1 Comparison of Radon Mass Exhalation Rate Measurements from Building Materials by two Different Methods Sandra Soares 1, 2,3 , Joaquim Kessongo 1, 2,3 , Yoenls Bahu 1, 2,3 , Luis Peralta 2,4 1 Departamento de Física, Faculdade de Ciências da Universidade da Beira Interior, Covilhã, Portugal 2 Laboratório de Instrumentação e Física Experimental de Partículas (LIP), Lisboa, Portugal 3 LabExpoRad – UBIMedical, Universidade da Beira Interior, Covilhã, Portugal 4 Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal Abstract: The aim of this study is to measure the radon mass exhalation rate from common granite building materials used in the East and Northeast part of Portugal. Twelve cubic shaped samples were measured. Nine of them without any coating and three coated with different materials (varnish, hydrorepellent and liquid silicone). The radon measurements were performed with two different techniques: one using passive detectors and other using an active detector. For the passive method CR-39 solid state nuclear track detectors were used. The active method used the RAD7 DURRIDGE detector. Radon mass exhalation rates obtained from both methods present relatively low values in the 11 to 45 mBq kg -1 h -1 range for the analyzed samples. Concerning the coated samples, the measured values are on average four times lower than the ones without coating. Overall, the measured values for both methods present a good agreement. 1. Introduction The human species has always been exposed to ionizing radiation of natural origin, which can be found in soils, some rocks and water. From the late nineteenth century onwards, so-called artificial radiation was added to this background radiation. About 80% of the background radiation arise from natural sources, in which we can include the naturally occurring radioactive materials (NORM) present in the Earth's crust (1,2) , in different types of food, beverages and in some building materials. This radiation is largely due to primordial radionuclides of the natural radioactive series of 232 Th and uranium isotopes 238 U; 235 U, and their progeny. A sizable contribution is also given by the radioisotope 40 K. In terms of human health and environment effects the radionuclide with major radiologic interest is radon ( 222 Rn) (3) . This radioactive isotope results from the disintegration of 226 Ra, a decay product of the 238 U series and responsible for the largest source of natural radiation to which population is subject, contributing with approximately 50% for the total dose of radiation (4) . Radon concentration can reach high levels in dwellings depending not only on exhalation from soil but also on the building material used like concrete, bricks, phosphogypsum or granite (5) . In general, either to improve waterproofness or indoor performance namely comfort and desegregation of the natural stone limiting the particle loss, some coatings are normally used over natural stone. These coatings can also be used as a way of impermeabilization to radon exhalation. There are several parameters to assess, in a satisfactory way, the levels of airborne radon resulting from soil beneath