Silicosis and smoking strongly increase lung cancer risk in silica-exposed workers

Our plain-English summary of Silicosis and smoking strongly increase lung cancer risk in silica-exposed workers, by N. Kurihara, O. Wada (Industrial Health, 2004).

The study

• Authors: N. Kurihara, O. Wada
• Journal: Industrial Health
• Published: 2004;42(3):303-14
• Type of article: Meta-analysis, Review
• Cited by: 70 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

It remains controversial whether silica is a human lung carcinogen. In this study, we estimated the relative risks of lung cancer due to silica and silicosis by meta-analysis. We collected papers published from 1966-2001 which epidemiologically reported on the relationship between silica/silicosis and lung cancer. We removed papers which did not exclude the effects of asbestos and radioactive materials including radon. We selected the most recent one if some papers were based on the same cohort. Based on the selected papers, we summarized the lung cancer risks from silica, silicosis and non-silicosis with exposure to silica, by meta-analysis using a random effects model. The pooled relative risks were 1.32 (95% confidence interval (CI), 1.23-1.41) for silica, 2.37 (95% CI, 1.98-2.84) for silicosis and 0.96 (95% CI, 0.81-1.15) for non-silicosis with exposure to silica. Since some papers on silica did not exclude silicosis, the risk due to silica itself may be smaller than 1.32. It was less possible that silica exposure directly increases lung cancer risk. On the other hand, the relative risk, 2.37 for silicosis suggested that silicosis increases lung cancer risk. Meta-analysis also revealed that cigarette smoking strongly increased the lung cancer risk in silicotic patients (relative risk, 4.47; 95% CI, 3.17-6.30). Thus, the present study suggested the great importance of preventing silicosis and smoking cessation in reducing lung cancer incidence in silica-exposed workers.

In plain English

This review was a meta-analysis pooling published studies of crystalline silica, silicosis and lung cancer. The authors searched MEDLINE (January 1966 to October 2001) and three Japanese journals. Studies not adequately accounting for smoking, radon, asbestos and PAH were excluded. Pooling 30 studies of silica-exposed workers gave a lung cancer relative risk of 1.32 (95% confidence interval 1.24 to 1.41). Pooling 16 studies of people with silicosis gave 2.37 (1.98 to 2.84). In 8 studies of silica-exposed workers without silicosis, the pooled risk was 0.96 (0.81 to 1.15). Among people with silicosis, it was 4.47 (3.17 to 6.30) in smokers and 2.24 (1.46 to 3.43) in non-smokers. Lung cancer risk showed no significant trend across radiographic categories I, II and IV of silicosis (P=0.86); category III was excluded for possible publication bias. The authors concluded that silicosis is a lung cancer risk factor, that crystalline silica appears to act indirectly, and that smoking strongly increased risk in silicotic patients. They noted that most silica studies did not say whether workers had silicosis, and that few studies covered non-silicotic workers.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Review article: meta-analysis of published cohort and case-control studies, pooled with a random effects model
• Who was studied: Published epidemiological studies found in MEDLINE (January 1966 to October 2001) and three Japanese journals. The silica analysis used 30 studies (17 cohort, 13 case-control) from 25 papers. The silicosis analysis used 16 studies (11 cohort, 5 case-control). The analysis of non-silicotic silica-exposed workers used 8 studies. The radiographic-category analysis used 7 studies, and the smokers versus non-smokers analysis used 6 cohort and 2 case-control studies.
• Exposure: Occupational exposure to crystalline silica, and silicosis, in workers such as granite, stone, foundry, ceramic, brick, pottery, sand and quarry workers and gold and other miners. Smoking was examined among silicotic patients. Studies were excluded if they did not adequately account for smoking, radon, asbestos and PAH.
• Main results:
  • Silica-exposed workers (all 30 studies): pooled relative risk of lung cancer 1.32 (95% CI, 1.24–1.41)
  • Silica-exposed workers, by study type: cohort studies 1.29 (95% CI, 1.20–1.40); case-control studies 1.42 (95% CI, 1.22–1.65)
  • People with silicosis (all 16 studies): pooled relative risk of lung cancer 2.37 (95% CI, 1.98–2.84)
  • Silica-exposed workers without silicosis (8 studies): pooled relative risk 0.96 (95% CI, 0.81–1.15)
  • People with silicosis: pooled relative risk 4.47 (95% CI, 3.17–6.30) in smokers and 2.24 (95% CI, 1.46–3.43) in non-smokers
  • Radiographic categories I, II and IV of silicosis: no significant trend between category and lung cancer risk (P=0.86). Category III was left out of the trend test because of possible publication bias.
  • Publication bias tests: possible bias in the category III analysis only, not in the other categories
• The authors' conclusion: The authors concluded that silicosis is a risk factor for lung cancer. They found a small risk in silica-exposed subjects, a group that included silicotic patients, and took this to suggest that crystalline silica induces lung cancer indirectly in humans. They also concluded that smoking strongly increased lung cancer risk in silicotic patients.
• Limitations the authors state:
  • Most silica studies did not say whether subjects had silicosis, so the silica-exposed groups may include silicotic patients
  • Few studies (8) were available for the analysis of non-silicotic silica-exposed subjects
  • Publication bias may exist in the studies used for radiographic category III
  • The data could not establish whether the effects of silicosis and smoking are additive or multiplicative

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer.

Medical terms in this study, explained in our Lexicon: Silicosis, Lung Cancer, Relative Risk, Meta-Analysis, confidence interval.

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• Silica exposure and altered regulation of autoimmunity — S. Lee, H. Matsuzaki, et al. (2014)
• Radiographic evidence of silicosis risk in the diatomaceous earth industry — J.M. Hughes, H. Weill, et al. (1998)
• Silica, asbestos, man-made mineral fibers, and cancer — K. Steenland, L. Stayner (1997)
• Silicosis and autoimmunity — S. Lee, H. Hayashi, et al. (2017)

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