Mortality experience among Minnesota taconite mining industry workers

Our plain-English summary of Mortality experience among Minnesota taconite mining industry workers, by E.M. Allen, B.H. Alexander, R.F. MacLehose, et al (Occupational and Environmental Medicine, 2014).

The study

• Authors: E.M. Allen, B.H. Alexander, R.F. MacLehose, G. Ramachandran, J.H. Mandel
• Journal: Occupational and Environmental Medicine
• Published: 2014;71(11):744-9
• Cited by: 20 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Objective: To evaluate the mortality experience of Minnesota taconite mining industry workers.

Methods: Mortality was evaluated between 1960 and 2010 in a cohort of Minnesota taconite mining workers employed by any of the seven companies in operation in 1983. Standardised mortality ratios (SMR) were estimated by comparing observed deaths in the cohort with expected frequencies in the Minnesota population. Standardised rate ratios (SRR) were estimated using an internal analysis to compare mortality by employment duration.

Results: The cohort included 31,067 workers with at least 1 year of documented employment. Among those, there were 9094 deaths, of which 949 were from lung cancer, and 30 from mesothelioma. Mortality from all causes was greater than expected in the Minnesota population (SMR=1.04, 95% CI 1.02 to 1.04). Mortality from lung cancer and mesothelioma was higher than expected with SMRs of 1.16 for lung cancer (95% CI 1.09 to 1.23) and 2.77 for mesothelioma (95% CI 1.87 to 3.96). Other elevated SMRs included those for cardiovascular disease (SMR=1.10, 95% CI 1.06 to 1.14), specifically for hypertensive heart disease (SMR=1.81, 95% CI 1.39 to 2.33) and ischemic heart disease (SMR=1.11, 95% CI 1.07 to 1.16). Results of the SRR analysis did not show variation in risk by duration of employment.

Conclusions: This study provides evidence that taconite workers may be at increased risk for mortality from lung cancer, mesothelioma, and some cardiovascular disease. Occupational exposures during taconite mining operations may be associated with these increased risks, but non-occupational exposures may also be important contributors.

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In plain English

This mortality study followed 31 067 Minnesota taconite mining workers born in 1920 or later. Each had at least 1 documented year of employment with one of the seven companies operating in 1983. Deaths from 1960 through 2010 were compared with those expected from Minnesota population rates. A standardised mortality ratio (SMR) above 1 means more deaths than expected. Taconite mining can expose workers to non-asbestiform amphibole and non-amphibole elongate mineral particles, silica and cleavage fragments. Length of employment was the exposure measure. Of 9094 deaths, 949 were from lung cancer and 30 from mesothelioma. Deaths were higher than expected from all causes (SMR 1.04, 95% CI 1.02 to 1.06), lung cancer (SMR 1.16, 95% CI 1.09 to 1.24), mesothelioma (SMR 2.77, 95% CI 1.87 to 3.96), hypertensive heart disease (SMR 1.81, 95% CI 1.39 to 2.33) and ischemic heart disease (SMR 1.11, 95% CI 1.07 to 1.16). Risk did not appear to rise with longer employment. The authors concluded workers may be at increased risk but, given their limited exposure assessment, could not say for sure what role workplace exposures played.

