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Introduction
Silicosis in the United States has become a relatively rare disease since exposure levels were lowered in the 1930s and 1940s. Nevertheless, mortality and morbidity as a result of silicosis continue to occur here. There were approximately 1000 deaths per year from silicosis in the late 1960s, falling to 500 per year in the late 1980s.(1) Data are generally unavailable on silicosis incidence. However, four states that initiated reporting systems in the late 1980s found 400 cases between 1987 and 1990.(2)
An estimated 1.7 million US workers outside of the mining industry are exposed to crystalline silica.(1) Among miners, silica exposure is common but is highly variable, depending on the silica content of the ore. Approximately 200 000 miners have potential silica exposure.
Despite the abundance of historical literature on silica and silicosis, surprisingly little work has been done on the quantitative relationship between exposure and disease. In 1993, Hnizdo and Sluis-Cremer(3) published data on 2235 South African gold miners, 313 (14%) of whom developed silicosis. Silicosis was ascertained longitudinally with annual chest x-rays, which were taken even after workers left the workforce. Silicosis was defined as the presence of rounded opacities with profusion of at least 1/1 according to International Labor Organization (ILO) categories. Converting dust measurements to silica measurements (respirable dust was approximately 30% silica), these authors found that the average cumulative exposure for the whole cohort was about 2 mg/m sup 3 -years over an average 25 years of exposure. No silicosis occurred when cumulative exposures were below 0.9 mg/m sup 3 -years. On the other hand, the cumulative risk of silicosis was approximately 25% at 2.7 mg/m sup 3 -years and 77% at the highest observed levels of 4.5 mg/m sup 3 -years.
Muir et al.(4) found a much lower risk of disease per unit of cumulative exposure. These authors studied 2109 gold and uranium miners in Canada hired between 1940 and 1959, with an average cumulative silica exposure of 2.5 mg/m sup 3 -years over an average 16 years of exposure.(5) Thirty-two (1.5%) of these men developed silicosis, as determined by multiple chest x-rays and defined by a finding of rounded opacities with a reading of at least 1/1 according to ILO categories. The cumulative risk based on a...