45. The Chair of the PMP provided an overview of the latest activities of the informal working group, focusing primarily on emissions of sub 23 nm particles and particles from tyre and brake wear. He focused first on sub 23 nm particles (GRPE-68-21), reporting that the PMP informal working group does not see an urgent need to revise the current measurement cut-off size of 23 nm. He mentioned that the literature review on the emission of particles from tyre and brake wear (GRPE-68-20) has been completed. He cautioned about the numerous measurement techniques, the important differences between light and heavy duty vehicles, the importance of driving conditions and the lack of a clear definition of non-exhaust emissions. Overall, he reported that exhaust and non-exhaust sources are estimated to contribute almost equally to total traffic-related PM₁₀ emissions. The relative contribution of non-exhaust sources is expected to increase in the coming years due to the tendency of decreasing exhaust emissions. He provided information on the mass and number size distributions, the chemical characterization of particles from tyre and brake wear, providing estimations of the emission factors applicable to them. He underlined that brake particles are always linked to the vehicles, while particles from tyre wear result from the interaction of the vehicle and the road. Having mentioned current research activities being carried out by the industry, he concluded that knowledge at present is not sufficient in these areas and several open questions remain. He warned that, should the PMP meeting keep working on tyre and brake wear, it will be necessary to revise the mandate of the informal working group, since this is currently limited to the literature work presented at this GRPE session.