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. Deaths were compared with Minnesota population rates (standardised mortality ratios), and an internal comparison was made by duration of employment (standardised rate ratios).
• Who was studied: 31 067 Minnesota taconite mining industry workers. All were born in 1920 or later and had at least 1 year of documented employment with any of the seven mining companies operating in 1983. They were followed for deaths from 1960 through 2010. The cohort was 93% male and contributed 1 152 966 person-years.
• Exposure: Work in taconite iron ore mining and processing. The article describes potential exposure to non-asbestiform amphibole and non-amphibole elongate mineral particles (EMP), respirable silica, quartz, dust and cleavage fragments. The eastern zone 4 ore contains iron-rich amphibole EMPs, primarily cummingtonite-grunerite. No exposure measurements were used: duration of employment was the exposure measure.
• Main results:
  • The cohort had 9094 deaths. Of these, 949 were from lung cancer and 30 from mesothelioma.
  • In the whole cohort, deaths from all causes were higher than expected compared with the Minnesota population: SMR 1.04, 95% CI 1.02 to 1.06.
  • In the whole cohort, lung cancer deaths were higher than expected (SMR 1.16, 95% CI 1.09 to 1.24). Mesothelioma deaths were also higher than expected (SMR 2.77, 95% CI 1.87 to 3.96).
  • In the whole cohort, the following were elevated:
• cardiovascular disease deaths: SMR 1.10, 95% CI 1.06 to 1.14
• hypertensive heart disease: SMR 1.81, 95% CI 1.39 to 2.33
• ischemic heart disease: SMR 1.11, 95% CI 1.07 to 1.16
  • Only one death each from asbestosis and from silicosis was observed in the cohort.
  • Among the 20 282 workers who ever worked in zone 1:
• mesothelioma SMR 1.85, 95% CI 0.98 to 3.16
• lung cancer SMR 1.18, 95% CI 1.09 to 1.27
  • Among the 5580 workers who ever worked in zone 2:
• mesothelioma SMR 7.38, 95% CI 4.30 to 11.82
• lung cancer SMR 1.43, 95% CI 1.26 to 1.63
  • Among the 6501 workers who ever worked in zone 4:
• mesothelioma SMR 3.17, 95% CI 1.37 to 6.25
• lung cancer SMR 1.23, 95% CI 1.07 to 1.40
  • In the internal comparison by length of employment, the reference group was workers with 2 to 5 years. Rate ratios for hypertensive heart disease were elevated but imprecise for 6 to 14 years (SRR 1.29, 95% CI 0.63 to 2.63) and 15 or more years (SRR 1.84, 95% CI 0.82 to 4.11). Duration of employment did not appear to be associated with mortality risk.
• The authors' conclusion: The authors concluded that the study provides evidence that taconite workers may be at increased risk of death from lung cancer, mesothelioma and some cardiovascular disease. They stated that occupational exposures may be associated with these risks, but non-occupational exposures may also contribute.
• Limitations the authors state:
  • Duration of employment was used as a stand-in for exposure instead of specific exposure measurements.
  • Employment duration was calculated as last date minus start date and ignored gaps in work history, which could misclassify time worked.
  • No smoking data were available. The authors said smoking may explain at least some of the excess lung cancer, though smoking is not a risk factor for mesothelioma.
  • Mesothelioma risk may be underestimated because a specific code for the disease was not available until 1999.
  • Only underlying causes of death were counted, not new (incident) cases or contributing causes, so total disease burden is not reflected.
  • The zone analysis counted any work in a zone. It does not allow risks to be compared between zones.
  • Because exposure was assessed in a limited way, the authors could not say for sure what role actual workplace exposures played in the excess deaths.

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: Taconite, rate ratio, Lung Cancer, Mesothelioma, Occupational Exposure, Non-Occupational Exposure.

Related studies

• Cancer incidence among Minnesota taconite mining industry workers — E.M. Allen, B.H. Alexander, et al. (2015)
• Occupational exposures and lung cancer risk among Minnesota taconite mining workers — E.M. Allen, B.H. Alexander, et al. (2015)
• Comprehensive assessment of exposures to elongate mineral particles in the taconite mining industry — J. Hwang, G. Ramachandran, et al. (2013)
• 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)
• A case-control study of mesothelioma in Minnesota iron ore (taconite) miners — C.S. Lambert, B.H. Alexander, et al. (2016)
• The search for asbestos within the Peter Mitchell Taconite iron ore mine, near Babbitt, Minnesota — M. Ross, R.P. Nolan, et al. (2008)

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