To address the issue of residential radon risk we conducted two distinct experiments actively monitoring radon in 25 apartments in Rome and suburban areas for two weeks, and in 3 apartments in the historic center for several months. The action levels of 300 and 100 Bq/m3 are overcome in 17% and 60% of the cases, respectively, and these percentages rise to 20% and 76% for average radon measured overnight (more relevant for residents’ exposure). Active detectors allowed us to identify the seasonal behavior of radon, strongly dependent on the gradient of indoor-to-outdoor temperature, and of the way it travels from the ground to upper floors. High levels of radon are not limited to the lowest floors when the use of heating and ventilation produces massive convection of air. The importance of these factors is also evidenced by the very different values of gas concentration measured at different floors of the same building or in distinct rooms of the same apartment, that cannot be ascribed to the characteristics of the premises. However, the finding that high residential radon levels tend to concentrate in the historic center proves the significant influence of building age, typology, construction materials, and geogenic radon.