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Silica and Lung Cancer

Staynor, Leslie

doi: 10.1097/EDE.0b013e318064672b
LETTERS: Letters to the Editor

Division of Epidemiology and Biostatistics University of Illinois Chicago School of Public Health Chicago, IL

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The author responds:

Erren and his coauthors1 agree that there is sufficient evidence to conclude that “exposure to silica is causally associated with an increased risk of lung cancer.” However, they add that there is still a “critical ‘but,’” which is whether silicosis is required for silica-associated lung cancer. I agree with the authors that this is indeed an important unresolved question, which has large medico-legal and regulatory implications. If silicosis were necessary for silica-induced lung cancer, then a case of lung cancer without silicosis might be dismissed as being unrelated to silica. If occupational standards for silica are set at a level that would prevent silicosis, then this would also eliminate excess lung cancer. This is somewhat moot given that the OSHA standard for silica is associated with a substantial risk of silicosis2–4 as well as lung cancer.5,6 The question has greater relevance for environmental exposures, which are generally below levels where silicosis has been reported to occur. If their conjecture is true, this may suggest that there is a threshold for silicosis and hence for lung cancer. However, the absence of evidence of silicosis for environmental exposures does not necessarily imply a threshold, and might be explained by limitations of the diagnostic tests or low power of studies to detect weak effects.

Most studies have not found an association between silica and lung cancer among individuals without silicosis,7–10 while some have.11 Interpretation of these mostly negative findings is complicated by the facts that silicosis may also be regarded as a marker of heavy exposure to silica and that the diagnosis of lung cancer may increase the likelihood of silicosis detection. Study results indicating an increase in lung cancer among persons without silicosis might be attributable to underascertainment of silicosis based on chest radiographs. Comparisons of findings from pathologic exams at autopsy with chest radiographs have shown that the use of chest radiographs may grossly underestimate the prevalence of silicosis.12

Finally, Erren et al1 suggest that further research is needed to resolve this important issue in order to be able to “make sound public health decisions concerning silica at the low end of the exposure scale.” In 2000 Checkoway and Franzblau13 suggested that the question of whether or not silicosis required for elevated lung cancer risk “is virtually unanswerable.” Unfortunately this situation has not improved since their review and is unlikely to change in the near future.

Leslie Staynor

Division of Epidemiology and Biostatistics, University of Illinois, Chicago School of Public Health, Chicago, IL

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© 2007 Lippincott Williams & Wilkins, Inc.