Reanalysis of mortality from lung cancer among diatomaceous earth industry workers, with consideration of potential confounding by asbestos exposure

Our plain-English summary of Reanalysis of mortality from lung cancer among diatomaceous earth industry workers, with consideration of potential confounding by asbestos exposure, by H. Checkoway, N.J. Heyer, P.A. Demers, et al (Occupational and Environmental Medicine, 1996).

The study

• Authors: H. Checkoway, N.J. Heyer, P.A. Demers, G.W. Gibbs
• Journal: Occupational and Environmental Medicine
• Published: 1996;53(9):645-7
• Cited by: 28 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objective: To evaluate the potential for confounding from asbestos exposure, primarily chrysotile, on the relation between crystalline silica and mortality from lung cancer among diatomaceous earth (diatomite) workers.

Methods: A reanalysis of a cohort mortality study of diatomite workers was performed to take into account quantitative estimates of asbestos exposure. The reanalysis was limited to a subset of the original cohort, comprising 2266 white men for whom asbestos exposure could be reconstructed with greatest confidence. Comparisons between mortality from lung cancer (standardised mortality ratios (SMR)) were made between rates for 1942-87 for United States white men, and workers cross classified according to cumulative exposures to crystalline silica and asbestos. Comparisons of internal rates, involving Poisson regression modeling, were conducted for exposure to crystalline silica, with and without adjustment for asbestos exposure. Exposures were lagged by 15 years to take into account disease latency.

Results: There was an overall excess of lung cancer (SMR 1.41; 52 observed). The SMRs for four categories of increasing crystalline silica among the workers not exposed to asbestos were 1.13, 0.87, 2.14, 2.00. An SMR of 8.31 (three observed) was found for workers with the highest cumulative exposure to both dusts. Internal analysis, after adjustment for asbestos exposure, yielded rate ratios for categories of exposure to crystalline silica: 1.00 (reference), 1.37, 1.80, and 1.79.

Conclusions: Asbestos exposure was not an important confounder of the association between crystalline silica and mortality from lung cancer in this cohort. Although based on a small number of deaths from lung cancer, the data suggest possible synergy between these exposures. An extended follow up of this cohort is in progress and should enable better assessments of independent and combined effects on risk of lung cancer.

In plain English

This study reanalysed a cohort mortality study of diatomaceous earth (diatomite) workers, whose main exposure was crystalline silica. The authors asked whether asbestos, primarily chrysotile, could explain the link between silica and lung cancer. They studied 2266 white men with estimable asbestos exposure from one southern California plant, comparing their lung cancer deaths with United States white men's rates for 1942-87. Overall, 52 lung cancer deaths gave a standardised mortality ratio (SMR) of 1.41 (95% confidence interval 1.05 to 1.85). Among men without asbestos exposure, SMRs for four rising silica categories were 1.13 (0.63 to 1.86), 0.87 (0.18 to 2.53), 2.14 (0.86 to 4.41) and 2.00 (0.73 to 4.35); every interval included 1. Adjusting for asbestos changed the silica rate ratios only slightly. Men most exposed to both dusts had an SMR of 8.31 (1.71 to 24.3), from three deaths. The authors concluded that, to the extent the asbestos estimates were valid, asbestos did not confound the silica and lung cancer association in this cohort. They suggested possible synergy between the dusts but called that inference probably premature given so few deaths.

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: Reanalysis of a cohort mortality study, using standardised mortality ratios against national rates and internal Poisson regression comparisons
• Who was studied: 2266 white men from the larger of two diatomite plants in southern California. They had worked at least one day between 1 January 1942 and 31 December 1987 and had at least 12 months of service. Workers first employed before 1930 were excluded. The group includes 89 men known to have worked in asbestos mixing jobs.
• Exposure: Cumulative exposure to crystalline silica (principally cristobalite; a semiquantitative index) and to asbestos, primarily chrysotile (in fibres/ml x years). Exposures were lagged 15 years. Chrysotile-containing products were made at the plant from the 1920s to 1977. Other asbestos types may have occurred episodically during maintenance.
• Main results:
  • Whole subset of 2266 men: 52 lung cancer deaths, SMR 1.41 (95% CI 1.05 to 1.85) compared with United States white men
  • Confidence interval for the overall SMR of 1.41: 1.05 to 1.85
  • Workers not exposed to asbestos: SMRs for four categories of increasing crystalline silica were 1.13 (95% CI 0.63 to 1.86), 0.87 (0.18 to 2.53), 2.14 (0.86 to 4.41) and 2.00 (0.73 to 4.35)
  • Confidence intervals for the two lowest silica categories among workers not exposed to asbestos (15 deaths, SMR 1.13; 3 deaths, SMR 0.87)
  • Confidence interval for silica category 100-199 among workers not exposed to asbestos (7 deaths, SMR 2.14)
  • Confidence interval for silica category over 200 among workers not exposed to asbestos (SMR 2.00)
  • Workers with the lowest crystalline silica exposure: the trend with asbestos exposure was irregular and difficult to interpret
  • Internal analysis adjusted for asbestos: rate ratios for crystalline silica categories were 1.00 (reference), 1.37, 1.80 and 1.79. Adjustment for asbestos made only minor differences.
  • Workers with the highest cumulative exposure to both crystalline silica and asbestos: SMR 8.31 (95% CI 1.71 to 24.3), based on three observed deaths
  • Confidence interval for the SMR of 8.31 in the highest combined exposure category: 1.71 to 24.3
• The authors' conclusion: The authors concluded that, to the extent the asbestos estimates are valid, asbestos did not confound the association between crystalline silica and lung cancer in this cohort. They also said the data suggest possible synergy between the two dusts.
• Limitations the authors state:
  • The conclusion depends on the asbestos exposure estimates being valid.
  • Any inference about synergy is probably premature, because only three lung cancer deaths occurred in the highest combined exposure category.
  • Exposure to asbestos types other than chrysotile may have occurred, but there were no measurement data by fibre type.
  • The smaller plant was excluded because the information needed to reconstruct its asbestos exposures was not available.
  • The silica gradient was weaker than in the original study, probably because workers hired before 1930 were excluded; in the original study that group had the largest lung cancer excess (SMR 2.63).
  • The crystalline silica index was semiquantitative. The planned update is expected to allow more direct measurement of silica exposure.

Topics

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

Medical terms in this study, explained in our Lexicon: Lung Cancer, Confounding, Asbestos Exposure, Chrysotile, Crystalline, Cumulative Exposure, Latency, rate ratio.

Related studies

• Radiographic evidence of silicosis risk in the diatomaceous earth industry — J.M. Hughes, H. Weill, et al. (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)
• Dose-response associations of silica with nonmalignant respiratory disease and lung cancer mortality in the diatomaceous earth industry — H. Checkoway, N.J. Heyer, et al. (1997)
• 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)
• Residential proximity to naturally occurring asbestos and mesothelioma risks: further consideration of exposure misclassification and occupational confounding — M.A. Kelsh, D.W. Berman, et al. (2006)

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