The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part II. Mortality

Our plain-English summary of The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part II. Mortality, by H.E. Amandus, R. Wheeler (American Journal of Industrial Medicine, 1987).

The study

• Authors: H.E. Amandus, R. Wheeler
• Journal: American Journal of Industrial Medicine
• Published: 1987;11(1):15-26
• Cited by: 89 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. 000295).

Abstract

The vermiculite ore and concentrate of a mine and mill located near Libby, Montana was found to be contaminated with a fiber of the tremolite/acetinolite series. A study was conducted to estimate the exposure-response relationship for mortality for 575 men who had been hired prior to 1970 and employed at least 1 year at the Montana site. Individual cumulative fiber exposure (fiber-years) was calculated. Results indicated that mortality from nonmalignant respiratory disease (NMRD) and lung cancer was significantly increased compared to the U.S. white male population. For those workers more than 20 years since hire, the standard mortality rate (SMR) for lung cancer (ICDA 162-163) was 84.7, 225.1, 109.3, and 671.3 for less than 50, 50-99, 100-399, and more than 399 fiber-years respectively. Corresponding results for NMRD (ICDA 460-519) were 327.8, 283.5, 0, and 278.4. Based on a linear model for greater than 20 years since hire, the estimated percentage increase in lung cancer mortality risk was 0.6% for each fiber-year of exposure. At 5 fiber-years, the estimated percentage was 2.9% from an unrestricted (nonthreshold) linear model and 0.6% from a survival model.

In plain English

This study followed 575 men hired before 1970 who worked at least one year at a vermiculite mine and mill near Libby, Montana, whose ore was contaminated with fibers of the tremolite-actinolite series. The authors estimated each man's cumulative fiber exposure in fiber-years and compared deaths through 1981 with U.S. white male death rates. Of 161 deaths, lung cancer caused 20 against 9.0 expected (SMR 223.2) and nonmalignant respiratory disease 20 against 8.2 expected (SMR 243.0), both significantly raised. Among men hired more than 20 years earlier, the lung cancer SMR was significantly raised only above 399 fiber-years (671.3). For these men, the authors' linear models estimated that lung cancer death risk rose about 0.6% per fiber-year. No association was found between cumulative exposure and nonmalignant respiratory disease deaths, though the authors said conclusions on this were unclear. Two mesothelioma cases occurred. Current and former smokers made up 84.0% of a sample of later workers versus 68.5% of U.S. white males in 1975; the authors called their smoking adjustment imprecise. They said the risk below 50 fiber-years remained unclear.

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 (man-years analysis with standardized mortality ratios, linear regression and Cox survival models of exposure-response)
• Who was studied: 575 men hired before 1970 and employed at least 1 year at the vermiculite mine and mill near Libby, Montana. Vital status was determined for 569 men as of December 31, 1981, with 161 deaths and 13,502 man-years of follow-up. Deaths were compared with U.S. white male death rates.
• Exposure: Vermiculite ore and concentrate contaminated with fiber of the tremolite/actinolite series. Exposure was measured as individual cumulative fiber exposure (fiber-years), built from job exposure estimates and work histories. The cohort average was 200 fiber-years, with an average net tenure of 8.3 years.
• Main results:
  • Whole cohort compared with U.S. white males: lung cancer deaths were 20 observed vs 9.0 expected, SMR 223.2 (95% CI 136.3 to 344.7). Nonmalignant respiratory disease deaths were 20 observed vs 8.2 expected, SMR 243.0 (95% CI 148.4 to 375.3). Both were significantly increased. Other selected causes were not significantly increased, including all causes (161 observed vs 146.1 expected, SMR 110.2).
  • Men with more than 20 years since hire: the lung cancer SMR was 84.7 (less than 50 fiber-years), 225.1 (50-99), 109.3 (100-399) and 671.3 (more than 399). It was statistically significant only in the more than 399 fiber-years group.
  • For workers with at least 20 years since hire, the linear models estimated a 0.6% increase in lung cancer mortality risk for each fiber-year of exposure.
  • At 5 fiber-years, the estimated increase in lung cancer risk was 0.6% from the survival model and 2.9% from the nonthreshold linear model.
  • No association was found between cumulative fiber exposure and deaths from nonmalignant respiratory disease, and the authors said conclusions about the exposure-response association were unclear. For more than 20 years since hire, the nonmalignant respiratory disease SMR was 327.8, 283.5, 0 and 278.4 across the four exposure groups, and was significant only in the less than 50 fiber-years group.
  • Two mesothelioma cases were observed, making up 1.2% (2/161) of all deaths. Both men were hired in 1946, had more than 300 fiber-years of exposure, and worked mostly in the dry mill.
• The authors' conclusion: The authors concluded that lung cancer mortality risk among workers with at least 20 years since hire increased by an estimated 0.6% per fiber-year. They added that the risk at low exposure (below 50 fiber-years) remained unclear.
• Limitations the authors state:
  • Smoking was not directly controlled. Current and former smokers combined made up 84.0% of a sample of 161 men employed from 1975 to 1982 with at least 5 years tenure, against 68.5% of U.S. white males in 1975. Current smoking alone was 35.0% in the worker sample versus 39.3% among U.S. white males in 1975.
  • The authors called their crude smoking adjustment imprecise and said estimates below 50 fiber-years may be confounded.
  • The linear exposure-response models were fitted to only four data points, so statistical tests on the coefficients are of limited value.
  • Results for nonmalignant respiratory disease were hard to interpret because few deaths were certified as asbestos-related. Fibrosis was listed in only 10 of the 20 cases.
  • Air samples were unavailable for some work areas before 1968, so a range of exposure estimates was derived, and the high estimate was used.
  • The choice of comparison population can bias results. Lung cancer SMRs were higher using Montana or regional county rates than using U.S. rates.

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, Fiber Year, Lung Cancer.

Related studies

• 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)
• The morbidity and mortality of vermiculite miners and millers exposed to tremolite-actinolite: Part I. Exposure estimates — H.E. Amandus, R. Wheeler, 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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