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

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. 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%). 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.