Over the past two decades numerous research projects have investigated an array of characteristics of ultrafine particles (UFP), or more generally, particles of the lower submicrometre size range. There is a reasonably good scientific understanding of the particle concentration levels, modality of size distributions associated with various sources, and their spatial heterogeneity in the urban environments. However, despite the progress made, there has only been very limited progress in understanding the risk to human health posed by UFP, and therefore whether the particles should be controlled. Part of the reason relates to the challenges in conducting epidemiological studies on the impact of UFP, and lack of consistency between the outcomes of some of the studies. In 2018 a group of exposure experts, toxicologist and epidemiologist joint forces to think outside the box, and to use the wealth of scientific knowledge on UFP physico-chemistry, toxicology and epidemiology to develop an approach that will create the basis for protection against the particles. The group has been working on developing a White Paper to inform decision makers on the state of knowledge on UFP and on conducting meta-analysis of data from epidemiologic studies to identify any evidence that can already be used to recommend exposure limits for UFP. ... mehrThe outcome of this work will contribute to the current debates on UFP, and to the work conducted on this topic by national and international bodies, including the World Health Organization or the European Union. The presentation will summarize the progress of this work and the picture that begins to emerge.