Inhaling finely divided crystalline silica dust in very small quantities (OSHA allows 0.1 mg/m3) over time can lead to silicosis, bronchitis or (much more rarely) cancer, as the dust becomes lodged in the lungs and continuously irritates them, reducing lung capacities (silica does not dissolve over time). This effect can be an occupational hazard for people working with sandblasting equipment, products that contain powdered crystalline silica and so on. Children, asthmatics of any age, allergy sufferers and the elderly (all of whom have reduced lung capacity) can be affected in much shorter periods of time. Amorphous silica, such as fumed silica is not associated with development of silicosis.[38] Laws restricting silica exposure with respect to the silicosis hazard specify that the silica is both crystalline and dust-forming.
In respects other than inhalation, pure silicon dioxide is inert and harmless. Because some silicas take on water, extended exposure may cause local drying of the skin or other tissue. Pure silicon dioxide produces no fumes and is insoluble in vivo. (in the body) It is indigestible, with zero nutritional value and zero toxicity. When silica is ingested orally, it passes unchanged through the gastrointestinal (GI) tract, exiting in the feces, leaving no trace behind. Small pieces of silicon dioxide are equally harmless, as long as they are not large enough to mechanically obstruct the GI tract, or jagged enough to lacerate its lining.
A study which followed subjects for 15 years found that higher levels of silica in water appeared to decrease the risk of dementia. The study found that for every 10 milligram-per-day intake of silica in drinking water, the risk of dementia dropped by 11%.[39] - Wikipedia