An international team of experts published a White Paper on “Ambient ultrafine particle evidence for policy makers”, which was led by Professor Lidia Morawska (International Laboratory for Air Quality and Health, Queensland University of Technology), Professor Annette Peters (Institute of Epidemiology, Helmholtz Center Munich – German Research Center for Environmental Health), and Professor Flemming R. Cassee (National Institute for Public Health and the Environment, and the Institute of Risk Assessment Sciences, Utrecht University), and prepared by the ‘Thinking outside the box’ team. The White Paper has been published on the European Federation of Clean Air and Environmental Protection Associations (EFCA) website and can now be downloaded here.