Radiological survey of past and present vermiculite miners exposed to tremolite

Our plain-English summary of Radiological survey of past and present vermiculite miners exposed to tremolite, by J.C. McDonald, P. Sebastien, B. Armstrong (British Journal of Industrial Medicine, 1986).

The study

• Authors: J.C. McDonald, P. Sebastien, B. Armstrong
• Journal: British Journal of Industrial Medicine
• Published: 1986;43(7):445-9
• Cited by: 48 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. 000420).

Abstract

Chest radiographs taken by a standard technique were obtained from 173 current employees (164 men, 9 women) of a vermiculite mine in Montana, from 80 of 110 past employees resident within 200 miles, and from 47 men from the same area without known exposure to dust. In 43 of the 80 and 24 of the 47 an earlier chest x ray film was retrieved from the hospital archives. All 367 films were assessed blind and independently by three experienced readers using the ILO 1980 classification. Median radiographic assessment scores were analysed in relation to estimated cumulative exposure to the amphibole fibres that contaminate the vermiculite. Logistic regression analyses showed independent effects of age, smoking, and exposure on the prevalence of small opacities and of age and probably of exposure on pleural thickening. Overall, the data suggest that by retirement age the increase in prevalence of small opacities (greater than or equal to 1/0) lies between 5% and 10% per 100 f/ml years. This gradient may be somewhat steeper than for chrysotile miners and millers, but not much so.

In plain English

This cross-sectional study x-rayed workers at a Libby, Montana vermiculite mine whose ore contained fibrous amphibole of the tremolite series. It included all 173 people employed in July 1983 (164 men, nine women), 80 of 110 surviving past male employees living within 200 miles, and 47 unexposed men, who were included only to check the x-ray reading. Three readers classified the films blind using the ILO 1980 classification. Among 244 male workers, the age-standardised prevalence was 18.4% for small lung opacities graded 1/0 or more and 27.9% for chest-wall pleural thickening. These small opacities became more common as estimated cumulative fiber exposure rose. Statistical modelling found independent effects of age, smoking and exposure on small opacities, and of age and probably exposure on pleural thickening. Among men aged 60 or more, 24 of 64 had small opacities graded 1/0 or more. The authors concluded that by retirement age, the prevalence of these opacities rose by an estimated 5 to 10 percentage points per 100 f/ml years. They noted that exposure estimates before about 1972 were approximate, x-ray technique and reading varied, and the sample was small.

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: Cross-sectional radiological (chest x-ray) survey of current and past employees. An unexposed comparison group was used to control the reading process. Three readers read the films blind using the ILO 1980 classification.
• Who was studied: The study had three groups. The first was all 164 men and nine women employed at the Libby, Montana vermiculite mine on 1 July 1983. The second was 80 of 110 surviving past male employees living within 200 miles of the mine, who had been employed at least one year and hired before 1 January 1963. The third was 47 men without known dust exposure who attended the local hospital for routine chest x-rays. Earlier films were found for 43 of the 80 ex-employees and 24 of the 47 unexposed men, giving 367 films in total.
• Exposure: Fibrous amphibole of the tremolite series that contaminated the vermiculite ore. Exposure was expressed as estimated cumulative exposure in f/ml years, built from work histories and from year-by-year estimates of airborne fibre concentrations at each work location.
• Main results:
  • Among the 244 men in the exposure analysis (mean cumulative exposure 65.9 f/ml y), 45 had small opacities of ILO profusion 1/0 or more, an age-standardised prevalence of 18.4%. 68 had pleural thickening of the chest wall, an age-standardised prevalence of 27.9%.
  • In the age-standardised analysis, the rise in prevalence of small opacities with rising cumulative exposure was unlikely to be due to chance. The corresponding trend for pleural thickening of the chest wall was less strong.
  • Among men aged 60 or more (64 men, mean cumulative exposure 97.9 f/ml y), 24 had small opacities of profusion 1/0 or more (37.5%). 36 had pleural thickening of the chest wall (56.3%).
  • Logistic regression showed independent effects of age, smoking and cumulative exposure on the prevalence of small opacities.
  • Logistic regression showed effects of age, and probably of exposure, on pleural thickening.
  • After allowing for age, smoking and exposure, rates of change were higher in past employees than in present employees, but the difference was not statistically significant.
  • Among 43 ex-employees who also had an earlier film, small opacities of 1/0 or more were read in 18 (42%) of the 1983 films and in nine (21%) of the earlier films. Cumulative exposure had barely changed between the two films. The authors attributed the increase presumably in part to aging and in part to differences in radiographic technique.
  • Of the 47 unexposed men (32 of them aged 40 or more), small opacities were read in one and pleural thickening on the chest wall in four.
• The authors' conclusion: The authors concluded that by retirement age, the increase in prevalence of small opacities of profusion 1/0 or more lies between 5% and 10% per 100 f/ml years. They judged that this gradient may be somewhat steeper than for chrysotile miners and millers, but not much so. They also stated that at present Libby exposure levels, which the company reported to average 0.1 f/ml, no excess of radiological change should be detectable after a 40-year working life.
• Limitations the authors state:
  • Exposure estimates were approximate, especially for the period before about 1972.
  • Radiographic techniques and radiograph readings are major sources of variation. Agreement among the three readers fell to about 80% for small opacities and for pleural plaques or diffuse thickening.
  • The study sample was quite small.
  • Because numbers were small, standardised rates in the higher exposure groups had rather large standard errors.
  • The unexposed group was not selected or matched to the mine workers and was intended only to control the reading process.
  • Age, smoking and probably other environmental factors all contribute to radiographic change.

Mines named here

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

• Libby vermiculite mine, Montana

All asbestos mines

Topics

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

Medical terms in this study, explained in our Lexicon: vermiculite, Tremolite, Chest X-Ray, Cumulative Exposure, Amphibole Group, Pleural Thickening, Chrysotile.

Related studies

• Cohort study of mortality of vermiculite miners exposed to tremolite — J.C. McDonald, A.D. McDonald, et al. (1986)
• Health of vermiculite miners exposed to trace amounts of fibrous tremolite — J.C. McDonald, A.D. McDonald, et al. (1988)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part III. Radiographic findings — H.E. Amandus, R. Althouse, et al. (1987)
• Pulmonary changes after exposure to vermiculite contaminated with fibrous tremolite — J.E. Lockey, S.M. Brooks, et al. (1984)
• An epidemiological study in an Anatolian village environmentally exposed to tremolite asbestos — Y.I. Baris, N. Bilir, et al. (1988)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part II. Mortality — H.E. Amandus, R. Wheeler (1987)

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