A case-control study of mesothelioma in Minnesota iron ore (taconite) miners

Our plain-English summary of A case-control study of mesothelioma in Minnesota iron ore (taconite) miners, by C.S. Lambert, B.H. Alexander, G. Ramachandran, et al (Occupational and Environmental Medicine, 2016).

The study

• Authors: C.S. Lambert, B.H. Alexander, G. Ramachandran, R.F. MacLehose, H.H. Nelson, A.D. Ryan, J.H. Mandel
• Journal: Occupational and Environmental Medicine
• Published: 2016;73(2):103-9
• Cited by: 22 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objectives: An excess of mesothelioma has been observed in iron ore miners in Northeastern Minnesota. Mining and processing of taconite iron ore generate exposures that include elongate mineral particles (EMPs) of amphibole and non-amphibole origin. We conducted a nested case-control study of mesothelioma in a cohort of 68,737 iron ore miners (haematite and taconite ore miners) to evaluate the association between mesothelioma, employment and EMP exposures from taconite mining.

Methods: Mesothelioma cases (N=80) were identified through the Minnesota Cancer Surveillance System (MCSS) and death certificates. Four controls of similar age were selected for each case with 315 controls ultimately eligible for inclusion. Mesothelioma risk was evaluated by estimating rate ratios and 95% CIs with conditional logistic regression in relation to duration of taconite industry employment and cumulative EMP exposure [(EMP/cc)×years], defined by the National Institute for Occupational Safety and Health (NIOSH) 7400 method. Models were adjusted for employment in haematite mining and potential exposure to commercial asbestos products used in the industry.

Results: All mesothelioma cases were male and 57 of the cases had work experience in the taconite industry. Mesothelioma was associated with the number of years employed in the taconite industry (RR=1.03, 95% CI 1.00 to 1.06) and cumulative EMP exposure (RR=1.10, 95% CI 0.97 to -1.24). No association was observed with employment in haematite mining.

Conclusions: These results support an association between mesothelioma and employment duration and possibly EMP exposure in taconite mining and processing. The type of EMP was not determined. The potential role of commercial asbestos cannot be entirely ruled out.

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In plain English

This nested case-control study examined mesothelioma in a cohort of 68,737 people employed in northeastern Minnesota iron ore mining before 1982. It compared 80 mesothelioma cases, all pleural and all in men, with 315 controls. Exposure to elongate mineral particles (EMPs) longer than 5 micrometres was estimated from work histories and air measurements. Employment-duration results were adjusted for age and haematite mining. Exposure results were also adjusted for possible commercial asbestos exposure. Each year of taconite employment was associated with a rate ratio of 1.03 (95% CI 1.00 to 1.06). Each additional (EMP/cc)×year of exposure gave a rate ratio of 1.10 (95% CI 0.97 to 1.24), which was not statistically significant. Workers in the high exposure category, split at the controls' median of 0.40 (EMP/cc)×years, had a mesothelioma rate 2.25 times that of the low category (95% CI 1.13 to 4.50). Risk rose with employment in Zones 1 and 2 but not Zone 4. No association was seen with haematite mining. The authors concluded that the results support an association with employment duration and possibly EMP exposure. They said commercial asbestos could account for the association.

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: Nested case-control study within a cohort of iron mining workers
• Who was studied: Cohort of 68,737 people employed in iron ore mining in northeastern Minnesota sometime before 1982 (the MRHAP cohort), from which mesothelioma cases and age-similar controls were drawn
• Exposure: Years of employment in taconite mining, and cumulative exposure to elongate mineral particles (EMPs) measured in (EMP/cc)×years. EMPs were counted by the NIOSH 7400 method: longer than 5 micrometres, with an aspect ratio of 3:1 or more. Haematite mining employment and years in jobs with a high commercial asbestos score were considered as other exposures.
• Main results:
  • 80 cases and 315 controls were included. All cases were pleural mesothelioma.
  • All cases and 95% of controls were male
  • The taconite employment-duration estimates were adjusted for age and years of haematite employment
  • The cumulative EMP exposure estimates were adjusted for age, haematite employment, and potential for commercial asbestos exposure
  • Mesothelioma risk was associated with years of taconite employment: rate ratio 1.03 per year (95% CI 1.00 to 1.06)
  • Each additional (EMP/cc)×year of exposure was associated with an elevated but non-significant rate ratio of 1.10 (95% CI 0.97 to 1.24)
  • Cumulative exposure was split into high and low categories at the controls' median. Table 3 gives the low category as below 0.40 (EMP/cc)×years and the high category as 0.40 or more. Workers in the high category had a mesothelioma rate about 2.25 times that of workers in the low category (95% CI 1.13 to 4.50); there were 40 cases and 92 controls in the high category and 17 cases and 92 controls in the low category.
  • Risk increased with duration of employment in Zone 1 and Zone 2 but was not associated with employment duration in Zone 4
  • No association was observed between employment in haematite mining and mesothelioma risk
  • No asbestiform amphibole EMPs were found in the onsite air samples
• The authors' conclusion: The authors concluded that the results support an association between mesothelioma and employment duration, and possibly EMP exposure, in taconite mining and processing. The type of EMP was not determined, and the potential role of commercial asbestos cannot be entirely ruled out.
• Limitations the authors state:
  • The NIOSH 7400 definition does not distinguish amphibole from non-amphibole EMPs or asbestiform from non-asbestiform EMPs
  • Counts under different EMP definitions were highly correlated, so the independent effects of each definition could not be estimated
  • No quantitative data existed on commercial asbestos exposure, so a qualitative job-based scale was used instead
  • The commercial asbestos classifications likely did not capture all past asbestos exposure, and commercial asbestos exposure could account for the association observed
  • Historical exposure data were sparse in the early years, which could have caused exposure misclassification
  • Cases may have been missed among cohort members who lived outside Minnesota and died before 1999
  • The zone pattern did not match estimated exposure levels, which could suggest uncontrolled confounding
  • The facility with the highest mesothelioma rate had closed, and no exposure estimates were determined for it
  • Smoking information was not available

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer, Mesothelioma.

Medical terms in this study, explained in our Lexicon: Case-Control Study, Mesothelioma, Taconite, elongate mineral particles (EMPs), Amphibole Group, nested case-control study, Death Certificate, rate ratio.

Related studies

• Occupational exposures and lung cancer risk among Minnesota taconite mining workers — E.M. Allen, B.H. Alexander, et al. (2015)
• Cancer incidence among Minnesota taconite mining industry workers — E.M. Allen, B.H. Alexander, et al. (2015)
• The relationship between various exposure metrics for elongate mineral particles (EMP) in the taconite mining and processing industry — J. Hwang, G. Ramachandran, et al. (2014)
• Mortality experience among Minnesota taconite mining industry workers — E.M. Allen, B.H. Alexander, et al. (2014)
• The search for asbestos within the Peter Mitchell Taconite iron ore mine, near Babbitt, Minnesota — M. Ross, R.P. Nolan, et al. (2008)
• Mesothelioma in iron ore (taconite) miners — D. Mirabelli (2016)

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