Fumes, silica

Endpoint summary

Additional information

The data on the kinetics of synthetic amorphous silica in the lungs show rather consistently that in contrast to crystalline silica, which exhibits a marked tendency to accumulate, amorphous silica reaches a plateau level at which elimination equates with deposition. After the cessation of exposure, synthetic amorphous silica is rapidly eliminated from the lung tissue. This difference between crystalline and amorphous silica can be at least partly explained by the better solubility of amorphous silica in lung tissue.

Solubility is the main determinant affecting the pulmonary elimination of particles. The solubility of different forms of amorphous silica is affected by particle characteristics. For example, particle size and surface area can modify the solubility of amorphous silica in biological fluids. The particle surface area of synthetic amorphous silica is larger than that of silica fume. The solubility of different forms of amorphous silica has been studied in in vitro systems. The available in vitro dissolution data on the amorphous silica shows that both forms of amorphous silica are dissolved in neutral physiological solutions, although the dissolution of silica fume is slightly slower than that of synthetic amorphous silica. This is supported by an early study by Swensson (1967) showing that although the lung kinetics of synthetic amorphous silica and silica fume resemble each other and differ significantly from that of quartz, the reml of silica fume from the lungs is slightly slower than that of pyrogenic silica. However, the dissolution kinetics of silica fume is not considered to be so different from synthetic amorphous silica that it would hamper the possible read across from synthetic amorphous silica to silica fume. The slight differences can be explained by the differences in particle size and surface area.

The dissolution of impurities from silica fume into biological fluids (artificial gastric juice and PBS) has also been studied. The dissolved amount is considered to represent the bioaccessible and, therefore, possibly bioavailable fraction. The main impurities released from silica fume were iron, zinc and magnesium. Also, some lead was dissolved from low-grade silica fume. Lead may be present in low-grade silica fume at rates of up to 0.3%.

When the dissolution of these metallic impurities from silica fume was compared to the dissolution of the same elements from pyrogenic silica (Aerosil Ox50), it was noted that iron release was very similar between silica fume and pyrogenic silica. The main metallic impurities that were released from pyrogenic silica (Aerosil particles) included aluminium, calcium, iron (in GST) and magnesium (in GST) (Herting et al.2009; report on synthetic amorphous silica). The release of calcium was higher from Aerosil particles than from silica fume particles. Also, aluminium was released at a similar or slightly higher rate from Aerosil particles than from silica fume particles.

On the other hand, the release of magnesium and zinc was higher from silica fume than from Aerosil particles. Both magnesium and zinc are essential elements, present in significant amounts in the normal human body. For example, the normal magnesium requirements for normal adults are 310-400 mg/day (Food and Nutrition Research 1997). In the case of zinc, the normal dietary intakes are 8.8–14.4 mg/day for adults and, for example, the normal blood zinc levels are approximately 1 mg/L (IPCS 2001). It is one of the most abundant trace metals in humans. The absorption of zinc from the gastrointestinal tract is regulated by homeostatic mechanisms. It should be noted that, in any case, the released levels of elements were very low. Therefore, the minor levels of these essential elements present in and released from silica fume are unlikely to contribute to the body burden of these elements or to the toxicity of silica fume.

The release of other elements was low/non-detectable from all amorphous silica particles.

Thus, although silica fume may contain small quantities of different elements, their release is very similar to their release from pyrogenic silica particles and, therefore, they are not considered to hamper the read across from synthetic amorphous silica.

After ingestion, synthetic amorphous silica seems to have an insignificant effect on tissue silica levels. Also, the urinary excretion of silica is not significantly affected by oral ingestion of synthetic amorphous silica. There is significant variability in the urinary excretion of silica between individuals. This is likely to be due to variability in dietary intake. Since silicon ion in different forms is ubiquitous in the environment, various foods and drinking water contain various amounts of silicon (for further information see recent reviews by Jugdaohsingh 2007 and Martin 2007). Our normal dietary intake of silicon is between 20-50 mg Si/day (EFSA 2004). The absorption of dietary silicon is largely affected by the exact form of the silicon in the food (Jugdaohsingh 2007; Martin 2007). In addition, it has been suggested that silicon may also have some desirable effects in humans; it has already been demonstrated to be essential in some primitive organisms (Jugdaohsingh 2007; Martin 2007). Especially beneficial effects on bone growth have been suggested. In blood, silicon exists as a monosilicic acid (Martin 2007). The EFSA (2009) reviewed the use of silica in food additives and concluded that adding up to 1,500 mg SiO₂/day (equal to 700 mg/day) of silicon dioxide to food supplements is not a safety concern.