Physical properties of brake wear particles Brake wear generates particles across a wide range of diameters from a few hundred nanometres (nm) to a few tens of micrometres (µm). Whilst brake wear emissions are predominantly the result of a mechanical process and thus expected to release particles in the coarse size fraction, it appears that a significant proportion of brake debris may be emitted in the fine size mode (Wahlin et al., 2006; Iijima et al.,2007).