Fiber length of some selected asbestos samples

Our plain-English summary of Fiber length of some selected asbestos samples, by A.G. Wylie (1979).

The study

• Author: A.G. Wylie
• Published: Annals of the New York Academy of Sciences p605-610 (1979)
• Type of article: Journal article
• Our copy: from Johns-Manville's own medical library. The article is still under copyright, so we describe it here rather than post it.

In plain English

This laboratory study measured asbestos particle size and shape. It did not study people or disease. Using a scanning electron microscope, the author measured 1200 to 2000 particles from each of four commercial samples: short-fiber chrysotile from California, long-fiber chrysotile from Quebec, amosite from Africa (about 95% grunerite asbestos and 5% actinolite asbestos), and crocidolite from Africa. In every sample, aspect ratio (length divided by width) rose with length in a straight-line pattern on logarithmic scales. Width stayed about constant with length in both chrysotiles. It rose slightly in crocidolite and more in amosite. More than 96% of measured particles longer than 5 micrometers had aspect ratios above 10:1, and many exceeded 50:1. The author proposed the slope of this pattern as a possible "fibrosity index" needing further testing. The author concluded that the usual 3:1 aspect ratio for defining a fiber is not justified on mineralogic grounds. Once asbestos is known to be present, the author stated, counting only particles with aspect ratios above 20:1 would include most asbestos particles longer than 5 micrometers. Aspect ratio alone cannot identify asbestos.

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 mineralogical study: size and shape measurements of asbestos samples by scanning electron microscope, with regression analysis
• Who was studied: Four commercial asbestos samples: short-fiber chrysotile from the New Idria Serpentinite Body, California (COF-25); long-fiber chrysotile from the Jeffrey Mine, Asbestos, Quebec (Plastibest 20); amosite from Africa (S-33); crocidolite from Africa (ML 6). Each mineral had 1200 to 2000 particle measurements from 12 to 20 separate preparations.
• Exposure: Not a human exposure study. The fiber types examined were chrysotile (two samples, not milled), amosite (about 95% grunerite asbestos and 5% actinolite asbestos) and crocidolite. The amosite and crocidolite samples were air-jet milled.
• Main results:
  • In all four samples, the log of aspect ratio was linearly related to the log of length. The linear models explained 97.1% of the variation for long-fiber chrysotile, 98.2% for short-fiber chrysotile, 99.2% for crocidolite and 98.9% for amosite.
  • For both chrysotile samples, the slope M was close to 1 (1.01 and 0.99), meaning width stayed about constant across lengths. Width increased slightly with length for crocidolite (M = 0.88) and more for amosite (M = 0.77).
  • More than 96% of all measured particles longer than 5 micrometers had aspect ratios above 10:1, and many had aspect ratios above 50:1.
  • For particles shorter than 1 micrometer, the predicted aspect ratios approached those typical of many nonfibrous silicates (less than 5:1).
  • Preliminary results from a separate study of massive tremolite suggested that M values for nonfibrous amphiboles are less than 0.5.
• The authors' conclusion: The author concluded that the 3:1 aspect ratio used to define a fiber is not justified on mineralogic grounds and should be abandoned. Once asbestos presence is established, counting only particles with aspect ratios above 20:1 would include most asbestos particles longer than 5 micrometers and probably exclude most nonfibrous fragments. The slope M was proposed as a possible "fibrosity index."
• Limitations the authors state:
  • Aspect ratio cannot be the only criterion for identifying asbestos. A single particle with a high aspect ratio is not necessarily an asbestos fiber.
  • The fibrosity index model needs further testing before it can be considered a reliable measure.
  • More work is needed to describe the shape and size of the fibrils more accurately.
  • Using a 10:1 cut-off instead of 20:1 might lead to some nonfibrous cleavage fragments being counted as asbestos.
  • In most cases the width measured was the intermediate dimension of the particle, not the smallest.
  • All bulk samples contained small but significant mineral impurities. X-ray analysis was used to check particles with low aspect ratios.

Topics

Medical terms in this study, explained in our Lexicon: Fiber Length, Chrysotile, Amosite, Grunerite, Actinolite, Crocidolite, Aspect Ratio (Length-to-diameter ratio).

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