Radon risk mitigation in urban environments: experiences from active monitoring

The primary stone building material of ancient Rome from its initial settlement until recent times is constituted by the tuff; easy to cut, resistant to weathering, and an effective thermal insulator, with the disadvantage of a significant radionuclide content. An accurate monitoring of indoor radon in workplaces and residential dwellings constitutes a first step towards mitigating the exposure to the population. Since radon diffusion dynamics involves complex interactions among many environmental parameters on different time scales, a proper assessment of radon concentration variations can be better achieved by means of active monitoring approaches. We present here the results of continuous measurements conducted in 35 residential dwellings located in the municipality of Rome and its suburban area, and in a public building and workplace: the geophysical museum of Rocca di Papa.
The use of active devices makes it possible to discriminate between average indoor radon measured during the day, when workers and visitors are more likely present, and overnight, more relevant for the exposure of residents. Collecting long time series of radon concentration enables us to identify fluctuations over seasonal scales, with radon generally decreasing in the warm season. The simultaneous tracking of different floors of the same building shows an inversion of the dynamics of gas convection during the warm season compared to the cold one, likely depending on the chimney effect. Monitoring different rooms of the same dwelling reveals that values of gas concentration may greatly differ, indicating the importance of ventilation and/or heating system. Considering several dwellings allows us to question the general belief of a constantly higher exposure of the lowest floors to the indoor radon risk with respect to elevated floors (radon would enter mainly through foundation walls). Finally, the clustering of houses with high indoor radon levels in the historic center of Rome indicates the influence of geogenic radon and of building characteristics like age, typology, and construction materials.
With so many endogenous and exogenous factors affecting the healthiness of indoor environments in terms of radon concentration, a proper assessment of health hazard requires the knowledge of the dynamics of the gas generation and transport inside the buildings, and of its temporal fluctuations; our analysis provides the instruments to disclose the characteristics of such dynamics, with the final goal to select the most suitable preventive measures to reduce radon exposure.