Characterizing layout-dependent breathing-zone contaminant concentration in office spaces with mixing ventilation using an occupant-to-exhaust distance index: a parametric CFD study

Office layouts can affect the accumulation and removal of occupant-generated contaminants, yet these effects remain insufficiently characterized across diverse occupant layouts and inlet/outlet arrangements using simple geometric descriptors. This study characterizes layout-dependent variations in ventilation performance using an occupant-to-exhaust distance index. We analyzed 198 cases covering different room sizes, ventilation airflow rates, desk arrangements, and two reciprocal inlet/outlet configurations. Using previously validated CFD airflow fields [A. Takii et al., Results Eng., 2026], we solved a steady passive-scalar transport equation for an idealized continuously emitted, air-following contaminant and evaluated ventilation-performance indices. The concentration distributions varied widely among layouts, affecting the magnitudes of the indices and their sensitivity to ventilation conditions. The occupant-to-exhaust distance index captured first-order variations in breathing-zone concentration. Within the present dataset, layouts above the estimated change point, , tended to exhibit higher normalized breathing-zone concentrations; thus, this value is interpreted as a dataset-specific screening boundary rather than a universal design criterion. Configurations that deviated from this trend were associated with wall proximity and occupant clustering, exhibiting recirculating or stagnant regions and elevated breathing-zone concentrations. These findings demonstrate the potential of a simple geometric index for preliminary screening of occupant–ventilation layouts. The applicability of these findings is limited to the tested windowless, relatively densely occupied offices with ceiling-mounted mixing ventilation, two reciprocal inlet/outlet configurations, and no air-conditioning recirculation.