Brake wear particles (BWP) are a significant source of urban air pollution, yet the toxicity linked to their chemical composition remains poorly understood. While studies have examined either chemical composition or toxicity, comprehensive investigations combining both remain limited. Here, we conducted an in-depth physicochemical characterization of airborne, size-separated BWP from two brake pad types and comprehensively assessed their in vitro toxicity using human lung epithelial cells (A549). Iron, primarily in the form of iron oxide, was the most abundant element in the wear particles (33-50% by mass), with evidence pointing to the brake disc as the main source. A surprisingly high resemblance in elemental composition at the nano- and microscale was observed. This, along with an absence of clear differences in metal profiles or toxicological responses between size fractions, suggests that brake wear microparticles may form through compaction of vapor-condensed nanoparticles on the friction surfaces, followed by their release through mechanical shearing. Acellular and cellular assays showed the concentration-dependent ability of all studied particles to induce reactive oxygen species production, antioxidant depletion, and oxidative stress-mediated DNA damage. The nonasbestos organic pad, with more than 50-fold higher copper levels than the low-metallic pad, induced stronger DNA damage and acellular antioxidant depletion, suggesting copper as a potential source for the enhanced toxicity.
Hyman et al. (2026) studied this question.