Measurement of elongate mineral particles: What we should measure and how do we do it?

Our plain-English summary of Measurement of elongate mineral particles: What we should measure and how do we do it?, by E.J. Chatfield (Toxicology and Applied Pharmacology, 2018).

The study

• Author: E.J. Chatfield
• Journal: Toxicology and Applied Pharmacology
• Published: 2018;361:36-46
• Cited by: 15 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The length distributions of single fibrils of Coalinga, UICC-B and wet dispersed chrysotile were measured by transmission electron microscopy (TEM). It was found that the distributions significantly diverged above approximately 10 μm (μm) in length, corresponding to differences in published results of animal experiments. This result is in contrast to published data in which counting of an insufficient number of fibers resulted in an erroneous conclusion that the length distribution of Coalinga chrysotile fibrils was indistinguishable from those of other sources of chrysotile. The size distributions of the respirable particle size fractions from acknowledged tremolite asbestos samples were found to be dominated by elongate particles longer than 5 μm that are within the dimensional range of non-asbestiform amphiboles. Prior studies have shown that these elongate particles obscure a correlation between a specific size range of particles and results of animal implantation studies that used tremolite of various morphologies. In the prior studies, a reference protocol was developed from four crushed non-asbestiform amphiboles to differentiate the size range of amphibole particles that correlates with the mesothelioma frequencies observed in the animal studies. In the work reported here, this correlation was tested with TEM analyses of amphiboles from Libby, MT, Sparta, NJ and Homestake mine, Lead, SD, which represent known environmental/occupational situations. Further TEM analyses of the tremolite samples used in the original animal implantation studies have also shown that the numbers of elongate tremolite particles with lengths ≤5 μm implanted into the animals are not correlated with the observed mesothelioma frequencies.

Copyright © 2018. Published by Elsevier Inc.

In plain English

In this paper, the author reported laboratory measurements of mineral particles by transmission electron microscopy (TEM). No new animal or human studies were done. The length distribution of Coalinga chrysotile single fibrils, built from 52,226 fibrils, differed significantly from those of UICC-B and wet dispersed chrysotile only above about 10 µm. The author re-examined six tremolite samples from earlier animal implantation experiments by Davis et al. (1991) and compared particle counts with the mesothelioma frequencies those experiments reported. Particles 5 µm or shorter outnumbered longer ones by factors up to 84.5. Yet Ala di Stura tremolite delivered marginally fewer short particles than Shinness tremolite, while the reported frequencies were 66.7% and 5.6%. The author's earlier work had set a method that counts only particles more elongated than those of crushed non-asbestiform amphibole. Using it, the author predicted a risk for Libby, Montana amphibole somewhat higher than for Jamestown tremolite. For Sparta tremolite and Homestake grunerite, the predicted risk was of the same order as for the Shinness and Carr Brae tremolites. The author concluded that short particles had negligible impact on mesothelioma in the Davis experiments. The author noted that those experiments had no control series.

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 TEM measurement study of mineral samples, including re-examination of tremolite samples used in earlier animal implantation experiments (Davis et al., 1991). It compared these measurements with previously published animal and human findings and reported no new animal or human data.
• Who was studied: Mineral samples, not people or animals:
• three chrysotile sources: Coalinga (Calidria), UICC-B Canadian, and wet dispersed chrysotile;
• six tremolite samples from the Davis et al. (1991) animal studies: Shinness, Carr Brae, Ala di Stura, Swansea, Jamestown and Korea;
• amphiboles from the Libby, MT vermiculite mine (mainly richterite-winchite);
• tremolite from the Sparta, NJ marble quarry;
• grunerite from the former Homestake gold mine, Lead, SD.
• Exposure: What was measured:
• the lengths of chrysotile single fibrils;
• the lengths and widths of particles in the respirable fraction of crushed amphiboles, both longer than 5 µm and 5 µm or shorter.

The amphibole particles were judged against a reference size range for crushed non-asbestiform amphibole, developed in the author's earlier work (2010) from four prismatic amphiboles: two actinolite, one riebeckite and one grunerite. Particle numbers were expressed per 10 mg of respirable dust, the fixed dose implanted into each animal in the Davis et al. (1991) experiments.

• Main results:
  • The Coalinga chrysotile length distribution was based on a total of 52,226 fibrils.
  • No single fibrils longer than approximately 30.2 µm were found in Coalinga chrysotile. Fibrils longer than 30.2 µm were found in UICC-B and wet dispersed chrysotile.
  • The fibril length distributions of UICC-B and wet dispersed chrysotile began to diverge from Coalinga chrysotile above 5 µm. They diverged significantly only above approximately 10 µm.
  • In the respirable fraction of Jamestown tremolite asbestos, more than 66% of particles had dimensions within the range of non-asbestiform amphibole.
  • For the Shinness, Carr Brae and Ala di Stura tremolites, the number of particles within the non-asbestiform size range (sub-criteria particles) implanted per animal was roughly constant at about 4 x 10^7. Over these samples, the mesothelioma frequencies reported by Davis et al. (1991) ranged from 5.6% (Shinness) to 66.7% (Ala di Stura).
  • Libby richterite-winchite had 1.161 x 10^8 extra-criteria particles per 10 mg of respirable dust. Sparta tremolite had 1 extra-criteria particle out of 202 counted, and Homestake grunerite had 1 out of 210.
  • Plotted against the Davis et al. (1991) animal results:
• Libby richterite-winchite was predicted to be a somewhat higher risk than Jamestown tremolite.
• Homestake grunerite and Sparta tremolite were predicted to pose the same order of mesothelioma risk as the Shinness and Carr Brae tremolites.
  • No particles meeting the Lippmann (1988) criteria for mesothelioma were found in the Shinness, Carr Brae, Ala di Stura and Swansea tremolites. Yet the mesothelioma frequencies Davis et al. reported for these samples ranged from 5.6% to 97.2%.
  • In the six Davis tremolite doses, particles 5 µm or shorter outnumbered particles longer than 5 µm by factors up to 84.5 (Carr Brae). The ratio was much lower for the Korea tremolite (7.76).
  • Ala di Stura tremolite delivered marginally fewer particles 5 µm or shorter than the Shinness and Carr Brae tremolites. Its reported mesothelioma frequency was higher: 66.7% versus 5.6% for Shinness.
• The authors' conclusion: The author concluded that particles 5 µm or shorter had negligible impact on the mesothelioma frequencies in the Davis et al. (1991) animal studies. The author also concluded that the protocol, which counts only particles outside the non-asbestiform amphibole size range, appeared to offer a way to assess amphiboles for their potential to produce mesothelioma by implantation in animals. The author questioned whether measuring short particles was worth the effort.
• Limitations the authors state:
  • The Davis et al. animal study had no control series and used a single fixed mass dose of 10 mg.
  • For the asbestiform tremolites, the 10 mg dose was a variable level of over-dose.
  • Further interpretation would require dose-response studies in which the mass dose is varied.
  • More research is needed to resolve discrepancies between this work and the Lippmann asbestos exposure indices.
  • Chrysotile fibril widths were not measured accurately.
  • The UICC-B and wet dispersed chrysotile distributions were based on fewer fibrils than the Coalinga distribution.

Topics

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

Medical terms in this study, explained in our Lexicon: elongate mineral particles (EMPs), Fibril, UICC, Chrysotile, Transmission Electron Microscopy (TEM), Tremolite, Non-Asbestiform, Amphibole Group.

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