Integrated CFD-Quantitative Microbial Risk Assessment analysis of thermal comfort, particle deposition and indirect infection risk in a conference room under different occupancy scenarios

Abstract Conference rooms experience large and rapid variations in occupancy, which alter internal heat gains and the interaction between supply jets and occupant thermal plumes. This study investigates how occupancy affects thermal comfort, the deposition of exhaled droplets on occupants and the associated indirect (deposition–hand–mucosa) infection-risk indicators in a mechanically ventilated hotel conference room in Naples, Italy. A steady RANS model of the airflow and temperature fields is coupled with one-way Lagrangian tracking of 10 µm droplets released by the speaker, and the predicted deposition on each occupant is used as input to a Quantitative Microbial Risk Assessment (QMRA) chain. The model is verified through grid and time-step sensitivity analyses and validated against benchmark data from the literature. Four occupancy levels are compared: 25%, 50%, full and overfull condition with standing attendees. Increasing occupancy shifts the flow from a jet-dominated to a plume-influenced regime and raises the PMV from about −0.5 at 25% occupancy to +0.41 on average, and up to +0.75 in individual rows, under overfull conditions. In all scenarios, the first row receives the largest share of droplets among seated occupants (3.1–3.7% of the emitted droplets), whereas under overfull conditions the standing row along the [windowless] side receives 10.6%. Under conservative assumptions, the corresponding QMRA indicators reach about 0.44% for the first row and 1.45% for the standing row. The results indicate that deposition-related exposure is governed by the airflow pathways between the source and the main outlet rather than by distance alone, and that occupancy management and ventilation design should jointly consider thermal comfort and deposition hotspots.