Ultrafine particles (UFP), the nano fraction of airborne particulate matter, are recognised as a major health risk factor in the WHO Guidance, considered to cause serious environmental effects and have a significant climate impact. The most important emission sector is transport of all kinds through direct particle emissions from vehicles, ships and aircraft engines but also by producing volatile organic pollutants which are converted in the atmosphere through photochemical reactions. UFPs health effects are costantly demonstrated at all scales through indoor and ambient exposure. A further interest in UFP’s results from their specific role in atmospheric processes such as cloud formation and precipitation and in climate. In particular, UfPs contribute to the Short-Lived Climate Pollutants (SLCPs), in particular black carbon (BC) and organic aerosols. The relation between UFP and human health and that of UFP and climate are both areas of active research and cross-links between these fields are more and more ...