Final results from a 90-day quantitative inhalation toxicology study evaluating the dose-response and fate in the lung and pleura of chrysotile-containing brake dust compared to TiO2, chrysotile, crocidolite or amosite asbestos: Histopathological examination, confocal microscopy and collagen quantification of the lung and pleural cavity

Our plain-English summary of Final results from a 90-day quantitative inhalation toxicology study evaluating the dose-response and fate in the lung and pleura of chrysotile-containing brake dust compared to TiO2, chrysotile, crocidolite or amosite asbestos: Histopathological examination, confocal microscopy and collagen quantification of the lung and pleural cavity, by D.M. Bernstein, B. Toth, R.A. Rogers, et al (Toxicology and Applied Pharmacology, 2021).

The study

• Authors: D.M. Bernstein, B. Toth, R.A. Rogers, P. Kunzendorf, J.I. Phillips, D. Schaudien
• Journal: Toxicology and Applied Pharmacology
• Published: 2021;424:115598
• Cited by: 13 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The final results from this multi-dose, 90-day inhalation toxicology study in the rat with life-time post-exposure observation have shown a significant fundamental difference in pathological response and tumorgenicity between brake dust generated from brake pads manufactured with chrysotile or from chrysotile alone in comparison to the amphiboles, crocidolite and amosite asbestos. The groups exposed to brake dust showed no significant pathological or tumorigenic response in the respiratory track compared to the air control group at exposure concentrations and deposited doses well above those at which humans have been exposed. Slight alveolar/interstitial macrophage accumulation of particles was noted. Wagner grades were 1-2 (1 = control group), similar to the TiO2 particle control group. Chrysotile was not biopersistent, exhibiting in the lung a deterioration of its matrix which results in breakage into particles and short fibers which can be cleared by alveolar macrophages and which can continue to dissolve. Particle-laden macrophage accumulation was observed, leading to a very-slight interstitial inflammatory response (Wagner grade 1-3). There was no peribronchiolar inflammation, occasional very-slight interstitial fibrosis (Wagner grade 4), and no exposure-related tumorigenic response. The pathological response of crocidolite and amosite compared to the brake dust and chrysotile was clearly differentiated by the histopathology and the confocal analysis. Crocidolite and amosite induced persistent inflammation, microgranulomas, persistent fibrosis (Wagner grades 4), and a dose-related lung tumor response. Confocal microscopy quantified extensive inflammatory response and collagen development in the lung, visceral and parietal pleura as well as pleural adhesions. These results provide a clear foundation for differentiating the innocuous effects of brake dust exposure from the adverse effects following amphibole asbestos exposure.

Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.

In plain English

In this experiment, 1020 male rats in nine groups breathed, for 13 weeks, filtered air, brake dust containing chrysotile (three levels), titanium dioxide, chrysotile (two levels), crocidolite or amosite. At two years (31 rats per group, some euthanized early), the table listed lung tumors as 1 each in air, high brake dust, titanium dioxide and low chrysotile groups, 5 for crocidolite and 9 for amosite. Only the crocidolite and amosite counts were marked statistically significant. No mesotheliomas were found in the chest. Crocidolite and amosite caused lasting lung fibrosis, more collagen and thicker pleura; amosite-exposed rats showed pleural adhesions. Brake dust caused no fibrosis; chrysotile caused very slight fibrosis in some rats, including 4 of 4 high-dose rats examined 6 months after exposure, but 1 of 31 at 24 months. Fibers over 20 micrometres cleared with half-times of 23 and 37 days for chrysotile, versus 518 and 13,446 days for crocidolite and amosite. The authors concluded the results separate what they called innocuous brake dust effects from adverse amphibole asbestos effects. They noted rats differ from humans and doses exceeded brake workers' exposures.

