Silica is comprised of silicon and oxygen (SiO2) and is one of the most abundant compounds on earth. It occurs in either a crystalline or non-crystalline (amorphous) form. Crystalline silica, in which the silicon and oxygen atoms are arranged in a tetrahedron structure, exists in several forms, or polymorphs. The most commonly encountered polymorph is quartz; cristobalite, tridymite, tripoli, are other naturally occurring polymorphs of crystalline silica. The importance of the crystalline feature is that it is highly stable, insoluble in water, and possesses on its surface reactive oxygen species when fractured (Castranova et al., 1996).
The presence of these reactive species on the crystal surface is largely responsible for the toxicity of crystalline silica particles when inhaled. The Occupational Safety and Health Administration (OSHA) currently enforces an exposure limit for respirable and total dust that contains quartz, as well as for dust that contains cristobalite or tridymite for manufacture, construction, and maritime industries. The Mine Safety and Health Administration (MSHA) enforces the same occupational exposure limit for both aboveground and underground mining operations.
Quartz is a common component of rock and soils. Consequently, workers may be exposed to quartz-containing dust when performing operations that disturb or break up soils and rock, or materials derived from them such as concrete and masonry. Cristobalite and tridymite are also naturally occurring but to a much lesser degree than quartz. These polymorphs are formed from quartz that has been subjected to high temperatures. Work operations that involve subjecting quartz-containing materials to high temperatures may cause quartz to morph physically and chemically, resulting in worker exposures to dusts containing cristobalite. Tripoli is a colorless microcrystalline form of quartz that is derived by the alteration of chert, chalcedony, or novaculite ore, or the leaching of highly siliceous limestone. Due to its inert nature and its fine-grained texture, tripoli has numerous applications; mainly it is used as an abrasive in polishing, buffing, and burnishing compounds; scouring soaps and powders; a filler or extender in plastics, rubber, and sealants like caulks and epoxy resins; and a pigment in paints.