Occupational exposures and lung cancer risk among Minnesota taconite mining workers

Our plain-English summary of Occupational exposures and lung cancer risk among Minnesota taconite mining workers, by E.M. Allen, B.H. Alexander, R.F. MacLehose, et al (Occupational and Environmental Medicine, 2015).

The study

• Authors: E.M. Allen, B.H. Alexander, R.F. MacLehose, H.H. Nelson, A.D. Ryan, G. Ramachandran, J.H. Mandel
• Journal: Occupational and Environmental Medicine
• Published: 2015;72(9):633-9
• Cited by: 16 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objective: To examine the association between employment duration, elongate mineral particle (EMP) exposure, silica exposure and the risk of lung cancer in the taconite mining industry.

Methods: We conducted a nested case-control study of lung cancer within a cohort of Minnesota taconite iron mining workers employed by any of the mining companies in operation in 1983. Lung cancer cases were identified by vital records and cancer registry data through 2010. Two age-matched controls were selected from risk sets of cohort members alive and lung cancer free at the time of case diagnosis. Calendar time-specific exposure estimates were made for every job and were used to estimate workers' cumulative exposures. ORs and 95% CIs were estimated using conditional logistic regression. We evaluated total lung cancer risk and risk of histological subtype by total work duration and by cumulative EMP, and silica exposure by quartile of the exposure distribution.

Results: A total of 1706 cases and 3381 controls were included in the analysis. After adjusting for work in haematite mining, asbestos exposure and sex, the OR for total duration of employment was 0.99 (95% CI 0.96 to 1.01). The ORs for quartile 4 versus 1 of EMP and silica exposure were 0.82 (95% CI 0.57 to 1.19) and 0.97 (95% CI 0.70 to 1.35), respectively. The risk of each histological subtype of lung cancer did not change with increasing exposure.

Conclusions: This study suggests that the estimated taconite mining exposures do not increase the risk of developing lung cancer.

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In plain English

This nested case-control study examined lung cancer in 46,170 Minnesota mining industry workers born in 1920 or later. It compared 1706 lung cancer cases, found from death and cancer records to 2010, with 3381 age-matched controls; 738 cases and 1532 controls had worked only in haematite, not taconite, mining. From work records and air measurements and models, the authors estimated each worker's taconite years and cumulative exposure to elongate mineral particles (EMPs, mostly non-asbestiform) and respirable silica. Analyses adjusted for haematite mining, years in jobs likely to involve commercial asbestos, and sex. For taconite employment duration the odds ratio was 0.99 (95% CI 0.96 to 1.01). Comparing top with bottom exposure quartiles, odds ratios were 0.82 (95% CI 0.57 to 1.19) for EMPs and 0.97 (95% CI 0.70 to 1.35) for silica. No subtype showed a statistically significant association, though silica odds ratios rose imprecisely for squamous cell and non-specified cancers. The authors concluded that non-asbestiform EMPs and silica were not lung cancer risk factors in these miners. They noted likely exposure misclassification and lacked smoking data for the study population.

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 Minnesota iron mining industry workers (part of the Taconite Workers Health Study), analysed with conditional logistic regression
• Who was studied: A cohort established in 1983 of individuals with any employment in the Minnesota mining industry, limited to those born in 1920 or later (46 170 individuals). Not all had worked in taconite mining: 738 of the 1706 cases and 1532 of the 3381 controls had worked only in haematite mining. Lung cancer cases were identified through death records (1960 to 2010) and the Minnesota cancer registry (1988 to 2010). Each case had two age-matched controls. The final analysis included 1706 lung cancer cases and 3381 controls, mostly male (96% of cases, 94% of controls).
• Exposure: Three measures of taconite mining exposure:
• years of employment in taconite mining;
• cumulative exposure to elongate mineral particles (non-asbestiform amphibole and non-amphibole EMPs), in (EMP/cc)×years;
• cumulative respirable silica exposure, in (mg/m3)×years.

Exposures were estimated from a job-exposure matrix built on 2010 to 2011 personal air sampling and historical monitoring data. Haematite mining years and years in job groups with a high probability of commercial asbestos exposure were included as adjustment factors.

