The relationship between various exposure metrics for elongate mineral particles (EMP) in the taconite mining and processing industry

Our plain-English summary of The relationship between various exposure metrics for elongate mineral particles (EMP) in the taconite mining and processing industry, by J. Hwang, G. Ramachandran, P.C. Raynor, et al (Journal of Occupational and Environmental Hygiene, 2014).

The study

• Authors: J. Hwang, G. Ramachandran, P.C. Raynor, B.H. Alexander, J.H. Mandel
• Journal: Journal of Occupational and Environmental Hygiene
• Published: 2014;11(9):613-24
• Cited by: 12 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Different dimensions of elongate mineral particles (EMP) have been proposed as being relevant to respiratory health end-points such as mesothelioma and lung cancer. In this article, a methodology for converting personal EMP exposures measured using the National Institute for Occupational Safety and Health (NIOSH) 7400/7402 methods to exposures based on other size-based definitions has been proposed and illustrated. Area monitoring for EMP in the taconite mines in Minnesota's Mesabi Iron Range was conducted using a Micro Orifice Uniform Deposit Impactor (MOUDI) size-fractionating sampler. EMP on stages of the MOUDI were counted and sized according to each EMP definition using an indirect-transfer transmission electron microscopy (ISO Method 13794). EMP were identified using energy-dispersive x-ray and electron diffraction analysis. Conversion factors between the EMP counts based on different definitions were estimated using (1) a linear regression model across all locations and (2) a location-specific ratio of the count based on each EMP definition to the NIOSH 7400/7402 count. The highest fractions of EMP concentrations were found for EMP that were 1-3 μm in length and 0.2-0.5 μm in width. Therefore, the current standard NIOSH Method 7400, which only counts EMP >5 μm in length and ≥ 3 in aspect ratio, may underestimate amphibole EMP exposures. At the same time, there was a high degree of correlation between the exposures estimated according to the different size-based metrics. Therefore, the various dimensional definitions probably do not result in different dose-response relationships in epidemiological analyses. Given the high degree of correlation between the various metrics, a result consistent with prior research, a more reasonable metric might be the measurement of all EMP irrespective of size. [Supplementary materials are available for this article. Go to the publisher's online edition of Journal of Occupational and Environmental Hygiene for the following free supplemental resource: figures detailing EMP concentration.].

In plain English

This study measured airborne elongate mineral particles (EMP) in Minnesota's six operating taconite mines, one in the eastern geological zone of the Mesabi Iron Range and five in the western zone. It did not study workers' health. The authors took 4-hour area air samples, sized the particles by electron microscopy, and compared four size-based particle definitions, including the standard NIOSH 7400 count of particles longer than 5 μm. Only five chrysotile particles were found among 2931 identified particles, and these were excluded. The amphiboles found were amosite/cummingtonite-grunerite and actinolite. Concentrations were much higher in the eastern zone. There, particles 1–3 μm long and 0.2–0.5 μm wide made up the largest share at every location, and results under the different definitions were highly correlated (R2 0.90–0.98 for total EMP and 0.84–0.99 for amphibole EMP). Most correlations in the western zone were low. The authors concluded that NIOSH 7400 counts are much lower than counts of shorter particles and that measuring all EMP regardless of size might be more reasonable. The microscopy could not distinguish asbestiform from non-asbestiform particles.

