  Particulate air pollution results from different sources, among which those
related to road traffic have a significant impact on human health.
Combustion-derived particles emitted by thermal engines have been incriminated
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 (BWP) 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,
optical and electron microscopy, the particle size and elemental composition
are revealed. The BWP 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 its nano-sized fraction, 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 BWP. 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 to reactive oxygen species
generation, but with limited pro-inflammatory effects. On an actual
cell-deposited mass-dose basis, the cytotoxicity of the nanosized fraction is
similar to that of BWP, suggesting that the cytotoxicity is particle size
independent. To conclude, brake wear dust contains substantial amount of
metallic nanoparticles exhibiting toxicity for lung cells, and should warrant
further consideration.
