Natural history of pleural thickening after exposure to crocidolite

Our plain-English summary of Natural history of pleural thickening after exposure to crocidolite, by N.H. de Klerk, W.O. Cookson, A.W. Musk, et al (British Journal of Industrial Medicine, 1989).

The study

• Authors: N.H. de Klerk, W.O. Cookson, A.W. Musk, B.K. Armstrong, J.J. Glancy
• Journal: British Journal of Industrial Medicine
• Published: 1989;46(7):461-7
• Cited by: 26 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 000068).

Abstract

Serial plain chest radiographs of 384 men who worked at the Wittenoom crocidolite mine and mill between 1943 and 1966 and who applied for pneumoconiosis compensation between 1948 and 1982 have been examined independently by two trained observers for pleural disease using the 1980 ILO-UICC classification of radiographs to record width and extent of pleural disease. Radiographs covering follow up periods of from two to 38 years were examined (median number of films per subject was nine). The degree of crocidolite exposure was estimated from employment records and a survey of airborne fibre concentrations performed in 1966. Agreement between the observers on the presence and degree of pleural disease in the final film for each subject was moderately close (Kendall's tau B = 0.62) and was least for subjects with thickening less than 5 mm in width. Diffuse pleural thickening extending for greater than 50% of the lateral chest wall was the most common type recorded by both observers. Minor pleural thickening frequently progressed in extent along the lateral chest wall but progression beyond 5 mm in thickness was less common. Pleural plaques were not seen to progress beyond their initial thickness or extent. The rate of onset of thickening in this population increased continually from the time of first exposure and also increased slightly with age. There was evidence that the level of total cumulative exposure to crocidolite increased the rate of onset of pleural thickening in the period between five and 15 years after first exposure. Rate of progression of established thickening was greatest in subjects who first developed thickening early after first exposure. The relative rate of progression decreased slowly with time from first signs of thickening and there was no evidence of any progression more than 15 years after onset.

In plain English

This study examined serial chest X-rays of men who worked at the Wittenoom crocidolite mine and mill in Western Australia and applied for asbestosis compensation between 1948 and December 1982. Of 384 such men, X-rays were found for 303, covering two to 38 years from first exposure. Two readers graded the films independently, with moderately close agreement. Reader 1 found that 170 men (56%), and reader 2 that 137 (45%), eventually developed pleural thickening. Diffuse thickening was much more common than pleural plaques, which did not progress beyond their initial stage. The rate of new thickening rose about 63% per five years since first exposure. Higher total cumulative exposure was linked to a faster onset rate, about 20% per log unit, but only between five and 15 years after first employment. Progression was fastest soon after thickening appeared, with no evidence of progression beyond 15 years after onset. The authors concluded the thickening appeared unlikely to progress enough to impair lung function without parenchymal fibrosis or pleural effusion. They noted limited power to detect effects on progression.

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: Follow-up study of serial chest radiographs from compensation claimants. Two observers read the films independently, and the data were analysed with stratified Poisson regression.
• Who was studied: 384 men who had worked at the Wittenoom crocidolite mine and mill (operated 1943 to 1966) and applied for asbestosis compensation between 1948 and December 1982. Radiographs were recovered for 303 of them, with 1 to 27 (median 9) films each, covering follow-up of two to 38 years from the start of exposure.
• Exposure: Crocidolite. Exposure was estimated from company employment records and a 1966 Mines Department survey of airborne respirable crocidolite fibres longer than 5 microns. The measures were intensity (fibres/cc), duration, total cumulative exposure (fibre/cc years) and place of work (mill or elsewhere).
• Main results:
  • Among the 303 men with radiographs, reader 1 found that 170 (56%) and reader 2 that 137 (45%) eventually developed some radiographic evidence of pleural thickening.
  • Agreement between the two readers on each man's final film gave a Kendall's tau B of 0.62 (printed as 0-62), which the authors described as moderately close agreement. Disagreement was greatest for thickening under 5 mm extending over more than half of the lateral chest wall.
  • The most common type of thickening at all times since first employment was diffuse pleural thickening extending over more than 50% of the lateral chest wall.
  • Pleural plaques (defined in this study as thickening over 5 mm extending less than a quarter of the lateral chest wall) were not seen to progress beyond their initial stage of thickness or extent.
  • There were 2777 person-years of observation for onset of disease, with 79 events, and 638 person-years of observation for progression, with 26 events.
  • Linear trend lines showed: approximately a 63% increase in the incidence (onset) rate for each five-year period since first exposure; a 32% fall in the rate of progression for each five-year period since first exposure; and a 22% fall in the rate of progression for each five-year period since onset.
  • The rate of progression was greater in men who first developed thickening soon after first exposure to crocidolite than in men whose thickening developed later.
  • The rate of progression was highest in the five years right after thickening appeared and fell slowly afterwards. There was no evidence of progression more than 15 years after onset.
  • When each covariate (exposure measures, year of first exposure, site of work, age) was examined individually, no large or consistent effects on the rate of onset were found.
  • No exposure variable appeared to have a substantial effect on the rate of progression.
  • Total cumulative exposure was examined in combination with time since exposure. Taken together, the added terms were not statistically significant. The individual terms showed total cumulative exposure significantly increasing the onset rate by about 20% per log (fibre/cc year) between five and 15 years after first employment, but not at other times.
• The authors' conclusion: The authors concluded that increasing numbers of men previously exposed to crocidolite at Wittenoom Gorge may be expected to develop diffuse pleural thickening. On present evidence, they considered it unlikely to progress enough to impair lung function in the absence of parenchymal fibrosis or pleural effusion.
• Limitations the authors state:
  • Because relatively few men had progression, the study's power to detect effects on progression was much lower than its power to detect effects on onset.
  • Mild pleural thickening is hard to tell apart from other shadows on a plain chest radiograph, which the authors gave as the reason for disagreement between the readers.
  • Mild diffuse pleural thickening may be mistaken for subpleural parenchymal fibrosis on chest radiographs. The authors judged that this misclassification was unlikely to explain the exposure effects.
  • No radiographs could be found for 81 of the 384 men. These men differed in year of birth, year of starting work and age at starting work, which the authors attributed to better retrieval of more recent films. Measures of exposure did not differ appreciably between the two groups.
  • There were not enough data to test whether the effects of cumulative exposure were linear within time periods.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Wittenoom, Western Australia

All asbestos mines

Topics

The National Library of Medicine indexes this study under subjects our site covers: Crocidolite, Pleural plaques and thickening.

Medical terms in this study, explained in our Lexicon: Pleural Thickening, Crocidolite, Chest X-Ray, Wittenoom, Pneumoconiosis, UICC, Diffuse Pleural Thickening, Pleural Plaques.

Related studies

• The presentation and natural history of asbestos-induced diffuse pleural thickening — V. Jeebun, S.C. Stenton (2012)
• The natural history of asbestosis in former crocidolite workers of Wittenoom Gorge — W. Cookson, N. De Klerk, et al. (1986)
• Radiographic abnormalities and mortality in subjects with exposure to crocidolite — N.H. de Klerk, A.W. Musk, et al. (1993)
• Benign and malignant pleural effusions in former Wittenoom crocidolite millers and miners — W.O. Cookson, N.H. De Klerk, et al. (1985)
• Prevalence of radiographic asbestosis in crocidolite miners and millers at Wittenoom, Western Australia — W.O. Cookson, N.H. De Klerk, et al. (1986)
• Asbestos-related diffuse pleural thickening — N. Fujimoto, K. Kato, et al. (2014)

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