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

J Occup Environ Hyg. 2014;11(9):613-24. doi: 10.1080/15459624.2014.890287.


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.].

Keywords: NIOSH 7400; elongate mineral particles (EMP); size-based definitions for EMP exposures; taconite.

Publication types

  • Comparative Study
  • Evaluation Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Air Pollutants, Occupational / analysis*
  • Air Pollutants, Occupational / chemistry
  • Asbestos, Amphibole / analysis
  • Asbestos, Amphibole / chemistry
  • Environmental Monitoring / instrumentation
  • Environmental Monitoring / methods*
  • Humans
  • Inhalation Exposure / analysis*
  • Iron*
  • Linear Models
  • Mining*
  • Minnesota
  • Occupational Exposure / analysis*
  • Particle Size
  • Particulate Matter / analysis*
  • Particulate Matter / chemistry
  • Silicates*


  • Air Pollutants, Occupational
  • Asbestos, Amphibole
  • Particulate Matter
  • Silicates
  • taconite
  • Iron