Radon and its short-lived progenies (218Po, 214Pb, 214Bi and 214Po) are well known radioactive indoor pollutants identified as the major radiation burden component of the thermal spa users. Monitoring of short-lived progeny concentration is of great importance for short-term dose estimations both for bathers and working personnel. A prediction model of the short-lived progeny concentration variations was developed and applied on published data of the thermal spas of Lesvos Island. The physical procedures involved were modeled in a set of differential equations describing radon progeny concentration variations on the basis of radon measurements. Published daughter data were fitted on model predictions adjusting non-measured parameters, e.g. attachment and deposition rate constants for attached and unattached progenies. Attachment rate constants were estimated between 50 and 200 h-1 while the deposition rate constants between 0.25 and 5 h-1 for attached progenies and 0.5 and 170 h-1 for the unattached ones. In addition, unattached 218Po, 214Pb and 214Bi progenies were found to be shifted forward in respect to radon approximately 0.001 h, 0.05 h and 0.40 h respectively, while attached 218Po, 214Pb and 214Bi progenies 0.05 h, 0.45 h and 0.65 h respectively.