Silica, Silicosis, and Lung Cancer: An Epidemiological Update

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Abstract
Excluding reports on coal miners and foundrymen, which are difficult to interpret, there have been at least 39 useful studies of mortality since 1978 in workers potentially exposed to crystalline silica, and some 16 studies of persons compensated or hospitalized for silicosis. Virtually all of the latter show evidence of an approximately threefold increase in lung cancer risk, perhaps higher in the hospitalized subjects. Mainly by reason of various probable biases associated with case selection and confounding, it cannot be confidently concluded from these findings that crystalline silica alone was responsible for the elevated risk. Of the 39 reports on silica-exposed workers—34 of cohort design—18 were in metal miners, 8 in quarrymen, 9 in ceramic and other manufacturing processes, and 4 in mixed industries. Detailed review of the 39 studies suggests that any lung cancer risk in metal miners could well be explained by radon or arsenic, and whether or not there is an excess in quarrymen is uncertain. The evidence in pottery work, and especially in refractory brick manufacture and calcining of diatomaceous earth, is more sure. It has long been suggested that cristobalite, and perhaps tridymite, both produced by high temperature treatment of quartz or amorphous silica, are the most toxic forms of the crystalline mineral. The epidemiological findings on lung cancer to date appear compatible with this gradient and point to the need for closer examination of this hypothesis. Meanwhile, although quantitative data on exposure—response are extremely scanty, there is little to suggest that any excess risk of lung cancer would be detectable at exposure levels sufficient to control the occurrence of radiologically apparent silicosis. McDonald, C.: Silica, Silicosis, and Lung Cancer: An Epidemiological Update. Appl. Occup. Environ. Hyg. 10(12):1056–1063; 1995.

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