Assessment of the physicochemical properties of chrysotile-containing brake debris pertaining to toxicity

Our plain-English summary of Assessment of the physicochemical properties of chrysotile-containing brake debris pertaining to toxicity, by M.S.P. Boyles, C.A. Poland, J. Raftis, et al (Inhalation Toxicology, 2019).

The study

• Authors: M.S.P. Boyles, C.A. Poland, J. Raftis, R. Duffin
• Journal: Inhalation Toxicology
• Published: 2019;31(8):325-342
• Cited by: 4 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Grinding and drilling of chrysotile asbestos-containing brake pads during the 20th century led to release of chrysotile, resulting in varying levels of workplace exposures of mechanics. Despite exposures, excess risk of mesothelioma remains in doubt. Objectives: The toxicity of particulates is primarily derived through a combination of physicochemical properties and dose and as such this study aimed to determine properties of asbestos-containing brake debris (BD) which may influence pathogenicity and potential of mesothelioma. Materials and Methods: Chrysotile-containing brake pads were ground - to reflect occupational activities, aerosolized, and size-fractionated to isolate respirable fractions. Analysis of morphology, biodurability, surface charge, and interactions with macrophages were undertaken. Results: The respirable fraction of BD contained ∼15-17% free chrysotile fibers thereby constituting a small but relevant potential long fiber dose. Acellular biodurability studies showed rapid dissolution and fragmentation of chrysotile fibers that was consistent for pure chrysotile control and BD samples. Conclusions: The long, free, respirable chrysotile fibers were present in BD, yet were of low bio-durability; incubation in artificial lysosomal fluid led to destruction of free fibers.

In plain English

This laboratory study, with no people or animals, examined dust made by grinding chrysotile-containing automotive drum brake shoes. The authors isolated the breathable (respirable) part of three brake debris samples. They compared it with pure UICC chrysotile B, amosite asbestos and a glass fiber called MMVF10. Of the particles and fibers counted in the respirable fraction, 15–17% were free chrysotile fibers meeting the WHO fiber definition, averaging about 11 micrometers long. No amphiboles were detected. A computer model predicted that 9.52% to 10.00% of the respirable debris would deposit in the deep (alveolar) lung. In artificial lysosomal fluid, which imitates conditions inside lung scavenger cells, only 40% of each debris sample remained after 2 weeks, much as with pure chrysotile. Amosite lost no more than 20%. After 168 days no free fibers were seen in the debris. In a pilot test, pure chrysotile was more cytotoxic to macrophage-like cells than the debris. Fibers stuck out of 38.1% of chrysotile-treated cells but only 0.8–1.8% of debris-treated cells. The authors called the fiber length a cause for concern. They concluded the fibers were respirable but of relatively low biodurability.

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 characterization study: acellular dissolution tests in artificial lysosomal fluid, electron microscopy, X-ray diffraction, surface charge and free-radical tests, computer modeling of lung deposition, and a pilot cell-culture (macrophage) experiment
• Who was studied: Three brake debris samples (BD-A, BD-B, BD-C) generated by RJ Lee Group from chrysotile-containing automotive drum brake shoes. Comparison materials were UICC chrysotile B (pure chrysotile), UICC amosite asbestos (a biodurable amphibole) and MMVF10 (a bio-soluble glass fiber). Cell experiments used differentiated THP-1 macrophage-type cells.
• Exposure: Respirable fraction of debris from brake shoes ground with a commercial arc grinder to reflect mechanics' work. Samples were incubated in artificial lysosomal fluid for up to 168 days. Macrophage-type cells were treated with doses from 0 to 20 micrograms per square centimeter.
• Main results:
  • Of the particles and fibers counted in the respirable fraction of the three brake debris samples, 15–17% were free fibers meeting the WHO criteria (17.08% BD-A, 14.90% BD-B, 15.16% BD-C by count). They averaged around 11 micrometers in length.
  • Electron microscopy found no evidence of amphibole contamination in the brake debris. X-ray diffraction also did not show the amosite peak pattern.
  • Lung deposition modeling (4 mg/m3, light exercise) predicted that 10.00% of BD-A, 9.95% of BD-B and 9.52% of BD-C would deposit in the alveolar region.
  • In artificial lysosomal fluid, only 40% of each brake debris sample was recovered after 2 weeks, with no further leaching after that. Pure chrysotile also fell to less than 50% recovered within the first 2 weeks. Amosite lost no more than 20% over the whole incubation. MMVF10 had 30% remaining at 168 days.
  • In BD-A, free respirable fibers fell from 22% at Day 0 to 4% at Day 126, and none were seen at 168 days. After 168 days, no free fibers were observed in any of the brake debris samples by TEM.
  • None of the test materials, brake debris or reference fibers, produced a significant increase in superoxide compared with the vehicle control.
  • In the pilot cell test, pure UICC chrysotile B was significantly more cytotoxic to macrophage-type cells than brake debris samples BD-A and BD-C across the doses tested (p less than 0.001). Debris caused significant cell damage only at doses above 10 micrograms per square centimeter.
  • Incomplete uptake (fibers protruding from cells, akin to frustrated phagocytosis) was seen in 38.1% of cells treated with UICC chrysotile B.
  • Incomplete uptake was seen in 0.8–1.8% of cells treated with brake debris (1.8% BD-A, 0.8% BD-C). Most debris-treated cells instead fully took up particles (50.6% BD-A, 61.6% BD-C), mostly resin fragments.
• The authors' conclusion: The authors concluded that long, free, respirable chrysotile fibers were present in the brake debris, and that their average length is a cause for concern as a fiber-type hazard. They also concluded the fibers were of relatively low biodurability, and that incubation in low-pH artificial lysosomal fluid destroyed the free fibers.
• Limitations the authors state:
  • At some time points dissolution left too few free fibers to count. Where counts were below 100, the authors said the robustness of the size data should be considered.
  • The static dissolution system allows only a limited assessment of dissolution rates.
  • X-ray diffraction was done only on the original samples, because too little material remained after incubation.
  • The cytotoxicity and uptake tests were a pilot-level assessment. Sample BD-B was not tested in cells because too little respirable sample was available.
  • For the mixed brake debris, it is unclear whether the measured surface charge applies to all particles or is an average of differing charges.
  • Attempts to detect a resin surface coating on free and protruding fibers with high-resolution TEM were unsuccessful.

Topics

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

Medical terms in this study, explained in our Lexicon: Chrysotile, Brake Lining, Mesothelioma, Respirable Fraction, Macrophage.

Related studies

• The toxicology of chrysotile-containing brake debris: implications for mesothelioma — C.A. Poland, R. Duffin (2019)
• Assessing specific causation of mesothelioma following exposure to chrysotile asbestos-containing brake dust — M.D. Freeman, S.S. Kohles (2012)
• 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)
• Assessment of airborne asbestos exposure during the servicing and handling of automobile asbestos-containing gaskets — C.L. Blake, G.S. Dotson, et al. (2006)
• Exposure to chrysotile asbestos associated with unpacking and repacking boxes of automobile brake pads and shoes — A.K. Madl, L.L. Scott, et al. (2008)

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