Roelofs and Vogelsberger (2004) determined solubility of pyrogenic, precipitated and gel forms of synthetic amorphous silica (SAS), including surface-treated forms. At 37°C and pH values near 7 the solubility for different forms of SAS was between 138 to 162 mg/l in simulated biological medium, close to the solubility in water. Roelofs and Vogelsberger (2004) also confirmed that silica has a tendency to supersaturate, i.e., the dissolution rate is more rapid than the precipitation rate. Hence, the different forms of SAS dissolve both in water and in simulated biological systems beyond the equilibrium concentration. Total dissolution can be expected in biological systems where dissolved SAS is quickly removed, such as in the lungs.
In a rat 4-week inhalation study, Warheit et al (1991) tested the pulmonary toxicity of colloidal silica Ludox. Treatment-related effects were observed at 50 and 150 mg/m³, which were reversible within 3-month recovery period (Warheit et al, 1991, Lee and Kelly 1992). The exposure led to inhaled doses of 489 μg/lung (10 mg/m³ group), 2418 μg/lung (50 mg/m³), and 7378 μg/lung (150 mg/m³), respectively. Although high lung burden were achieved at 50 and 150 mg/m³, half-times of 40 to 50 days were determined in rats (Lee and Kelly, 1992). The half-time at such high lung burden was shorter than physiological AM-derived clearance half-time of approximately 60 days. Therefore, contribution of dissolution is suggested. This view is strongly supported by the solubility of 138 to 162 mg/l in simulated biological medium. Based on the half-time of 40 to 50 days, amorphous SiO2 is partly soluble.