• Main results:
  • By ore type, the 1706 cases were:
• 668 taconite only;
• 738 haematite only;
• 300 both.

The 3381 controls were 1239 taconite only, 1532 haematite only and 610 both.

• Total duration of taconite employment was not associated with increased lung cancer risk: OR 0.99 (95% CI 0.96 to 1.01). The estimate was adjusted for haematite exposure, silica exposure, asbestos exposure and sex.
• Compared with the lowest exposure quartile (quartile 1), the OR for the highest quartile (quartile 4) was:
• 0.82 (95% CI 0.57 to 1.19) for cumulative EMP exposure;
• 0.97 (95% CI 0.70 to 1.35) for cumulative silica exposure.
  • An inverse association between risk and exposure was seen across quartiles for EMP and silica. The article states that none of the quartiles showed a significant increase in risk.
  • Workers with no taconite exposure (those who worked only in haematite production) had a lower lung cancer risk than quartile 1. The ORs were:
• 0.81 (95% CI 0.67 to 0.98) in the EMP analysis;
• 0.81 (95% CI 0.68 to 0.98) in the silica analysis.
  • Lung cancer risk did not appear to change by employment duration or cumulative EMP exposure in any zone of the iron range (zones 1, 2 and 4).
  • The subtype analysis used the 973 cases identified by the cancer registry. No statistically significant association was found between EMP or silica quartiles and any of five subtypes: squamous cell, adenocarcinoma, small cell, non-specified and other. The ORs were greatest for squamous cell and non-specified carcinomas, but none of these was statistically significant and most were imprecise.
  • For silica, the subtype ORs rose across quartiles 2 to 4, each compared with quartile 1:
• squamous cell: 1.11, 1.25 and 1.28 (quartile 4 95% CI 0.73 to 2.24);
• non-specified carcinoma: 1.15, 1.57 and 1.72 (quartile 4 95% CI 0.88 to 3.36).
  • The results did not change substantially when the analyses were repeated with 10-year and 20-year lags. They also did not change substantially when restricted to the 1397 cases identified by death certificate.
• The authors' conclusion: The authors concluded that exposure to non-asbestiform EMPs and to silica were not risk factors for developing lung cancer in this population of taconite miners. They also suggested that the excess lung cancer previously seen in this cohort may be related to non-occupational exposures, specifically smoking.
• Limitations the authors state:
  • Exposure misclassification was likely, because exposure measurements were sparse for some time periods and some job groups.
  • Historical exposure levels were estimated by imputation and regression modelling. The results depend on the assumptions of that reconstruction, such as a linear trend over time.
  • Selection bias was possible because cancer-specific mortality could not be ascertained before 1988.
  • Job groups may have been assigned incorrectly from work records, which varied in quality by mine. This could have masked a significant finding by zone.
  • There was no smoking information for the study population. Smoking was assessed only indirectly, through a 2010 survey of 1186 workers.
  • The smoking survey participants had to be alive in 2010 and were volunteers, so they were subject to selection bias. They also had greater cumulative exposure than the study population.
  • Commercial asbestos use was likely but poorly documented. There were no quantitative data on asbestos type or exposure level, so the asbestos adjustment may be misclassified.
  • Exposure durations in this population may not have been long enough to cause lung cancer.

Topics

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

Medical terms in this study, explained in our Lexicon: Occupational Exposure, Lung Cancer, Taconite, elongate mineral particles (EMPs), nested case-control study, Cancer Registry, Cumulative Exposure, Histological Subtype.

Related studies

• A case-control study of mesothelioma in Minnesota iron ore (taconite) miners — C.S. Lambert, B.H. Alexander, et al. (2016)
• Cancer incidence among Minnesota taconite mining industry workers — E.M. Allen, B.H. Alexander, et al. (2015)
• Mortality experience among Minnesota taconite mining industry workers — E.M. Allen, B.H. Alexander, et al. (2014)
• 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)
• Risk assessment due to environmental exposures to fibrous particulates associated with taconite ore — R. Wilson, E.E. McConnell, et al. (2008)

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