Effects of breathing pathway and phase difference on short-range airborne exposure in a ventilated indoor environment

Short-range airborne exposure in indoor environments is strongly influenced by the transient interaction between exhaled and inhaled respiratory flows. This study investigated how breathing pathway (mouth or nose) and inter-individual phase difference affect near-field contaminant transport between two occupants in a ventilated indoor space. A transient CFD model with physiologically realistic sinusoidal breathing boundary conditions was developed to simulate six face-to-face interaction scenarios at a separation distance of 0.75 m. The model was validated against benchmark experimental data for respiratory airflow and tracer-gas dispersion in a displacement-ventilated room. The results showed that synchronized mouth-to-mouth interaction produced the highest contaminant transfer, inhaled dose, and predicted infection risk. By contrast, phase offset, nasal pathways, and opposing jet interaction substantially reduced short-range exposure by weakening direct transport to the target breathing zone. Across the simulated cases, the predicted infection probability decreased by up to 87% relative to the worst-case condition. Overall, the findings show that short-range airborne exposure depends not only on distance, but also on breathing timing and pathway alignment within the indoor breathing microenvironment.