A predictive model for determining asbestos concentrations for fibers less than five micrometers in length

Our plain-English summary of A predictive model for determining asbestos concentrations for fibers less than five micrometers in length, by M.J. Kiefer, R.M. Buchan, T.J. Keefe, et al (Environmental Research, 1987).

The study

• Authors: M.J. Kiefer, R.M. Buchan, T.J. Keefe, K.D. Blehm
• Journal: Environmental Research
• Published: 1987;43(1):31-8
• Cited by: 1 article, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The controversy of whether small asbestos fibers are biologically significant has not been resolved. The present standard method for evaluating asbestos fiber concentrations in workroom air excludes fibers less than 5 micron long even though it has been shown that small fiber concentrations dominate in a dust cloud. This research project was conducted to develop a mathematical model whereby one could predict small (less than 5 micron length) asbestos fiber concentration based on the fiber count concentration determined by phase contrast microscope analysis. Dry chrysotile asbestos was aerosolized into a chamber and sampled by membrane filtration. Segments from each filter were analyzed by both the NIOSH technique using phase contrast microscopy (PCM) and scanning electron microscopy (SEM) at 2000 X for fiber concentrations. A linear relationship was found to exist between the natural logarithm of the SEM-determined concentration and the natural logarithm of the PCM-determined concentration (r = 0.852). Using these data, a mathematical model was developed to predict SEM concentrations based on PCM counts. This model may have application in retrospective epidemiological studies for estimating small fiber exposure levels to determine if small fibers play a role in disease production. The greatest utility would be in those retrospective studies where the only exposure information available is based on PCM counts.

In plain English

This was a laboratory study of fiber counting methods. The standard method, phase contrast light microscopy (PCM), counts only fibers longer than 5 micrometers. The authors asked whether PCM counts could be used to estimate counts by scanning electron microscopy (SEM), which also detects shorter fibers. They blew 99% pure chrysotile asbestos into a 1-cubic-meter chamber and kept 30 air filter samples for analysis. Each filter was counted by both methods. The SEM count included all fibers, both longer and shorter than 5 micrometers. PCM concentrations were significantly lower than SEM concentrations, and the average SEM-to-PCM ratio was 2.98. The mean geometric mean fiber length was 3.87 micrometers by PCM and 0.75 micrometers by SEM. The logarithms of the two concentrations were strongly correlated (r = 0.852). From this, the authors derived an equation predicting SEM concentration from PCM concentration, although the prediction range was wide. The authors concluded that the model was a first step and a rough tool for estimating past small-fiber exposures. They noted that it was developed under ideal laboratory conditions and must be refined through field validation studies.

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: Laboratory chamber experiment comparing two microscopy methods on the same filters (PCM at 400x by NIOSH rules; SEM at 2000x counting all fibers) and fitting a regression model to predict SEM counts from PCM counts
• Who was studied: Air filter samples, not people: 32 collected in eight chamber runs of four samples each, 2 lost, leaving 30 analyzed; laboratory at Colorado State University
• Exposure: Dry chrysotile asbestos (99% pure), aerosolized in a 1.0-cubic-meter Plexiglas chamber at generated masses of 0.5, 1.0 and 2.0 mg, aiming for concentrations near the TLV region of 2 fibers/cc
• Main results:
  • Across the chamber samples, PCM fiber concentrations were significantly lower than SEM fiber concentrations (P < 0.001)
  • The average SEM-to-PCM fiber concentration ratio was 2.98
  • In the size-distribution analyses, the mean geometric mean fiber length was 3.87 micrometers by PCM and 0.75 micrometers by SEM (difference significant, P < 0.001)
  • The natural logarithms of SEM and PCM concentrations were strongly correlated (r = .852, P < 0.01)
  • The regression equation was ln SEM concentration = 1.25 + 0.703 (ln PCM concentration), which the authors rewrote as SEM concentration = 3.49 times PCM concentration raised to the power 0.703
  • The prediction range around estimated SEM concentrations was wide, although the authors described the linear relationship between the log-concentrations as strong
• The authors' conclusion: The authors concluded that the model is a first step and a rough tool for estimating past small-fiber exposures from existing PCM data. They also stated that PCM is at best a rough estimate of the true concentration.
• Limitations the authors state:
  • The model was developed under ideal laboratory conditions
  • The model must be refined through field validation studies before it can be relied on
  • Fiber size varies with fiber type and process, so a model is valid only for a particular asbestos type and process
  • Size-distribution data did not improve the model, possibly because of the limited amount and narrow range of those data

Topics

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

Medical terms in this study, explained in our Lexicon: Chrysotile, NIOSH, phase contrast microscopy (PCM), Scanning Electron Microscopy (SEM).

Related studies

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• Exposures from chrysotile-containing joint compound: evaluation of new model relating respirable dust to fiber concentrations — G.P. Brorby, P.J. Sheehan, et al. (2013)
• Forsterite exposure causes less oxidative DNA damage and lung injury than chrysotile exposure in rats — A. Takata, H. Yamauchi, et al. (2009)
• Asbestos fiber concentrations in the lungs of brake repair workers: commercial amphiboles levels are predictive of chrysotile levels — G.M. Marsh, A.O. Youk, et al. (2011)
• Asbestos Fiber Type and Length in Lungs of Chrysotile Textile and Production Workers: Fibers Longer than 18 μm — B.W. Case, A. Dufresne, et al. (2000)
• Standardized methods for preparation and bi-variate length & diameter counting/sizing of aerosol and tissue digestion fiber samples — D.M. Bernstein, P. Kunzendorf (2018)

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