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: Industrial hygiene exposure assessment using area air sampling with a size-separating impactor and transmission electron microscopy (ISO 13794), carried out as part of a larger epidemiological study. No health outcomes were studied in this article.
• Who was studied: Area air samples taken at locations representing similar exposure groups (SEGs) in the six currently operating taconite mines on Minnesota's Mesabi Iron Range. Eastern zone: mine A, 23 samples at 17 locations. Western zone: mine B, 14 samples at 14 locations; mine C, 20 at 16; mine D, 12 at 11; mine E, 15 at 15; mine F, 8 at 8. Sampling dates are not given.
• Exposure: Airborne elongate mineral particles (EMP), classified as amphibole or non-amphibole/non-chrysotile. The amphibole types found were amosite/cummingtonite-grunerite and actinolite asbestiform/actinolite. Particles were counted under four size definitions: NIOSH (length >5 μm, aspect ratio of 3 or more); Suzuki et al. (width 0.25 μm or less, length 5 μm or less); Chatfield (width between 0.04 and 1.5 μm, aspect ratio between 20 and 1000); and cleavage fragment (aspect ratio 20 or less). Samples ran about 4 hours at about 10 L/min.
• Main results:
  • Only five chrysotile EMP were found among 2931 identified EMP, and they were excluded from analysis.
  • Total and amphibole EMP concentrations were much higher in the eastern zone (mine A) than in the western zone.
  • At every eastern-zone location, particles 1–3 μm long and 0.2–0.5 μm wide made up the highest fraction of total EMP.
  • In the eastern zone, concentrations were much lower under the NIOSH and Chatfield definitions than under the Suzuki definition, which counts shorter EMP.
  • The authors stated that EMP counts under the standard NIOSH 7400 method were much lower than counts based on shorter EMP.
  • In the eastern zone, log-transformed concentrations under the different definitions were highly correlated: R2 0.90–0.98 for total EMP and 0.84–0.99 for amphibole EMP.
  • In the western zone, correlations between definitions for total EMP were low (R2 0.14 to 0.58). The exception was Suzuki versus cleavage fragment (R2 0.88).
  • Measured against the NIOSH recommended exposure limit of 0.1 particles/cm3 (used only as a benchmark, since these were 4-hour area samples), total EMP was above the limit at most eastern-zone locations and below it at all but one western-zone location.
  • Example conversion factor: for total EMP in the eastern zone, the Suzuki-definition concentration equalled 3.83 times the NIOSH concentration raised to the power 0.744 (95% CI for the exponent 0.543 to 0.945).
  • Location-by-location ratios to the NIOSH count were generally largest for the cleavage fragment definition, followed by the Suzuki and then the Chatfield definitions. The regression approach and the location-specific ratio approach gave similar results.
  • The authors inferred that, because the metrics were highly correlated, the separate effects of different particle sizes will not be identifiable in epidemiological analysis.
• The authors' conclusion: The authors concluded that, given the high correlation between the size-based metrics, measuring all EMP irrespective of size might be a more reasonable metric.
• Limitations the authors state:
  • The study used area measurements, with a limited number of samples for each location in each mine.
  • Personal samples could not each be analyzed with ISO 13794 because of budget limits.
  • Impactor stages 1, 2, 12 and 13 were not analyzed, which might lead to underestimated EMP exposures.
  • The concentrations are not personal exposure samples and do not necessarily mean that workers were exposed above the exposure limit.
  • Asbestiform and non-asbestiform amphibole EMP are chemically identical, so the TEM analysis could not tell them apart.
  • The laboratory's label 'amosite' can mean either amosite (asbestiform) or cummingtonite-grunerite (non-asbestiform).
  • The authors' expectation that most amphibole EMP in the Iron Range are non-asbestiform rests on past studies, not on this study's data.

Topics

Medical terms in this study, explained in our Lexicon: elongate mineral particles (EMPs), Taconite, Mesothelioma, Lung Cancer, NIOSH, Iron Range, Transmission Electron Microscopy (TEM), NIOSH Method 7400.

Related studies

• Comprehensive assessment of exposures to elongate mineral particles in the taconite mining industry — J. Hwang, G. Ramachandran, et al. (2013)
• A review of mortality associated with elongate mineral particle (EMP) exposure in occupational epidemiology studies of gold, talc, and taconite mining — J.H. Mandel, B.H. Alexander, et al. (2016)
• Cancer incidence among Minnesota taconite mining industry workers — E.M. Allen, B.H. Alexander, et al. (2015)
• Risk assessment due to environmental exposures to fibrous particulates associated with taconite ore — R. Wilson, E.E. McConnell, et al. (2008)
• Occupational exposures and lung cancer risk among Minnesota taconite mining 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)

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