The classification of asbestos fibres by scanning electron microscopy and computer-digitizing tablet

Our plain-English summary of The classification of asbestos fibres by scanning electron microscopy and computer-digitizing tablet, by S.F. Platek, R.D. Riley, S.D. Simon (The Annals of Occupational Hygiene, 1992).

The study

• Authors: S.F. Platek, R.D. Riley, S.D. Simon
• Journal: The Annals of Occupational Hygiene
• Published: 1992;36(2):155-71
• Read it: PubMed · Publisher

Abstract

Because of the need to completely and accurately size asbestos bulk samples for toxicity studies, a method was developed to classify asbestos fibres using enlarged micrographs originally produced on the scanning electron microscope (SEM). Individual fibre length and width measurements were performed on a computer-assisted, digitizing tablet. This method, though time consuming, permitted the sizing of all fibres (length and width) and particles (area) in selected fields of view at SEM magnifications of x100 and x2500. Final enlargement of the micrographs permitted sizing magnifications of x10,000. Seven distinct asbestos samples were classified including five chrysotile and two crocidolite samples. Statistical analyses showed good interfilter fibre size correlation for all types of asbestos. In addition, it was determined that a representative sizing of all fibres and non-fibrous particles on a filter preparation could be performed using five sets of micrographs taken at a magnification of x2500 and enlarged to x10,000: an inverse square root transformation (-0.5 power) is needed to normalize the distributions of length and width.

In plain English

This laboratory study developed and tested a way to size asbestos fibres. The researchers, at the National Institute for Occupational Safety and Health, needed sized samples for a long-term (chronic) toxicity study. The authors photographed filter samples with a scanning electron microscope and enlarged the photographs. On a digitizing tablet, they measured each fibre's length and its width at the midpoint, and traced non-fibrous particles and compact fibre bundles to find their area. They measured seven samples: five chrysotile and two crocidolite. Differences between repeat filters of the same sample were small. In the NIEHS crocidolite, nearly 30% of fibres were longer than 5.0 micrometres, compared with 2.1% in the NIOSH crocidolite. Treating fibres as cylinders and particles as spheres, the authors calculated that fibres made up 48% (U.S.-modified chrysotile) to 99% (NIEHS crocidolite) of total volume. The authors concluded the method measured large numbers of fibres and particles relatively accurately. They noted it was time-consuming and could not tell asbestos from other fibrous materials. The study measured and counted fibres and particles; it did not report health effects.

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 methods-development study: scanning electron microscopy of filter preparations, with enlarged micrographs measured on a computer digitizing tablet
• Who was studied: Seven bulk asbestos samples, measured at the National Institute for Occupational Safety and Health (Cincinnati, Ohio, U.S.A.). Four independent filter samples were prepared from each bulk sample except the UICC chrysotile, for which one filter was used. Over 17,000 fibres were counted.
• Exposure: Five chrysotile samples: Canadian chrysotile base stock (CAB), a Canadian-modified version (CAM), a U.S.-modified version (USM), UICC chrysotile, and NIEHS chrysotile (NCRY). Two crocidolite samples: NIEHS crocidolite (NCRO) and NIOSH crocidolite (DCRO). Fibres were defined as particles with a length-to-width ratio greater than 3:1.
• Main results:
  • Fibre-length variability from filter to filter was small. The variance component for filters was almost two orders of magnitude smaller than that for individual fibres, and the statistically detected difference between filters was judged too small to matter in practice.
  • A composite distribution that combined low-magnification (x100) and high-magnification (x2500) fibres was very similar to the high-magnification distribution alone. The authors took this to indicate little or no bias in the selection of large fibres.
  • NIEHS crocidolite (NCRO) had a median length twice that of any other fibre type (2.92 micrometres composite, 2.89 high magnification only), and nearly 30% of its fibres (29.6% composite) were longer than 5.0 micrometres. NIOSH crocidolite (DCRO) had a median length of 1.02 micrometres, with 2.1% of fibres longer than 5.0 micrometres.
  • In all seven asbestos types, the median circular area equivalent diameter of non-fibrous particles was between 0.31 and 0.35 micrometres.
  • The authors calculated volumes by treating fibres as cylinders and non-fibrous particles as spheres. On that basis, fibre volume as a percentage of the total volume of fibres and non-fibrous particles ranged from 48% in the U.S.-modified chrysotile (USM) to 99% in the NIEHS crocidolite (NCRO).
  • The bivariate log-normal distribution did not fit the fibre size data well. An inverse square root (-0.5 power) transformation was needed to normalize fibre lengths and widths.
• The authors' conclusion: The authors concluded that their scanning electron microscopy method measures large numbers of asbestos fibres and non-fibrous particles relatively accurately and leaves a permanent record. They also concluded that it gave good filter-to-filter repeatability for homogeneous bulk samples.
• Limitations the authors state:
  • The method cannot tell asbestos apart from other fibrous materials.
  • The method is time-consuming.
  • The x100 micrographs resolved too few short fibres and too few long, thin fibres to overlap much with the high-magnification micrographs, which made any selection bias hard to identify and quantify.
  • The 90th and 99th percentiles were slightly longer in the composite distribution, suggesting that the selection process may not include enough of the large fibres.
  • The size cut-offs used to combine low- and high-magnification data were somewhat arbitrary.
  • Volumes rested on shape assumptions rather than direct measurement.

Topics

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

Medical terms in this study, explained in our Lexicon: Scanning Electron Microscopy (SEM), Chrysotile, Crocidolite.

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