Extended follow-up of lung cancer and non-malignant respiratory disease mortality among California diatomaceous earth workers

Our plain-English summary of Extended follow-up of lung cancer and non-malignant respiratory disease mortality among California diatomaceous earth workers, by L.G. Gallagher, R.M. Park, H. Checkoway (Occupational and Environmental Medicine, 2015).

The study

• Authors: L.G. Gallagher, R.M. Park, H. Checkoway
• Journal: Occupational and Environmental Medicine
• Published: 2015;72(5):360-5
• Cited by: 19 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Objectives: Millions of workers worldwide are employed in occupations involving potentiality hazardous exposure to crystalline silica. The diatomaceous earth industry can have particularly high exposures, but there is a lower likelihood of simultaneously occurring confounding exposures. We extended follow-up for diatomaceous earth industry workers previously studied for mortality.

Methods: The cohort included 2342 white men who were employed for at least 1 year at a diatomaceous earth plant in Lompoc, California beginning in 1942. Workers' vital status was updated using the National Death Index through 2011, an extension of 19 years from earlier studies. Detailed work history and quantitative air monitoring measurements estimated exposure intensity. Cox proportional hazards modelling estimated HRs and 95% CIs. SMRs were calculated.

Results: Elevated mortality was observed by quartile of cumulative crystalline silica exposure for lung cancer (HR=2.03, 95% CI 1.07 to 3.85, highest quartile, unlagged) and non-malignant respiratory disease (NMRD) (HR=3.59, 95% CI 1.94 to 6.67, highest quartile, unlagged), although trends were not statistically significant. Associations were attenuated when adjusted for smoking and asbestos exposure. Mortality from NMRD was significantly increased over the entire follow-up compared to the general population (SMR=1.37, 95% CI 1.17 to 1.60). An increase for lung cancer was confined to the earlier follow-up (SMR=1.29, 95% CI 1.01 to 1.61).

Conclusions: The risk of lung cancer and NMRD mortality remained elevated, although generally non-significant, and exposure-response trends with cumulative crystalline silica persisted on extended follow-up of this cohort. The findings support a generally consistently observed aetiological relation between crystalline silica and lung cancer.

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In plain English

This retrospective cohort study extended death records through 2011 for 2342 white men who had worked at least 1 year at a diatomaceous earth plant in Lompoc, California, from 1942 onward. The main exposure studied was respirable crystalline silica. The authors also estimated exposure to chrysotile asbestos from two small operations at the plants. Compared with US white men, deaths from non-malignant respiratory disease were higher over the whole follow-up (SMR 1.37, 95% CI 1.17 to 1.60). Lung cancer deaths were raised in the earlier follow-up (SMR 1.29, 95% CI 1.01 to 1.61) but not over the whole period (SMR 1.03, 95% CI 0.85 to 1.23). Compared with the lowest silica category, the highest category had a lung cancer HR of 2.03 (95% CI 1.07 to 3.85) and a non-malignant respiratory disease HR of 3.59 (95% CI 1.94 to 6.67). The lung cancer trend was not statistically significant. Allowing for asbestos and smoking weakened these estimates. The authors concluded that the findings continue to support aetiological relations of cumulative crystalline silica exposure with both outcomes.

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: Retrospective cohort mortality study: an extended follow-up of a previously studied cohort, using comparisons with general-population death rates (SMRs) and Cox proportional hazards models (HRs)
• Who was studied: 2342 white men employed at least 1 year at a diatomaceous earth plant in Lompoc, California, who worked at least 1 day between 1942 and 1987. Vital status was followed through 2011, giving 1219 deaths and 86 561 person-years of observation.
• Exposure: Cumulative respirable crystalline silica, estimated from job histories and air monitoring, in mg/m3-years (overall mean 2.16). Chrysotile asbestos from two small operations in the plants was also estimated, in fibers/mL-years (overall mean 1.44), and treated as a possible confounder.
• Main results:
  • Compared with US men, all-cause mortality in the cohort was statistically significantly reduced.
  • Compared with US men, mortality from non-malignant respiratory disease was increased over the entire follow-up: SMR 1.37 (95% CI 1.17 to 1.60).
  • Within non-malignant respiratory disease, the largest excesses were for pneumoconiosis and other respiratory conditions (SMR 2.16, 95% CI 1.56 to 2.91) and for COPD (SMR 1.30, 95% CI 1.03 to 1.61).
  • Lung cancer mortality compared with US men was increased in the first follow-up period (SMR 1.29, 95% CI 1.01 to 1.61) and decreased in the second period (SMR 0.66, 95% CI 0.46 to 0.92).
  • Over the entire follow-up there was no increased lung cancer mortality compared with US men: SMR 1.03 (95% CI 0.85 to 1.23).
  • Lung cancer by cumulative silica exposure, 1942 to 2011, no lag: the highest category (more than 5.6 mg/m3-years) had HR 2.03 (95% CI 1.07 to 3.85) compared with the lowest. The trend per 5 mg/m3-years was HR 1.13 (95% CI 0.99 to 1.29), p=0.08, and was not statistically significant.
  • Non-malignant respiratory disease (excluding infectious lung diseases) by cumulative silica exposure, 1942 to 2011, no lag: the highest category (more than 6.9 mg/m3-years) had HR 3.59 (95% CI 1.94 to 6.67). The trend per 5 mg/m3-years was HR 1.21 (95% CI 1.08 to 1.35), p=0.001. The results section reports this trend as statistically significant with and without adjustment for asbestos.
  • The non-malignant respiratory disease trend was driven more by pneumoconiosis and other respiratory diseases (HR 1.29, 95% CI 1.11 to 1.50 per 5 mg/m3-years) than by COPD and asthma (HR 1.14; only one confidence limit, 0.96, is printed).
  • When asbestos exposure (ever/never) was added to the lung cancer models, the estimates were weakened but the overall increased risk persisted.
  • Smoking was known (ever/never) for about half the cohort (N=1171). The authors estimated smoking adjustment indirectly, assuming smoking carried a 10-fold increased risk. On that basis, the highest-category HR would fall from 2.03 to 1.60 for lung cancer and from 3.59 to 2.82 for non-malignant respiratory disease.
  • Among workers hired after 1950, whose silica exposure estimates were based on measurements and who had minimal asbestos exposure, the lung cancer findings were very similar to the overall results but not statistically significant.
• The authors' conclusion: The authors concluded that the updated analyses continue to support aetiological relations of cumulative crystalline silica exposure with lung cancer and non-malignant respiratory disease, and that the findings should contribute to reviews of occupational exposure standards.
• Limitations the authors state:
  • Silica exposures for earlier periods were extrapolated, and the conversion factors for dust and respirable fractions were uncertain.
  • Exposure data were not updated after 1994.
  • Causes of death came from death certificates that were not validated against medical records or other sources.
  • More than half of the non-malignant respiratory disease category consisted of unspecified or other respiratory diseases.
  • The role of silicosis as a precursor of lung cancer could not be fully assessed.
  • In the analysis restricted to workers hired after 1950, few workers had exposures in the upper categories, which made the non-malignant respiratory disease estimates imprecise.

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: Lung Cancer, Crystalline, Confounding, Occupational History, Air Sampling, Asbestos Exposure.

Related studies

• 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)
• 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)
• Radiographic evidence of silicosis risk in the diatomaceous earth industry — J.M. Hughes, H. Weill, et al. (1998)
• 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 Mortality Follow-up of a Cohort of 25,460 Workers Exposed to Acrylonitrile — S. Koutros, J.H. Lubin, et al. (2019)

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