The in door ra don be hav ior has com plex dy nam ics due to the in flu ence of the large num ber of dif fer ent pa ram e ters: the state of in door at mo sphere (tem per a ture, pres sure, and rel a tive humid ity), aero sol con cen tra tion, the ex change rate be tween in door and out door air, con struction ma te ri als, and liv ing hab its. As a re sult, in door ra don con cen tra tion shows vari a tion, with the usual pe ri od ic ity of one day and one year. It is well-known that sea sonal vari a tion of the ra don con cen tra tion ex ists. It is par tic u larly in ter est ing to in ves ti gate in door ra don vari ation at the same mea sur ing lo ca tion and time pe riod, each year, due to es ti ma tion of in di vid ual an nual dose from ra don ex po sure. The long-term in door ra don mea sure ments, in a typ i cal fam ily house in Ser bia, were per formed. Mea sure ments were taken dur ing 2014, 2015, and 2016, in Feb ru ary and July, each year. The fol low ing mea sur ing tech niques were used: ac tive and char coal can is ters meth ods. Anal y sis of the ob tained re sults, us ing multivariate anal y sis meth ods, is pre sented.

The presented, very simplified model provides a possibility for estimation of surface Pb-210 activity, depending on the changes of Rn-222 concentration during the long-term radon presence inside the closed room. This can be useful for retrospective assessment of the average indoor radon concentration for certain historical period, based on the surface contamination by the radionuclide Pb-210 in a closed or poorly ventilated room over a long period of time. However, the surface Pb-210 contamination depends on the pattern of radon concentration changes, and in this model is supposed that the change of indoor radon concentration, which periodically enters the room, is affected only by the radioactive decay and the inserted amount of radon in each entry. So, each radon entry can be comprehended as a “net amount” of radon, or excess which remains inside the room due to radon’s periodical in-out flow. It is shown, that under the conditions of the model, the achieved average value of radon concentration of 275 Bq/m3, implies that the saturated surface contamination by the Pb-210 of 160 Bq/m2 after approximately 150 years. [Projekat Ministarstva nauke Republike Srbije, br. 171002: Nuclear Methods Investigations of Rare Processes and Cosmic Rays i br. 43002: Biosensing Technologies and Global System for Continuous Research and Integrated Management of ecosystems

Si mul ta neous in door mea sure ments of ra don, thoron and equi lib rium equiv a lent con cen trations, by three dif fer ent con tin u ous ra don mon i tors in real am bi ent con di tions, were per formed and com pared dur ing two weeks pe riod. Ra don con cen tra tions var ied from 153 Bqm-3 to 870 Bqm-3 with vari a tions of thoron con cen tra tions in the range (2.17-219) Bqm-3. Ob tained arith me tic means of equi lib rium equiv a lent ra don and thoron con cen tra tions were (130 ± 50) Bqm-3 and (23 ± 18) Bqm-3 , re spec tively. The intercom par ison of ac tive ra don de vices was fo cused on con sis tency of re sults be tween them selves and the in flu ence of en vi ron men tal con di tions on the be hav ior of each de tec tor sys tem. The ob tained sta tis ti cally sig nif i cant cor re la tion be tween the re sults of used mon i tors, val i dates them for ap pli ca tion in ra don di ag nos tic mea sure ments as the first step in ra don remediation and dose as sess ment.