The purposes of this study were a) to summarize measurements of airborne (respirable) crystalline silica dust exposure levels among U.S. workers, b) to provide an update of the 1990 Stewart and Rice report on airborne silica exposure levels in high-risk industries and occupations with data for the time period 1988-2003, c) to estimate the number of workers potentially exposed to silica in industries that the Occupational Safety and Health Administration (OSHA) inspected for high exposure levels, and d) to conduct time trend analyses on airborne silica dust exposure levels for time-weighted average (TWA) measurements. Compliance inspection data that were taken from the OSHA Integrated Management Information System (IMIS) for 1988-2003 (n = 7,209) were used to measure the airborne crystalline silica dust exposure levels among U.S. workers. A second-order autoregressive model was applied to assess the change in the mean silica exposure measurements over time. The overall geometric mean of silica exposure levels for 8-hr personal TWA samples collected during programmed inspections was 0.077 mg/[m.sup.3], well above the applicable American Conference of Governmental Industrial Hygienists threshold limit value of 0.05 mg/[m.sup.3]. Surgical appliances supplies industry [Standard Industrial Classification (SIC) 3842] had the lowest geometric mean silica exposure level of 0.017 mg/[m.sup.3], compared with the highest level, 0.166 mg/[m.sup.3], for the metal valves and pipe fitting industry (SIC 3494), for an 8-hr TWA measurement. Although a downward trend in the airborne silica exposure levels was observed during 1988-2003, the results showed that 3.6% of the sampled workers were exposed above the OSHA-calculated permissible exposure limit. Key words: crystalline silica dust, industries, occupations, OSHA IMIS, silica exposure. doi:10.1289/ehp.7384 available via http://dx.doi.org/[Online 6 December 2004]
Silica is a mineral compound made up of one silicon atom and two oxygen atoms (Si[O.sub.2]). It has a melting point of 1,600[degrees]C and is a colorless, odorless, and noncombustible solid [American Conference of Governmental Industrial Hygienists (ACGIH) 2001]. Crystalline silica is formed when silica molecules are lined up in order and in crystal form. It is an abundant mineral in rock, sand, and soil. Quartz is a term often used to refer to crystalline silica dust.
Crystalline silica has been used in many industries such as blast furnaces, cement manufacturing, glass and concrete mixing product manufacture, ceramics, clay, glass and china pottery, electronic, foundry, sandblasting and manufacturing abrasives, and many construction activities (Altindag et al. 2003; Flanagan et al. 2003; Irwin 2003; Rappaport et al. 2003). It is used as an abrasive agent in many industrial applications. Occupations having a high potential for exposure to crystalline silica dust (respirable quartz) are metal, coal, and nonmetal (except fuels) mining; foundry, stone clay, and glass production work; and agricultural, chemical production, highway repair, and tuck-pointing work [Akbar-Khanzadeh and Brillhart 2002; Occupational Safety and Health Administration (OSHA) 2004; Rappaport et al. 2003].
Silica dust is an inhalation hazard. Workers may be at risk of silicosis from exposure to silica dust when high-velocity impact shatters the sand into smaller, respirable (< 0.5 to 5.0 [micro]m in diameter) dust particles. Silicosis is a disease where scar tissue forms in the lungs and reduces the ability to extract oxygen from the air. Symptoms of silicosis can be acute, accelerated, or chronic. Acute silicosis may develop within weeks and up to 5 years after breathing large amounts of crystalline silica. Accelerated silicosis may develop shortly after exposure to high concentrations of respirable crystalline silica, whereas chronic silicosis occurs after [greater than or equal to] 10 years of exposure …