The project addresses the unsolved issue of the drivers of the variation of radon. High resolution monitoring of 222Rn activity concentration performed in different natural subsurface environments (boreholes, gallery, caves) in Slovakia yielded long time series. The temporal variation of radon, which contains periodic and non-periodic signals, spans variation of annual to diurnal scale. Time series of radon and environmental conditions will be systematically analyzed in the time and frequency domains. The derived statistical characteristics of the radon variation will be evaluated against those of the associated environmental parameters, to assess the eventual influence of the latter on radon in the gas phase. In addition, the characteristics of the radon time series will be evaluated in the light of recent conjectures attributing the influence of a component of solar irradiation on the periodic radon signals. The outcome of the investigation pertains also to the issue of the radon hazard to a human being.