The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part I. Exposure estimates

Our plain-English summary of The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part I. Exposure estimates, by H.E. Amandus, R. Wheeler, J. Jankovic, et al (American Journal of Industrial Medicine, 1987).

The study

• Authors: H.E. Amandus, R. Wheeler, J. Jankovic, J. Tucker
• Journal: American Journal of Industrial Medicine
• Published: 1987;11(1):1-14
• Cited by: 57 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 000296).

Abstract

The vermiculite ore and concentrate of a mine and mill near Libby, Montana, was found to be contaminated with fibrous tremolite-actinolite. Of 599 fibers (length greater than 5 microns and width greater than 0.45 micron) counted in eight airborne membrane filter samples, 96% had an aspect ratio greater than 10 and 16% had an aspect ratio greater than 50. Additionally, 73% of the fibers were longer than 10 microns, 36% were longer than 20 microns, and 10% were longer than 40 microns. Estimates of exposure before 1964 in the dry mill were 168 fibers/cc for working areas, 182 fibers/cc for sweepers, 88 fibers/cc for skipping, and 13 fibers/cc for the quality control laboratory. In 1964-1971, exposure estimates for these areas were 33, 36, 17, and 3 fibers/cc, respectively. Estimates of exposures in the mine before 1971 ranged from 9-23 fibers/cc for drillers and were less than 2 fibers/cc for nondrilling jobs. All 8-hr TWA job exposure estimates decreased from 1972-1976, and from 1977-1982 were less than 1 fiber/cc.

In plain English

This paper, Part I of a series, estimated fibrous tremolite-actinolite exposures for jobs at a vermiculite mine and mill near Libby, Montana. The authors used 336 older dust-count samples, mostly from the dry mill, and 4116 later filter samples. They divided the site into 25 work areas and converted dust counts to fiber counts with a ratio of 4.0. Some estimates before 1968 rested on assumptions the authors called guesstimates. Estimated exposures in the dry mill before 1964 were 168 fibers per cubic centimeter (fibers/cc) for working areas, 182 for sweepers, 88 for skipping and 13 for the quality control laboratory. In 1964-1971 they were 33, 36, 17 and 3. Before 1971, drillers' estimates ranged from 9 to 23 fibers/cc; other mine jobs were below 2. In 1977-1982 most areas were below 1.0 fiber/cc. Of 599 airborne fibers measured, all longer than about 5 and wider than about 0.45 micrometers, 73% were longer than 10 micrometers. The authors concluded that exposures were extremely high in the mine and mill before 1974. They recognized that estimates before 1968 were of questionable accuracy.

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: Exposure assessment study: historical air sampling data were used to estimate job exposures by year, and fiber type and dimensions were characterized. It is Part I of a series; mortality and radiographic findings are reported in separate papers.
• Who was studied: Jobs and work areas at a vermiculite mine and mill near Libby, Montana, divided into 25 location-operations. Data were 336 midget impinger samples collected by the state of Montana, mainly in the dry mill, before 1969, and 4116 membrane filter samples collected after 1967 by NIOSH, MESA, MSHA and the company.
• Exposure: Fibrous tremolite-actinolite contaminating vermiculite ore and concentrate, measured as fibers/cc (with dust counts in mppcf converted to fibers/cc using a ratio of 4.0).
• Main results:
  • Dry mill jobs before 1964: estimated exposures of 168 f/cc for working areas, 182 f/cc for sweepers, 88 f/cc for skipping, and 13 f/cc for the quality control laboratory.
  • Dry mill jobs in 1964-1971: estimates were 33, 36, 17, and 3 f/cc for the same four areas.
  • Mine jobs before 1971: estimates ranged from 9-23 fibers/cc for drillers and were less than 2 fibers/cc for nondrilling jobs.
  • In 1977-1982, exposures in most areas were less than 1.0 f/cc; in the mill they ranged from 0.6 to 1.0 f/cc.
  • The authors sized 599 fibers from eight airborne samples. Only fibers longer than 4.98 micrometers and wider than 0.44 micrometers were included.
  • Among those 599 fibers, 96% had an aspect ratio (length to width) greater than 10, 67% greater than 20, and 16% greater than 50.
  • Among the same 599 fibers, 73% were longer than 10 micrometers, 36% were longer than 20 micrometers, and 11% were longer than 40 micrometers.
  • Examination of individual fibers by SEM/EDAX gave iron-to-magnesium peak ratios of 42-57%. The authors said the fibers might be better described as actinolite.
  • A conversion ratio of 4.0 fibers/cc per mppcf was chosen, based on 336 impinger samples from 1965-1969 and 81 filter samples from 1967-1971.
  • In 203 company samples from 1984, 92% of 8-hr TWA exposures were less than 0.20 f/cc, and the average was 0.1 f/cc.
• The authors' conclusion: The authors concluded that the conversion ratio varied from 1.2 to 11.5 across subgroups. They judged their chosen ratio of 4.0 unlikely to differ from the true ratio by more than a factor of 2, and they concluded that exposures were extremely high in the mine and mill before 1974.
• Limitations the authors state:
  • The authors recognized that the exposure estimates before 1968 were of questionable accuracy.
  • For drilling, river dock and ore loading areas, no exposure data existed before 1968, so those estimates were considered guesstimates. The authors said these had a small effect because few workers were employed in those areas.
  • No side-by-side impinger and filter samples were taken, so the conversion ratio could not be evaluated precisely.
  • Before 1974, filter samples were short-term or general area samples and were not likely to reflect 8-hr TWA exposure.
  • Fiber dimensions were measured by optical microscopy, so it is unclear how they compare with electron microscopy studies of other fiber types.

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: Actinolite, Talc and vermiculite, Tremolite.

Medical terms in this study, explained in our Lexicon: vermiculite, Tremolite, Actinolite, Aspect Ratio (Length-to-diameter ratio).

Related studies

• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part II. Mortality — H.E. Amandus, R. Wheeler (1987)
• 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)
• 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)
• Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana — J.C. McDonald, J. Harris, et al. (2004)
• Radiological survey of past and present vermiculite miners exposed to tremolite — J.C. McDonald, P. Sebastien, et al. (1986)

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