Dose-response associations of silica with nonmalignant respiratory disease and lung cancer mortality in the diatomaceous earth industry

Our plain-English summary of Dose-response associations of silica with nonmalignant respiratory disease and lung cancer mortality in the diatomaceous earth industry, by H. Checkoway, N.J. Heyer, N.S. Seixas, et al (American Journal of Epidemiology, 1997).

The study

• Authors: H. Checkoway, N.J. Heyer, N.S. Seixas, E.A. Welp, P.A. Demers, J.M. Hughes, H. Weill
• Journal: American Journal of Epidemiology
• Published: 1997;145(8):680-8
• Cited by: 70 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The potential carcinogenicity of crystalline silica to humans remains a controversial issue. The authors conducted an historical cohort mortality study of 2,342 male workers exposed to crystalline silica, predominantly cristobalite, in a diatomaceous earth mining and processing facility in California. During the years 1942-1994, mortality excesses were detected for nonmalignant respiratory diseases (NMRD) (standardized mortality ratio = 2.01, 95% confidence interval (CI) 1.56-2.55) and lung cancer (standardized mortality ratio = 1.29, 95% CI 1.01-1.61). NMRD mortality rose sharply with cumulative exposure to respirable crystalline silica; allowing for a 15-year latency, the rate ratio for the highest exposure stratum (> or = 5.0 mg/m3-years) was 5.35 (95% CI 2.23-12.8). The rate ratio for lung cancer reached 2.15 (95% CI 1.08-4.28) in the highest exposure category. These associations were unlikely to have been confounded by smoking or asbestos exposure. The findings indicate a strong dose-response relation for crystalline silica and NMRD mortality. The lung cancer results, although less convincing, add further support to an etiologic role for crystalline silica.

In plain English

This historical cohort mortality study followed 2,342 white men employed at least a year at the larger of two diatomaceous earth plants in Lompoc, California, from 1942 to 1994. The plant's products contained crystalline silica, mainly cristobalite. Compared with US white males, mortality from nonmalignant respiratory disease, a category excluding pneumonia and infections, was raised (standardized mortality ratio 2.01, 95% CI 1.56-2.55). Lung cancer deaths were also raised (1.29, 95% CI 1.01-1.61; against local county rates, 1.44, 95% CI 1.14-1.80). With a 15-year lag, workers with cumulative respirable crystalline silica of 5.0 mg/m3-years or more had a nonmalignant respiratory disease mortality rate ratio of 5.35 (95% CI 2.23-12.8) compared with those below 0.5 mg/m3-years. Lung cancer trends were weaker and uneven; the highest group's rate ratio was 2.15 (95% CI 1.08-4.28). Adjusting for asbestos did not change the trends, and one probable pleural mesothelioma death occurred. The authors noted crude smoking data, unknown vital status for 9 percent, and uncertain exposure estimates. They concluded that the lung cancer results, though less convincing, added support to a causal role for crystalline silica.

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: Historical (retrospective) cohort mortality study with a 7-year extended follow-up of an earlier cohort, using internal dose-response (Poisson regression) analyses
• Who was studied: 2,342 white male workers (non-Hispanic and Hispanic) at the larger of two diatomaceous earth mining and processing plants in Lompoc, California. Each had at least 12 months of cumulative service and was employed for at least one day between 1942 and 1987. Mortality was followed from 1942 to 1994, compared with US white males and with local southern California county rates for lung cancer.
• Exposure: Cumulative exposure to respirable dust and to respirable crystalline silica (mg/m3-years), estimated from job histories and air monitoring data. The silica was mainly cristobalite in calcined (estimated 10 percent crystalline silica) and flux-calcined (20 percent) diatomaceous earth. Cumulative asbestos exposure (chrysotile used in two small plant operations, plus maintenance exposures of unknown fiber type) was estimated and included as a covariate.
• Main results:
  • Compared with US white males, mortality from nonmalignant respiratory disease (NMRD, a category that excluded pneumonia and infectious diseases) was elevated in the cohort (SMR 2.01, 95% CI 1.56-2.55), and lung cancer mortality was elevated (SMR 1.29)
  • The NMRD category excluded pneumonia and infectious diseases
  • The cohort's lung cancer SMR compared with US white males had a 95% confidence interval of 1.01-1.61
  • Compared with local county rates, the cohort's lung cancer SMR was 1.44 (95% CI 1.14-1.80)
  • NMRD mortality rose with cumulative respirable crystalline silica exposure (lagged 15 years). Workers in the highest stratum (5.0 mg/m3-years or more) had a rate ratio of 5.35 (95% CI 2.23-12.8) compared with the lowest category (less than 0.5 mg/m3-years).
  • Lung cancer dose-response trends were weaker and not consistently increasing. Workers in the highest respirable crystalline silica stratum (5.0 mg/m3-years or more) had a lung cancer rate ratio of 2.15 (95% CI 1.08-4.28).
  • Adjusting for cumulative asbestos exposure did not change the NMRD trend slopes; the authors reported the same for lung cancer
  • There was one death from pleural cancer, probably a mesothelioma, against 0.68 expected, and no deaths from peritoneal mesothelioma
  • With smoking data for 50 percent of the cohort and an assumed 20-fold lung cancer risk for smokers, the authors estimated that the lung cancer rate ratios in the highest strata (lagged 15 years) would fall to 1.59 for respirable dust and 1.67 for crystalline silica
• The authors' conclusion: The authors concluded that the findings indicate a strong dose-response relation between crystalline silica and NMRD mortality. They also concluded that the lung cancer results, although less convincing, add further support to a causal (etiologic) role for crystalline silica.
• Limitations the authors state:
  • Vital status could not be determined for 9 percent of the cohort
  • Exposure estimates are uncertain: particle counts had to be converted to gravimetric units, and respirable fractions and silica content had to be estimated
  • Asbestos exposure may have been misclassified, limiting control for asbestos as a confounder
  • Smoking data were incomplete and crude
  • Lung cancer comparisons with most other studies may not be fully appropriate, because the silica here was mainly cristobalite rather than quartz

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: Dose-response relationship, Lung Cancer, Crystalline, standardized mortality ratio (SMR), confidence interval, Cumulative Exposure, Latency, rate ratio.

Related studies

• Re: "Dose-response associations of silica with nonmalignant respiratory disease and lung cancer mortality in the diatomaceous earth industry" — G.W. Gibbs (1998)
• Re: "Dose-response associations of silica with nonmalignant respiratory disease and lung cancer mortality in the diatomaceous earth industry" — M.J. Mirliss (1998)
• Extended follow-up of lung cancer and non-malignant respiratory disease mortality among California diatomaceous earth workers — L.G. Gallagher, R.M. Park, et al. (2015)
• Radiographic evidence of silicosis risk in the diatomaceous earth industry — J.M. Hughes, H. Weill, et al. (1998)
• Reanalysis of mortality from lung cancer among diatomaceous earth industry workers, with consideration of potential confounding by asbestos exposure — H. Checkoway, N.J. Heyer, et al. (1996)
• Nonmalignant respiratory disease mortality among woodworkers participating in the American Cancer Society Cancer Prevention Study-II (CPS-II) — P.A. Demers, S.D. Stellman, et al. (1998)

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