The German Environment Agency (UBA) in October 2018 published a study entitled Health Effects of Ultrafine Particles:  Systematic literature search and the potential transferability of the results to the German setting.  The study notes that ultrafine particles represent the smallest size fractions of particles with sizes from one to about 100 nanometers in aerodynamic diameter.  Their specific health effects are thus related to their physical capacity to reach diverse organ systems.  Prepared on behalf of UBA and the Swiss Federal Office for the Environment, the aims of the study were to review systematically the scientific literature on the health effects of ultrafine particles, to evaluate the quality of the selected studies, and to assess the transferability of the results to the situation in Germany.  The search strategy yielded 85 references of original articles.  Most of the included studies were conducted in North America (n=37) or Western Europe (n=27), investigating short-term effects (n=75).  The short-term studies are dominated by panel studies (n=32), scripted exposure studies (n=16), and time-series studies (n=11).  Ten studies investigated long-term associations using exposure estimates averaged over a period of months to years.  The study states that the evidence on health effects remains inconclusive or insufficient for most of the studied outcomes.  Specifically, while a number of studies have investigated mortality and emergency department/hospital admission outcomes, the relatively few studies with co-pollutant adjustment reveal mixed and, up to now, inconclusive evidence.  According to the study, in terms of number of studies, most available evidence is from studies investigating subclinical outcomes.  Within this group, cardiovascular outcomes and outcomes of pulmonary and systemic inflammation show the most consistent patterns with associations generally pointing in the direction of the adverse health outcome.  The study states that a future challenge is the development of enhanced spatiotemporal models that can contribute to a more precise exposure assessment across larger areas, as well as incorporating multipollutant models to become clear of independent effects.