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Particulate matter (PM) emitted from road traffic causes adverse health effects upon inhalation and respiratory deposition. Non-exhaust emissions will eventually become the dominant source of traffic PM upon transition to electric vehicles; however, non-tailpipe PM is currently unregulated as its health impacts are still unclear. In this study, we generated brake wear particles (BWPs) with non-asbestos organic, ceramic, and semimetallic brake pads using custom dynamometers and measured aqueous-phase formation of reactive oxygen species (ROS). We found that BWPs do not contain environmentally persistent free radicals (EPFRs), and all types of BWPs generate exclusively OH radicals in water. BWPs generated by ceramic and semimetallic brakes during heavier braking lead to higher OH yields compared to gentle braking conditions, suggesting higher OH formation potential from ultrafine BWPs. Chemical characterization reveals that organic and elemental carbon correlated positively with OH formation while exhibiting negative correlations with abundant metals including Fe and Mn. We suggest that the source of OH is thermal decomposition of organic hydroperoxides derived from phenolic resin. PM oxidative potential quantified with the dithiothreitol (DTT) assay exhibited a positive correlation with the OH yield. These results provide critical insights into the toxicity and adverse health effects of BWPs.