Brake wear (nano)particle characterization and toxicity on airway epithelial cells in vitro

Particulate air pollution resulting from different sources, among which are those related to road traffic, has a significant impact on human health. Combustion-derived particles emitted by thermal engines have been implicated and are now better controlled. In contrast, non-exhaust emission sources related to car wear and degradation processes are not yet regulated. Here we report on brake wear particles (BWPs) harvested in two test facilities operating in France, providing samples from different braking systems and driving/testing conditions. Using a combination of light scattering, X-ray fluorescence, and optical and electron microscopy, the particle size and elemental composition are revealed. The BWPs are shown to be in the nano-to micrometer range and to have a low carbonaceous content (6%), iron and copper being the main components (>40%). To evaluate the toxicity potential of their nano-sized fraction, the brake wear nanoparticles are isolated by sonication, filtration and ultra-centrifugation techniques, leading to stable colloidal dispersions. A significant outcome of this study is that the nano-sized fraction represents 26% by mass of the initial BWPs. Human bronchial epithelial cells (Calu-3) are used as relevant target cells to investigate their cytotoxicity. We observe a clear short-term loss of viability associated with reactive oxygen species generation, but with limited pro-inflammatory effects. On an actual cell-deposited mass-dose basis, the cytotoxicity of the nano-sized fraction is similar to that of BWPs, suggesting that the cytotoxicity is particle size-independent. To conclude, brake wear dust contains a substantial amount of metallic nanoparticles exhibiting toxicity for lung cells, and should warrant further consideration.