An inter-laboratory study to determine the effectiveness of procedures for discriminating amphibole asbestos fibers from amphibole cleavage fragments in fiber counting by phase-contrast microscopy

Our plain-English summary of An inter-laboratory study to determine the effectiveness of procedures for discriminating amphibole asbestos fibers from amphibole cleavage fragments in fiber counting by phase-contrast microscopy, by M. Harper, E.G. Lee, J.E. Slaven, et al (The Annals of Occupational Hygiene, 2012).

The study

• Authors: M. Harper, E.G. Lee, J.E. Slaven, D.L. Bartley
• Journal: The Annals of Occupational Hygiene
• Published: 2012;56(6):645-59
• Cited by: 13 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The US Occupational Safety and Health Administration (OSHA) and Mine Safety and Health Administration do not regulate cleavage fragments of amphibole and serpentine minerals as asbestos, even when particles meet the dimensional criteria for counting under standard phase-contrast microscopy methods. The OSHA ID-160 method cautions that discriminatory counting is difficult and should not be attempted unless necessary and no procedure is provided for differentiation. A standard published by the American Society for Testing and Materials (ASTM International D7200-06) includes an attempt to codify a procedure but recognizes that the procedure should be validated in an inter-laboratory study. The US National Institute for Occupational Safety and Health has carried out such a study with multiple laboratories using slides made from riebeckite and crocidolite, grunerite and amosite, tremolite and tremolite asbestos, and actinolite and actinolite asbestos using two different measurement aids (graticules). The asbestos fibers had dimensions consistent with those reported for air samples from actual amphibole asbestos operations, and the cleavage fragments were also dimensionally consistent with those found in non-asbestos mining and milling operations. The procedure for discriminating asbestos fibers from other mineral particles in the ASTM Standard calls for the recognition of characteristics supposedly common to asbestos. For the asbestos fibers created in this study, these characteristics were found not to be common and generally a function of length. More importantly, different laboratories did not recognize these features consistently. Laboratories were much more consistent in measuring dimensions, but excessive overlap in the lengths of asbestos fibers and cleavage fragments rendered length a poor criterion for discrimination. The ASTM discrimination procedure as written could not be supported on the basis of this study. Width was a much more consistent parameter for distinguishing the asbestos and non-asbestos fibers in this study and inclusion of aspect ratio, while considered important by some researchers, did not refine the discrimination further. The ability of the majority of microscopists in this study to discriminate fibers and cleavage fragments through measurement of particle widths was determined and found to be within limits of uncertainty typical for air sampling measurements. A width criterion might be a very simple and useful aid where discrimination between asbestos and non-asbestos fibers in fiber counting by phase-contrast microscopy is required for further investigation. Recognition of asbestos features can also be retained as excessive recognition by some laboratories will lead to a conservative decision for additional investigation.

In plain English

This was an inter-laboratory study testing a procedure in ASTM standard D7200-06 for distinguishing amphibole asbestos fibers from cleavage fragments of the same minerals under phase-contrast microscopy. Slides were made from riebeckite and crocidolite, grunerite and amosite, tremolite and tremolite asbestos, and actinolite and actinolite asbestos, in mixtures from 0% to 100% asbestos; 11 laboratories took part. Recognition of the standard's 'Class 1' features (curvature, split ends, fibrillar bundling) ranged from 0 to 96% on asbestos-only slides and from less than 1% to 85% on slides with no asbestos, and shorter fibers were less likely to show these features. Asbestos fibers and cleavage fragments overlapped too much in length for length to separate them. Width separated them better in these laboratory samples: 0.84 micrometers gave the best overall separation and 1.0 micrometer the fewest asbestos fibers missed. The authors concluded the ASTM procedure as written was not effective across all laboratories. They noted the width value might be an artifact of sample preparation, that chrysotile was not tested, and that they did not suggest width as an indicator of toxicity.

