Temporal variation of indoor 222Rn concentrations under contrasting room-opening conditions: A case study in Da Lat, Vietnam.

Indoor exposure to 222Rn depends not only on the mean concentration but also on temporal concentration variation in relation to room occupancy. Therefore, hourly radon data, when combined with space-use information, can provide a basis for occupancy-weighted exposure assessment. This study analysed hourly 222Rn concentration time series in two residential rooms in Da Lat: an upper-floor room without direct ground contact and with a direct opening to outdoors, and a lower-level, basement-like room with ground contact and indirect air exchange through an adjacent space. During the sequential monitoring segments, the enhanced natural air-exchange state had a lower mean 222Rn concentration than the closed state in both the basement-like room (154.6 versus 433.1 Bq m-3; 64.3% lower) and the upper-floor room (50.8 versus 89.3 Bq m-3; 43.1% lower). An exploratory one-zone mass-balance analysis yielded higher fitted effective removal parameters, aeff, under enhanced exchange (0.499 versus 0.315 h-1 and 0.338 versus 0.213 h-1, respectively), consistent with faster apparent radon removal. In the basement-like room, the effective source-related term, Eeff, was also lower under enhanced exchange (2348 versus 4945 Bq h-1); however, this parameter represents a model-inferred source-removal balance rather than an independently measured source strength. These findings support systematic, long-term investigations, prioritising lower-level, ground-contact and poorly ventilated rooms.