthetic biological fluids is slightly slower when compared to pyrogenic amorphous silica (Aerosil Ox50) (Herting et al. 2009a,b). This lower dissolution can be explained by lower specific surface area, which is higher in the case of synthetic amorphous silica. Based on these in vitro dissolution data, the bioavailabity of silicon from silicon particles is similar or slightly lower than from synthetic amorphous silica, and it is justified to use read-across from amorphous silicon dioxide to silicon. A detailed description of the justifications for read-across is available in Section 13 of the Iuclid dossier.

After ingestion, amorphous silicon dioxide has insignificant effect on tissue or urinary silicon levels. Since silicon in different forms is ubiquitous in the environment, various foods, drinking water and beverages contain silicon. Our normal dietary intake of silicon is between 20-50 mg Si/day and the silicon in diet seems to be in highly bioavailable form, as shown by a high proportion of dietary silicon excreted in the urine. The differences in dietary intake are likely to explain the variability in urine levels of silicon between different individuals. Although in neutral solutions elemental silicon and amorphous silicon dioxide are slowly dissolved, in acidic solutions is significantly lower. Thus, e.g. in the stomach, the release of silicon from silicon particles is likely to be low, which is likely to affect the absorption from the gastrointestinal tract.

Metallurgical silicon may contain small amounts of impurities, mainly iron (up to 3%), aluminium and calcium (up to 1.5%). In vitro dissolution data show that the release of these metals from silicon is at the same level as the release from pyrogenic silica particles (Aerosil Ox50). Therefore, these minor impurities of silicon are unlikely to hamper the read-across from amorphous silicon.

A subchronic inhalation study with silicon particles (MMAD 2.6 µm) was performed in Wistar rats according to OECD 413 (Fraunhofer ITEM 2014). The doses used in the test were 1, 4 and 16 mg/m3of silicon. The results showed that subchronic exposure by inhalation resulted in an overload effect in rat lungs at the highest dose level (16 mg/m3). The calculated half-times for silicon after exposure at 16 mg/m3were 128 days for male and 119 days for female rats.