Assessment of the Indoor Gamma Dose Rates in 15 Portuguese Thermal Spas A. S. Silva and M. Lurdes Dinis Abstract In Portugal, most of the average annual dose to which the population is exposed by natural sources is from radon (57%) and terrestrial gamma radiation (18%). In this study, the indoor gamma radiation dose rates were evaluated in 15 Portuguese thermal spas between 2011 and 2015. Gamma radiation dose rates were measured with a Geiger counter type GAMMA SCOUT ® (GS3). The readings of the dose rate were hourly collected during a period of time ranging between 25 and 45 days, in different treatment rooms within each one the selected facilities: inhalation treatment rooms, thermal pools and vapors areas. All registered values for the gamma dose rates were lower than 1 mSv/year, and therefore the contri- bution of the external dose to the calculation of the annual effective dose is negligible. Keywords Radon Á Thermal spa Á Gamma radiation Á Distribution 1 Introduction Radioactivity does not result exclusively from anthropogenic action, since the Earth has always been subject to cosmic radiation, forming part of some radionuclides. Natural radiation includes cosmic radiation as well as the radiation arising from the decay of naturally occurring radionuclides. These, include the primordial A. S. Silva (&) Á M. L. Dinis CERENA - Polo FEUP - Centre for Natural Resources and the Environment, Faculty of Engineering, University of Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal e-mail: pee11022@fe.up.pt; sofiajo2009@gmail.com M. L. Dinis e-mail: mldinis@fe.up.pt A. S. Silva Á M. L. Dinis 2PROA/LABIOMEP - Research Laboratory on Prevention of Occupational and Environmental Risks, Faculty of Engineering, University of Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal © Springer Nature Switzerland AG 2019 P. M. Arezes et al. (eds.), Occupational and Environmental Safety and Health, Studies in Systems, Decision and Control 202, https://doi.org/10.1007/978-3-030-14730-3_24 221