Radiographic evidence of silicosis risk in the diatomaceous earth industry

Our plain-English summary of Radiographic evidence of silicosis risk in the diatomaceous earth industry, by J.M. Hughes, H. Weill, H. Checkoway, et al (American Journal of Respiratory and Critical Care Medicine, 1998).

The study

• Authors: J.M. Hughes, H. Weill, H. Checkoway, R.N. Jones, M.M. Henry, N.J. Heyer, N.S. Seixas, P.A. Demers
• Journal: American Journal of Respiratory and Critical Care Medicine
• Published: 1998;158(3):807-14
• Cited by: 51 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

There is limited and conflicting evidence regarding the exposure-response relationship between exposure to crystalline silica and silicosis; the level of risk to current workers remains uncertain. We conducted an epidemiologic investigation of 1,809 workers in the diatomaceous earth industry, where exposures to crystalline silica are primarily to the cristobalite form. On the basis of the median of three independent readings, 81 (4.5%) workers were judged to have opacities on chest radiographs (small opacities, profusion >= 1/0, and/or large opacities). Age-adjusted relative risk of opacities increased significantly with cumulative exposure to crystalline silica. The concentration of respirable crystalline silica to which workers were exposed (highly correlated with period of hire) was an important determinant of risk after accounting for cumulative exposure. For workers with an average exposure to crystalline silica of <= 0.50 mg/m3 (or hired >= 1950), the cumulative risk of opacities for a cumulative exposure to crystalline silica of 2.0 mg/m3-yr was approximately 1.1%; for an average exposure > 0.50 mg/m3 (or hired < 1950), the corresponding cumulative risk was 3.7%. These findings indicate an exposure-response relationship between cumulative exposure to crystalline silica and radiographic opacities; moreover, the relationship was substantially steeper among workers exposed at the highest average concentrations of crystalline silica.

In plain English

The authors studied chest X-rays of men at a diatomaceous earth mining and processing plant in Lompoc, California, where dust contained crystalline silica, mainly cristobalite. Of 2,342 men employed at least a year, 1,809 had an X-ray taken more than a month after hire. Three readers classified each film. By majority reading, 81 of 1,809 men (4.5%) had lung opacities and 33 (1.8%) had pleural changes. Opacity rates rose with cumulative silica exposure. At a cumulative exposure of 2.0 mg/m3-yr, the observed cumulative risk was 3.7% for men with average exposure above 0.50 mg/m3 and 1.1% for those at or below it. Opacities were concentrated in men hired before 1950. Chrysotile asbestos was used in two plant operations, but analyses of estimated asbestos exposure found no relationship with X-ray changes. The authors noted that X-rays were mostly taken only during employment and that early exposures may have been underestimated. They concluded their results did not support the view that cristobalite is more fibrogenic (scar-forming) than quartz at comparable exposures.

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: Cohort study of workers using company chest X-rays, each worker's latest film read by three readers using the ILO 1980 classification, with Poisson regression and survival analysis against estimated exposure; part of a larger epidemiologic study that also included mortality follow-up
• Who was studied: 2,342 Caucasian men employed at least 1 year at one diatomaceous earth plant in Lompoc, California, with some employment between 1942 and 1987, and no known prior asbestos exposure elsewhere; 1,809 men with an X-ray taken more than 1 month after hire were included in exposure-response analyses
• Exposure: Respirable crystalline silica, primarily cristobalite, and respirable dust from diatomite (natural, calcined and flux-calcined), estimated from air monitoring since 1948 and work histories. Results are expressed as cumulative exposure (mg/m3-yr) and average concentration (mg/m3). Chrysotile asbestos was used at times in two operations, and maintenance workers may have had episodic asbestos exposure of unknown fiber type.
• Main results:
  • By median (majority) reading, 81 of 1,809 workers (4.5%) had opacities on chest X-ray (small opacities of profusion 1/0 or greater and/or large opacities).
  • By median reading, 33 of 1,809 workers (1.8%) had pleural changes.
  • Opacities by period of hire: 23% (35 of 151) of workers hired before 1940, 7% (35 of 482) of those hired 1940 to 1949, and 1% (11 of 1,176) of those hired in 1950 or later.
  • Incidence of opacities per 100,000 person-years rose across cumulative crystalline silica categories (under 1, 1 to 3, 3 to 4, 4 to 6, 6 to 8, and over 8 mg/m3-yr) from 63 to 2,204.
  • Opacities developed in 1.6% (23 of 1,452) of workers with average crystalline silica concentration of 0.50 mg/m3 or less, and in 16.2% (58 of 357) of those above 0.50 mg/m3.
  • Observed cumulative risk of opacities at 2.0 mg/m3-yr cumulative crystalline silica was 1.1% in the lower-concentration group and 3.7% in the higher-concentration group. At 4.0 mg/m3-yr the figures were 3.3% and 12.4%.
  • For the 1,176 workers hired in 1950 or later, observed cumulative risks of opacities were 1.0% at 2.0 mg/m3-yr and 2.3% at 4.0 mg/m3-yr cumulative crystalline silica.
  • Asbestos: among workers estimated to have no in-plant asbestos exposure, 2.6% developed pleural changes and 7.5% opacities. Among those with asbestos exposure, the figures were 1.3% and 2.3%. More detailed analyses using individual quantitative asbestos estimates found no relationships with opacities or pleural changes.
  • Among workers in the lower-concentration group with smoking information, 0.4% (1 of 269) of nonsmokers and 2.6% (20 of 756) of smokers developed opacities.
• The authors' conclusion: The authors concluded that their findings are consistent with an exposure-response relationship between crystalline silica and radiographic opacities, steeper at higher average concentrations. They also concluded that their results do not support the view that cristobalite is more fibrogenic than quartz at comparable exposures. They noted that a cumulative risk of about 1.0% at 2.0 mg/m3-yr among later workers is not appreciably higher than reported for many unexposed populations.
• Limitations the authors state:
  • Workers mostly had X-rays only while employed, so opacities developing after they left would not have been recorded. Information on opacities appearing more than 35 years after hire was limited.
  • The stronger risk in the high-concentration (mostly early-hire) group might instead reflect underestimated exposure levels for early workers.
  • Smoking information, recorded only as ever versus never, was available for about half the cohort and generally not for early workers.

Topics

Medical terms in this study, explained in our Lexicon: Silicosis, Crystalline, Chest X-Ray, Relative Risk, Cumulative 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)
• 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)
• 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)
• Mortality of a cohort of men in a silicosis register: further evidence of an association with lung cancer — T.P. Ng, S.L. Chan, et al. (1990)

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