Crystalline Silica, the most abundant mineral at the earth's surface, has been named to an Occupational Safety and Health Administration (OSHA) Special Emphasis Program. Over 2 million workers are exposed to this dust which claims 200 to 300 lives a year in the United States. (OSHA) Before we discuss anticipation, recognition, evaluation, and control of silica, I am going to review the definition and health effects of crystalline silica, the special emphasis program, and the current regulatory status. In this text the term silica refers to crystalline silica unless otherwise indicated.
Crystalline Silica - What is it? What's the big deal? For many of us silica is dirt, or the sand on the beach. Silica refers to the chemical compound silicon dioxide (SiO2), which occurs in a crystalline or noncrystalline (amorphous) form. Crystalline silica (Figure 1) may be found in more than one form (polymorphism). The polymorphic forms of crystalline silica are alpha quartz, beta quartz, tridymite, cristobalite, keatite, coesite, stishovite, and moganite (NIOSH).
Figure 1. Detailed microscopic view of silica particles on a filter.(Scanning electron micrograph by William Jones, Ph.D.) (NIOSH) (available in full paper).
When workers inhale the crystalline silica, the lung tissue reacts by developing fibrotic nodules and scarring around the trapped silica particles. This fibrotic condition of the lung is called silicosis. If the nodules grow too large or too numerous, breathing becomes difficult and death may result. Silicosis victims are also at high risk of developing active tuberculosis. Although many cases of silicosis result from years of exposure to a low dose of silica, there is also a disease form known as acute silicosis. Acute silicosis is a result of inhaling high concentrations of silica resulting in death within one to several years. (NIOSH) After reviewing the International Agency for Research on Cancer (IARC) findings and other studies, the National Institute for Occupational Safety and Health (NIOSH), has presented information to OSHA that recommends that silica be considered a potential occupational carcinogen. (NIOSH)
There are a number of other health-related symptoms from exposure to silica that can be found in the literature. It is recognized by the various interest groups and agencies that there is still a lot of work to be done in evaluating the health effects of silica. In a recent international forum on silica, there were a number of studies presented that discussed recent research as to how silica works in the body. For example, is freshly cut, ground, crushed (processed) silica more of a hazard than silica particles that have been processed days earlier? Why are smokers at a higher risk of developing silicosis than non-smokers?