Silica is the natural compound of the two most abundant elements on Earth - oxygen and silicon. It occurs in amorphous and crystalline forms, the most common of which is quartz. Quartz accounts for about 12 wt% of the Earth's crust and is present in all major rock types and top soils. Crystalline silica-containing airborne dusts are ubiquitous in the human environment, coming from un- consolidated materials on the Earth's surface, from natu ral raw materials processed by industry, and from many materials of construction.
