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Methodological Issues Related to Studies of Workers in the Diatomaceous Earth Industry

Department of Public Health Sciences, University of Alberta, Edmonton, Canada

The International Agency for Research on Cancer (IARC) recently reviewed the human and animal evidence relat ed to the carcinogenicity of silica, finding that there was sufficient evidence to conclude that crystalline silica in the form of quartz or cristobalite is a human carcinogen. A series of studies examining mortality of workers at several facilities processing diatomaceous earth was considered by the IARC committee to have provided strong evidence favouring their decision. A number of factors raise concerns about the results of these studies. These factors include modestly elevated risk estimates, the lack of adequate smoking information, bias in the estimates of exposures to crystalline silica and asbestos, and post hoc selection of historical exposure weightings and lag periods. Most of these shortcomings typify the literature available for the previous IARC review (1987) which concluded that there was only 'limited' evidence for the carcinogenicity of silica in humans, and suggest ed that studies be undertaken in populations of workers not exposed to known carcinogens. The limitations of

Key Words: Silica • Diatomaceous earth • Lung cancer • Epidemiology • Occupational health

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