IF YOU BELIEVE SILICA is no longer a significant occupational health hazard, think again. While silicosis-related deaths are declining and some industries with silica exposures employ fewer U.S.-based workers, other factors have increased the significance of this topic for SH&E professionals (CDC, 2005, pp. 401–402). Toxic tort litigation and a new exposure limit are the principal developments in this long-known hazard area. Reactions to these developments have involved the insurance industry as well as state and federal legislators and rule makers.
Silica consists of the chemical compound silicon dioxide (SiO2). Separately, silicon and oxygen comprise 75% of the earth's crust (U.S. Bureau of Mines, 1992, p. 4). Silica's physical structure can include crystalline and amorphous forms. The crystalline structure can be classified further into seven polymorphs. The three most common polymorphs are quartz, cristobalite and tridymite. Although silicon is a component in silicates and silicones, those are separate compounds from silica (Figure 1, p. 38). Crystalline silica can be inhaled deeply into the lungs when it is a respirable size of less than 10 _m (NIOSH, 2002, p. xvi). Crystalline silica of respirable size (hereafter referred to as "silica") is the occupational health concern of this article. Occupational exposure to silica can result in silicosis and other diseases. Respiratory effects of working with stone were observed in ancient times and by the pioneers of occupational health. The term silicosis dates from the 1870s (Rosenman, Reilly, Yoder, et al., 2006, p. 3). The disease consists of inflammation of the respiratory system tissues that eventually causes fibrosis, which reduces the ability to breathe efficiently (Spraycar, 1995, p. 1620). Although silicosis is most often associated with chronic exposures, acute silicosis can occur as well (NIOSH, 2002, p. 23). Other silica-related respiratory diseases include emphysema, pulmonary tuberculosis, bronchitis, asthma and lung cancer. Nonrespiratory diseases that appear to have an epidemiological relationship to silica exposures include autoimmune diseases, scleroderma and chronic renal disease (NIOSH, 2002, p. 2). In 1997, the International Agency for Research on Cancer (IARC) changed its classification of "inhaled crystalline silica in the form of quartz or cristobalite" to "carcinogenic to humans (Group 1)" (p. 7). In the U.S., the National Toxicology Program (NTP) followed suit in 2000 by classifying "silica, crystalline (respirable size)" as "known to be a human carcinogen" (2005, p. 1). Both the IARC and NTP classifications are binding for OSHA's hazard communications requirements for chemical hazard determination in regard to carcinogenicity (OSHA, 1996).
Exposures to silica exist in mining, agriculture, construction and some manufacturing sectors. OSHA estimates that more than 2 million employees are exposed to silica in general industry, construction and maritime industries (U.S. Department of Labor, 2007). NIOSH acknowledges that an unknown number of the 3.7 million workers engaged in agriculture have exposure to silica from dust-generating activities (NIOSH, 2002, p. 22). Silica is present in nearly all mining operations (U.S. Bureau of Mines, 1992, p. 15). The mere presence of silica does not necessarily constitute exposure.