Crystalline silica of respirable size (less than or equl to 10 micrometer) may cause lung inflammation or cancer when it is in air and enters the lungs. Crystalline silica is not used as a cosmetic ingredient.

Links to FDA Code of Federal Regulations for Silica (silicon dioxide)
http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRS...
http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRS...
http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRS...

Silica and Hydrated Silica may be used in cosmetics and personal care products marketed in Europe according to the general provisions of the Cosmetics Directive of the European Union.
Link to the EU Cosmetics Directive: http://ec.europa.eu/enterprise/cosmetics/html/consolidated_d...

A common use of Silica in cosmetics and personal care products is as an abrasive. Abrasives are used in cosmetics to remove unwanted tissue or foreign materials from various body surfaces. The removed materials may include dead skin surface cells, callus, or dental plaque. As a rule, abrasives are irregularly shaped fine or coarse solids. Harder abrasives include special forms of Hydrated Silica used for tooth cleansing, while softer abrasives such as oatmeal are employed to remove dead skin surface cells. Abrasive agents can also be used to produce gloss, e.g., on nails, by removal of surface imperfections.

Anticaking agents are ingredients used to prevent the agglomeration of a particulate solid into lumps or cohesive cakes. Caking of cosmetic products is generally due to an absorptive process, especially hygroscopicity. Anticaking agents are commonly included in loose powders that must remain free-flowing, especially when they are dispensed from aerosols or shaker-top containers. Anticaking agents are also useful processing aids whenever the flow of solids is hindered by agglomeration.