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: Animal inhalation experiment: 90-day (13-week) nose-only inhalation exposure in rats, followed by observation over the rats' lifetime (to about 20% survival), with interim and final sacrifices
• Who was studied: 1020 male CRL:WI Wistar Han rats in 9 groups of 106. 31 rats per group were included at the final sacrifice at 24.4 months, some of them euthanized early because of their condition. 4 rats per group were examined at interim time points through 18 months.
• Exposure: Exposure was 6 h/day, 5 days/week for 13 weeks. Groups and mean concentrations were as follows. Air control. Brake dust from brakes made with chrysotile: low 0.20 mg/m3 (2.4 WHO fibers/cm3; 0.2 fibers longer than 20 μm/cm3), mid 0.34 mg/m3 (4.9; 0.4), high 0.67 mg/m3 (6.6; 0.5). Titanium dioxide particle control 0.70 mg/m3. Chrysotile 7H19: low 0.27 mg/m3 (119 WHO f/cm3; 28.4), high 0.64 mg/m3 (233; 72.3). Crocidolite (Voorspoed mine) 1.28 mg/m3 (181; 23.5). Amosite 2.32 mg/m3 (281; 47.7). Crocidolite and amosite were each tested at one level, labelled 'high dose'.
• Main results:
  • In the 2-year table (31 rats per group examined), total lung tumors were listed as: air control 1, brake dust high 1, titanium dioxide 1, chrysotile low 1, crocidolite 5 and amosite 9. Brake dust low and mid and chrysotile high had 0. The text says the single affected high brake dust rat had two bronchiolo-alveolar carcinomas, while the table lists one. Only the crocidolite and amosite totals were marked p less than 0.05 (chi-squared test). Most tumors were bronchiolo-alveolar adenomas and carcinomas.
  • The authors considered the tumors in the crocidolite and amosite groups to be exposure-related.
  • No mesotheliomas or preneoplastic pleural lesions were found in the chest cavity in any group.
  • One crocidolite-exposed rat had a mesothelioma in the abdominal cavity (including the tunica vaginalis of the testis). The authors interpreted it as a background finding unrelated to the inhalation.
  • Interstitial fibrosis was not seen in any brake dust group at any time.
  • In the high-dose chrysotile group, very slight interstitial fibrosis was seen in 2 of 4 rats at 89 and 180 days. At 6 months after exposure it was seen in all rats of the group; 4 rats per group were evaluated at each time point through 18 months, so this was 4 of 4. The low-dose group had a single rat with fibrosis at 6 months.
  • At 12 months after exposure, there was a single rat with very slight interstitial fibrosis in each chrysotile group. At 18 and 24 months, no fibrosis was seen in the low-dose group. In the high-dose group, 1 of 31 rats had very slight fibrosis at 24 months.
  • Measured lung fibrosis on slides rose in the crocidolite group from 0.43% at 45 days to between 0.96% and 1.07% through 18 months. In the amosite group it went from 0.77% at 45 days to between 0.99% and 1.10%. Brake dust and titanium dioxide groups showed 0 at all time points.
  • In the chrysotile groups, measured fibrosis was 0.08% in the low-dose group at 6 months. In the high-dose group it was 0.28% at 89 days and 0.27% at 6 months, and 0 at other time points through 18 months.
  • Wagner grades were 1 to 2 in the brake dust groups (grade 1 is the air-control level). At 24 months in the high-dose chrysotile group, out of 31 rats, 4 had grade 1, 14 grade 2, 12 grade 3 and 1 grade 4 (fibrosis). In the low-dose chrysotile group, 12 had grade 1, 18 grade 2 and 1 grade 3.
  • In the crocidolite and amosite groups, Wagner grade 4 (fibrosis) was seen at every time point from 45 days to 24 months.
  • Confocal microscopy found no statistically significant difference from controls in lung collagen through 24 months for the brake dust, titanium dioxide or chrysotile groups. For crocidolite and amosite, lung collagen was 14 to 23% higher than control means (p less than 0.0001).
  • Through 18 months, collagen and mesothelial thickness of the visceral and parietal pleura were significantly increased in the crocidolite and amosite groups compared with controls (p less than 0.0001). They were not significantly different from controls in the brake dust, titanium dioxide or chrysotile groups.
  • Mean mesothelial thickness of the visceral pleura was 4.5 to 4.7 μm in controls against 11.6 to 19.2 μm in the amphibole groups. For the parietal pleura it was 2.7 to 5.0 μm in controls against 18.9 to 30.8 μm.
  • The pleural adhesions described in the findings were in amosite-exposed rats. Confocal images showed adhesions, one with a long amosite fiber in the pleural space, and on histopathology one amosite rat had an adhesion of the lung pleura to the chest wall.
  • After exposure ended, fibers longer than 20 μm cleared with half-times of 23 and 37 days in the low and high chrysotile groups, and 518 and 13,446 days in the crocidolite and amosite groups. In the brake dust groups, all such fibers had cleared by 180 days.
  • At 12 months, no fibers longer than 20 μm remained in the brake dust groups. The chrysotile groups had an estimated 40,000 to 70,000 such fibers per lung, and the crocidolite and amosite groups about 10,500,000 per lung.
• The authors' conclusion: The authors concluded that the results provide a clear foundation for differentiating what they called the innocuous effects of brake dust exposure from the adverse effects of amphibole asbestos exposure. They reported no sustained fibrotic response and no excess lung tumors in the brake dust groups at concentrations well above past human exposures.
• Limitations the authors state:
  • The study was done in rats, which, unlike humans, breathe only through the nose.
  • Rat lung branching differs from human lung branching.
  • Rat macrophages have been reported to be smaller than human macrophages.
  • The doses used were higher than those brake workers have been exposed to.

Topics

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

Medical terms in this study, explained in our Lexicon: Dose-response relationship, Pleura, Chrysotile, Crocidolite, Amosite, Collagen, Pleural Cavity, Brake Lining.

Related studies

• Evaluation of the dose-response and fate in the lung and pleura of chrysotile-containing brake dust compared to TiO2, chrysotile, crocidolite or amosite asbestos in a 90-day quantitative inhalation toxicology study - Interim results Part 2: Histopathological examination, Confocal microscopy and collagen quantification of the lung and pleural cavity — D.M. Bernstein, B. Toth, et al. (2020)
• Evaluation of the exposure, dose-response and fate in the lung and pleura of chrysotile-containing brake dust compared to TiO2, chrysotile, crocidolite or amosite asbestos in a 90-day quantitative inhalation toxicology study - Interim results Part 1: Experimental design, aerosol exposure, lung burdens and BAL — D.M. Bernstein, B. Toth, et al. (2020)
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• Quantification of the pathological response and fate in the lung and pleura of chrysotile in combination with fine particles compared to amosite-asbestos following short-term inhalation exposure — D.M. Bernstein, R.A. Rogers, et al. (2011)
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• The pathological response and fate in the lung and pleura of chrysotile in combination with fine particles compared to amosite asbestos following short-term inhalation exposure: interim results — D.M. Bernstein, R.A. Rogers, et al. (2010)

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