Characterization and toxicological assessment of size-fractionated brake wear particulate matter

Tailpipe emission controls and increasing prevalence of electric vehicles are altering air pollutant profiles, increasing the prominence of non-tailpipe pollutants such as brake and tire wear, with uncertain effects on health. Here we characterized physicochemical and toxicological profiles of wear particles derived from various brake pads.

Emissions from low metallic (LM), galvanized steel (ST) and ceramic (CE) brake pads installed in a light-duty compact SUV were sampled using an in-house designed Brake Emissions Capture and Sampling System (BECSS). Emissions were generated using a dynamometer through a standardized brake cycle (WLTP-brake). Real time particle-size characterization was performed (EEPS, APS), and samplers captured size-fractionated particulate matter [PM; Ultrafine (Uf), <0.1 µm; Fine (Fi), 0.1-2.5 µm; Coarse (Co), 2.5-10 µm] for toxicological assessment using a high volume cascade impactor, while additional fractions were captured for chemical analysis (gravimetric Impactor, ICP-MS). Human alveolar epithelial cells (A549) and murine monocytes (J774) were exposed to size-fractionated particles across a dose range (0 to 180 µg/cm2) for 24 h. Cell viability (LDH activity) and oxidative stress (H2O2) were assessed, and material was collected for subsequent assessment of inflammatory cytokines and mRNA profiles.

Particulate number size distribution was similar for all pads, peaking at ~10 nm for PM<0.5 µm (EEPS) and ~1 µm for PM0.5-20 µm (APS). LM produced the greatest numbers of nanosized particles (LM>ST≈CE), while CE showed the lowest numbers for micron-sized PM (LM≈ST>CE). Brake pads differed in mass released, with consistent trends across size fractions (LM>ST>CE). Chemical analyses revealed i) that Fe was the most abundant element for all pads (e.g. ST[~80%]>SM[~35%]>CE[~19%] for PM<1µm); ii) enrichment of Sb/Ba/Sr/Rb on CE pads and of Mn and Fe on ST pads; and iii) greater metal abundance in the smaller size fractions for ST. Cell viability was dose-dependently reduced in A549 cells (Co>Fi>Uf; ST≈CE>LM) and J774 cells (Co≈Fi≈Uf; ST>CE>LM) while similar H2O2 profiles were observed for all brake pads and size fractions.

Initial characterization of size-fractionated brake wear emissions under controlled conditions showed that chemical content and toxicity endpoints are determined by size fraction and brake pad composition. Future work will explore pathways of effects and employ more physiologically-relevant air-liquid interface exposures to complement this screening study.