Using Safecast data for estimating ambient dose rate in cities around the worldPetr KUČA1 and Peter Bossew21 National Radiation Protection Institute (SURO), Praha, Czech Republic 2 German Federal Office for Radiation Protection, Berlin Safecast [1] is a citizen science project, aimed to environmental monitoring. Its main activity is measuring ambient dose rate (ADR) all over the world. Motivated by the Fukushima NPP accident in March 2011, the project started soon after, and since, numerous citizens have contributed, carrying monitors with them.In this presentation, the Safecast project is introduced together with its standard instrument for ADR measurement, called bGeigie Nano. We discuss matters of quality assurance connected to data generation mainly by citizens who are generally no trained metrologists, and consequently, interpretation problems of Safecast data.The freely accessible data, currently (January 2020) over 120 million observations, were used to calculate mean ADR in various cities around the world where sufficient data is available. The resulting geographical pattern mainly reflects the variability of dose rate from terrestrial radiation, which is controlled by the one of geochemistry, namely the concentrations of uranium, thorium and potassium. Further influence comes from cosmic radiation, natural radionuclides in the air (a small contribution) and in a few cases, from anthropogenic radiation caused by nuclear fallout.In some cities at high altitude, such as Cusco (Peru), Nairobi (Kenia) or Denver (USA), secondary cosmic radiation clearly contributes strongly to ADR. In low to medium altitude, cosmic dose rate varies relatively little, so that it contributes little to the geographical pattern. Apart from the regional geological background, ADR is generated by building materials typical for an urban environment. Mean terrestrial ADR in cities around the world ranges between several 10 nSv/h and several 100 nSv/h. Anthropogenic radiation contributes little, except close to areas affected by the Chernobyl and Fukushima accidents. However, one can argue that also radiation from building materials, although originating from natural radionuclides, is anthropogenic, as buildings are anthropogenic objects and the choice of building materials is an anthropogenic one.We show maps displaying mean ADR for a number of cities. Geology and in some cases, altitude above sea level are clearly reflected in these maps. Besides, we address statistical issues related to spatial dispersion of ADR and of data clustering as resulting from varying and heterogeneous sampling density. Finally, we discuss merits of the Safecast project as well as inevitable limitations.[1] www.safecast.org ; Brown, A., Franken, P., Bonner, S., Dolezal, N., Moross, J. (2016): Safecast: successful citizen-science for radiation measurement and communication after Fukushima. Journal of Radiological Protection, 36 (2), S82 – S101; doi:10.1088/0952-4746/36/2/s82