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: Inter-laboratory study (ASTM ILS #0282) of a microscopy counting procedure, using prepared reference slides sent to participating laboratories in two parts
• Who was studied: Microscopists at 11 laboratories in total: nine laboratories in the first part (riebeckite/crocidolite and grunerite/amosite slides) and 10 in the second part (tremolite/tremolite asbestos and actinolite/actinolite asbestos slides); each slide had 100 marked particles meeting the fiber definition
• Exposure: Laboratory-prepared slides of amphibole asbestos (crocidolite, amosite, tremolite asbestos, actinolite asbestos) and cleavage fragments of the matching non-asbestos minerals (riebeckite, grunerite, tremolite, actinolite), at 100% asbestos, 100% cleavage fragments, and nominal 60:40 and 20:80 asbestos-to-fragment mixtures, at low and high loadings. Laboratories judged ASTM Class 1 features and measured length and width using the Walton–Beckett graticule, and in the first part also the RIB graticule.
• Main results:
  • Across laboratories, identification of Class 1 characteristics (curvature, split ends, fibrillar bundling) ranged from 0 to 96% for the asbestos-only slides and from less than 1% to 85% for the non-asbestos slides
  • For crocidolite, Class 1 recognition rates varied from 0 to 52% across laboratories
  • On 100% asbestos slides, shorter fibers were less likely to show Class 1 features
  • Laboratories #2 and #9 recognized a greater percentage of Class 1 features in the cleavage fragment slides than in the asbestos slides
  • Laboratories differed little in length measurements, but lengths of asbestos fibers and cleavage fragments overlapped, so length could not separate them
  • No laboratory's width measurements differed significantly from the mean of all laboratories, although laboratories #2 and #9 deviated by 15% from the group means
  • There was no significant difference between measurements made with the Walton–Beckett and RIB graticules; for laboratories #2 and #9, width ratios were closer to the group mean with the RIB graticule
  • With a 1 micrometer width criterion, 20% of cleavage fragments in this study would be thought of as asbestos; this dropped to almost zero with a 0.5 micrometer criterion
  • On the 100% asbestos slides, the proportion of particles meeting the 1 micrometer width criterion was 94% for crocidolite, 74% for amosite, 93% for tremolite asbestos and 95% for actinolite asbestos (all laboratories, low and high density slides combined)
  • A width criterion of 0.84 micrometers gave the best overall discrimination, while 1.0 micrometer gave the fewest false negatives for asbestos fibers
  • Adding an aspect ratio criterion to width gave almost the same percentages of fibers classed as asbestos as width alone
• The authors' conclusion: The authors concluded that the ASTM D7200 discrimination procedure was not effective across all participating laboratories for the materials prepared for this study. They concluded that a single width measurement between 0.5 and 1.0 micrometers gave effective discrimination in these laboratory samples, for deciding whether further analysis is needed. They did not suggest that any width parameter be used as an indication of toxicity.
• Limitations the authors state:
  • The nominal asbestos-to-cleavage-fragment ratios in the mixed slides could not be independently confirmed
  • Any width value chosen for discrimination may be an artifact of the laboratory sample preparation
  • Sample preparation may have reduced bundle size or the appearance of split ends
  • A single particle cannot be classified with confidence by width, because wide asbestos bundles and thin cleavage fragments both occur
  • The improvement seen with the RIB graticule may not last if it was only a psychological effect of the new graticule's novelty
  • Chrysotile asbestos and serpentine cleavage fragments were not tested

Topics

Medical terms in this study, explained in our Lexicon: Amphibole Asbestos, Amphibole Group, Cleavage Fragment, Fiber Counting, OSHA, Serpentine Group, Riebeckite, Crocidolite.

Related studies

• Evaluation of the dark-medium objective lens in counting asbestos fibers by phase-contrast microscopy — E.G. Lee, J.H. Nelson, et al. (2015)
• An evaluation of the risks of lung cancer and mesothelioma from exposure to amphibole cleavage fragments — J.F. Gamble, G.W. Gibbs (2008)
• Differential Counting of Asbestos Using Phase Contrast and Fluorescence Microscopy — T. Nishimura, M. Alexandrov, et al. (2016)
• Automated counting of airborne asbestos fibers by a high-throughput microscopy (HTM) method — M.O. Cho, S. Yoon, et al. (2011)
• Assessment of occupational exposure to asbestos fibers: Contribution of analytical transmission electron microscopy analysis and comparison with phase-contrast microscopy — C. Eypert-Blaison, A. Romero-Hariot, et al. (2018)
• The effect of a proposed change to fiber-counting rules in ASTM International Standard D7200-06 — M. Harper, E.G. Lee, et al. (2007)

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