Pulmonary toxicity of brake wear particles: effects of composition and surface reactivity

Our plain-English summary of Pulmonary toxicity of brake wear particles: effects of composition and surface reactivity, by G. Kim, S. Jeon, T.H. Shin, et al (Particle and Fibre Toxicology, 2026).

The study

• Authors: G. Kim, S. Jeon, T.H. Shin, Y. Park, K. Kim, S.H. Woo, Y.S. Huh, J.G. Lee, S. Lee, W.S. Cho
• Journal: Particle and Fibre Toxicology
• Published: 2026;23(1)
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Airborne brake wear particles (BWPs) are increasingly recognized as important non-exhaust contributors to urban particulate pollution; however, their inhalation toxicity and the key determinants of their pulmonary effects remain poorly defined.

Results: Two types of BWPs were generated using a brake dynamometer equipped with non-asbestos organic (NAO) and low-metallic (LM) brake pads on a cast-iron disc. The ≤ 2.5 μm fractions of BWPs generated from the NAO and LM brake pads were designated NAO2.5 and LM2.5, respectively. SRM 2975 (diesel exhaust particles) and Fe2O3 nanoparticles were included as reference particles. The test particles were characterized for their physicochemical properties, including morphology, size, surface area, crystallinity, colloidal properties, chemical composition, and solubility. The intrinsic oxidative potentials (IOPs) of the test particles, evaluated using a cell-free 2',7'-dichlorodihydrofluorescein diacetate assay, ranked as NAO2.5 > SRM 2975 > LM2.5 > Fe2O3 on a mass basis, and as NAO2.5 > LM2.5 > SRM 2975 > Fe2O3 on a surface area basis. Pulmonary neutrophilic inflammatory responses were evaluated by bronchoalveolar lavage fluid analysis at 24 h after pharyngeal aspiration of 25, 50, and 100 µg/mouse in female BALB/c mice. On a mass basis, the inflammatory response ranked SRM 2975 > NAO2.5 > Fe2O3 > LM2.5 on a mass basis, whereas on a surface area basis, the ranking was NAO2.5 > LM2.5 > SRM 2975 > Fe2O3. Lung burden analysis at days 0, 1, and 28 after a single pharyngeal aspiration of 100 µg/mouse showed relatively higher 28-day retention of NAO2.5 and LM2.5 (approximately 75% and 67%, respectively) compared to that of SRM 2975 (47%).

Conclusions: These data indicate that NAO2.5 is more inflammogenic than LM2.5. Given that BWPs were larger and had substantially lower surface area than the reference particles, their stronger responses under surface area-based comparison suggest greater inflammatory potency per unit surface area. Overall, the BWPs examined in this study exhibited a toxicity profile not fully explained by IOP alone, suggesting an important contribution of particle-specific physicochemical properties.

© 2026. The Author(s).

In plain English

In this mouse study, the authors made brake wear particles 2.5 micrometres or smaller on a test machine from non-asbestos organic (NAO) and low-metallic (LM) pads. Asbestos itself was not tested. They compared these with diesel exhaust particles (SRM 2975) and iron oxide nanoparticles, placing 25, 50 or 100 µg into female mice's lungs. One day later, by dose weight, the share of neutrophils (inflammatory cells) in lung fluid ranked diesel highest, then NAO, iron oxide and LM. By particle surface area, NAO ranked highest, then LM, diesel and iron oxide. In mice given 100 µg, inflammation had largely resolved by day 7 except with diesel particles, where some neutrophilic inflammation remained. At 28 days, 75% of NAO and 67% of LM particle mass remained, against 47% for diesel, relative to the amount measured right after dosing. Liquid soaked from the brake particles did not significantly change inflammation. The authors concluded that the particles' toxicity was not fully explained by oxidative potential. They noted their dosing method does not fully mimic breathing and findings cover only the samples tested.

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 experiment (single pharyngeal aspiration in mice) combined with laboratory physicochemical and cell-free characterization of particles
• Who was studied: Female BALB/c mice, six weeks old on arrival and acclimated for one week, studied under a Dong-A University (Korea) animal protocol. Each group had four mice for lung-fluid analysis and two for histology at each time point, plus four per group for lung-burden analysis.
• Exposure: Brake wear particles of 2.5 µm or smaller (NAO2.5 and LM2.5), generated on a dynamometer from non-asbestos organic (NAO) and low-metallic (LM) brake pads on a cast-iron disc. Reference particles were diesel exhaust particles (SRM 2975) and Fe2O3 nanoparticles. Doses of 25, 50 and 100 µg/mouse were given in 50 µL by pharyngeal aspiration.
• Main results:
  • Cell-free oxidative (ROS) potential on a mass basis ranked NAO2.5 highest, followed by SRM 2975, LM2.5 and Fe2O3.
  • At day 1, the percentage of neutrophils in lung lavage fluid on a mass-dose basis ranked SRM 2975 highest, then NAO2.5, then Fe2O3, then LM2.5.
  • On a surface-area dose basis, the neutrophil ranking was NAO2.5, then LM2.5, then SRM 2975, then Fe2O3.
  • Neutrophilic inflammation was significantly greater in the NAO2.5 group than in the LM2.5 group.
  • Of the cytokines tested, only IL-1β rose significantly, and only in the NAO2.5 and SRM 2975 groups at day 1.
  • Day 7 was assessed only after a single 100 µg/mouse dose. By then inflammation had largely resolved in all groups except SRM 2975, where residual neutrophilic inflammation remained.
  • At 28 days after 100 µg/mouse, 75% of NAO2.5 and 67% of LM2.5 deposited mass remained in the lung, compared with 47% for SRM 2975. Retention was expressed relative to the day 0 amount, not the nominal dose. Fe2O3 retained 63%, which was not statistically different from SRM 2975.
  • Neutrophil percentage correlated moderately with intrinsic ROS generation (r = 0.55), but this was not statistically significant (p = 0.07).
  • Leachates from both brake wear particle types did not significantly change lung-fluid inflammatory measures compared with the vehicle control.
  • No severe histopathological changes were seen in the lungs of the 100 µg/mouse groups.
• The authors' conclusion: The authors concluded that NAO2.5 was more inflammogenic than LM2.5. They also concluded that the brake wear particles' toxicity was not fully explained by intrinsic oxidative potential alone, which they took as a sign that particle-specific physicochemical properties contribute. They added that the particles' stronger responses under surface-area comparison suggest greater potency per unit surface area.
• Limitations the authors state:
  • Pharyngeal aspiration is a single bolus dose that does not fully mimic inhalation and may deposit particles unevenly.
  • Findings are limited to the specific NAO and LM samples tested.
  • ROS-normalized analyses show association, not causation.
  • The surface-area-based interpretation should be made cautiously, because other factors may influence responses.
  • Formal normality testing was not done because group sizes were small.

Topics

Medical terms in this study, explained in our Lexicon: Surface Reactivity, Brake Lining, Bronchoalveolar Lavage (BAL).

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