The toxicology of chrysotile-containing brake debris: implications for mesothelioma

Our plain-English summary of The toxicology of chrysotile-containing brake debris: implications for mesothelioma, by C.A. Poland, R. Duffin (Critical Reviews in Toxicology, 2019).

The study

• Authors: C.A. Poland, R. Duffin
• Journal: Critical Reviews in Toxicology
• Published: 2019;49(1):11-35
• Type of article: Review
• Cited by: 14 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The global use of "asbestos" in various commercial products has led to a wide range and pervasive legacy of disease. One such use of chrysotile asbestos was brake pads and was utilized commonly in automobiles and heavy vehicles. The result of incorporation of chrysotile into brake pads is associated with the exposure of mechanics fitting and servicing vehicles to liberated chrysotile fibers. Despite the proven exposure, the relative risk of malignant mesothelioma (MM) in this occupational population is broadly seen as low. The toxicity of particulates, including fibers such as chrysotile, is driven by a combination of dose and physicochemical properties. As such, it is plausible that chrysotile released from brake pads may have undergone modification, thereby altering the pathogenicity profile. The impact of high sheer stress causing shortening of long fibers, heat modification, binding of resin matrix to the fiber surface on the relative toxicity of brake debris with regards to MM is considered. It is apparent that released chrysotile can undergo significant modification, reducing the long fiber dose although not all modifications may lead to reduced toxicity.

In plain English

This article was a review, not a new study. The authors examined published research on chrysotile asbestos in vehicle brake pads. They asked whether changes to the fibers could explain why epidemiological studies, for the most part, have not supported an excess risk of malignant mesothelioma in mechanics despite documented exposure. They summarized selected exposure studies, which reported levels both above and below occupational limits. They cited a 2016 meta-analysis by Garabrant and colleagues that reported a summary relative risk of 0.80 (95% CI 0.61 to 1.05) for mesothelioma among mechanics. They also described a low but definite prevalence of pleural plaques in mechanics. Reviewing laboratory evidence on grinding, heat and resin binding, they noted that only a handful of experiments have tested brake debris itself. The authors concluded that released chrysotile can be significantly modified, reducing the long-fiber dose, though not all changes may lower toxicity, and that evidence rather than supposition is needed. The work was funded by Honeywell and Ford, which the article states are defendants in asbestos brake litigation.

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: Narrative review of published literature (not a systematic review or new data). Funded by Honeywell International Inc. and Ford Motor Company, which the article states are defendants in asbestos-product litigation involving automotive brakes.
• Who was studied: A selection of published studies on chrysotile release from brake pads, exposure of brake mechanics, disease in mechanics (mesothelioma, pleural plaques, lung cancer, asbestosis), and animal and cell studies of fiber toxicity. The authors summarized key studies rather than all studies.
• Exposure: Chrysotile asbestos in vehicle brake pads and linings, released during brake servicing such as grinding, drilling and beveling, and possibly altered by heat, shear stress and binding to resin.
• Main results:
  • From the studies they reviewed, the authors summarized that debris released from asbestos-containing brake pads contains chrysotile mostly in the form of shorter fibers (under 5 micrometres) with resin matrix attached.
  • The authors summarized that brake repair and servicing is associated with exposure, with levels below or above occupational exposure limits varying between sites and studies.
  • The authors judged that epidemiological studies, for the most part, do not support an excess risk of malignant mesothelioma in auto mechanics.
  • The authors cited a 2016 meta-analysis by Garabrant et al. of 17 studies that reported a summary relative risk estimate for malignant mesothelioma among mechanics of 0.80 (95% CI: 0.61 to 1.05). This is that meta-analysis's result, not a new analysis by the authors.
  • The authors concluded from limited studies that there is a low but definite prevalence of pleural plaques in mechanics working with chrysotile.
  • The authors found limited toxicological evidence on chrysotile-containing brake debris itself, with only a handful of experimental papers.
  • The authors stated that released chrysotile can undergo significant modification that reduces the long-fiber dose, but that not all modifications may reduce toxicity. For example, a resin coating could slow fiber breakdown, and iron in the resin matrix could add reactivity.
• The authors' conclusion: The authors concluded that, toxicologically, chrysotile released from brake pads may well be modified compared with bulk chrysotile, but that evidence rather than supposition is needed. They called for in-depth characterization of the respirable material released during brake repair and servicing.
• Limitations the authors state:
  • The authors stated that the review is not comprehensive, and that their summary table of mesothelioma studies does not cover every analysis.
  • The authors noted that the temperatures typically reached when grinding brake pads, which matter for heat modification of chrysotile, have not been established.
  • The authors noted that historical case and cohort data make meaningful new analysis difficult, especially where information on exposure characteristics is lacking.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Auto mechanics and brake work, Chrysotile, Mesothelioma.

Medical terms in this study, explained in our Lexicon: Chrysotile, Mesothelioma, Brake Lining, Relative Risk, Malignant Mesothelioma.

Related studies

• Assessment of the physicochemical properties of chrysotile-containing brake debris pertaining to toxicity — M.S.P. Boyles, C.A. Poland, et al. (2019)
• Evaluation of the dose-response and fate in the lung and pleura of chrysotile-containing brake dust compared to chrysotile or crocidolite asbestos in a 28-day quantitative inhalation toxicology study — D.M. Bernstein, B. Toth, et al. (2018)
• 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)
• 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 size and type of free particulates collected from unused asbestos-containing brake components as related to potential for respirability — M.A. Atkinson, M. O'sullivan, et al. (2004)
• Re: Evaluation of the size and type of free particulates collected from unused asbestos-containing brake components as related to potential for respirability — D.J. Paustenbach, B.L. Finley, et al. (2006)

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