As COVID‐19 spread worldwide, numerous outbreaks occurred in wards. This study, in addition to investigating the indoor air environment and ventilation characteristics of a ward that had experienced multiple outbreaks, quantitatively evaluated how ventilation characteristics and door operation affect infection risk. Supply and exhaust airflow rates were measured, and ventilation characteristics—including air changes per hour (ACH) in each patient room, the supply–exhaust airflow balance, and ward ventilation pathways—were investigated. Furthermore, assuming the presence of a single index case, computational fluid dynamics (CFD) simulations were performed for four cases combining two airflow conditions and two door‐operation conditions, and the probability of infection under 24 h of exposure was calculated. The results confirmed a substantial ward‐wide decrease in supply airflow, indicating that outdoor air was not sufficiently supplied to patient rooms, and CO ₂ concentrations exceeded 1,000 ppm at some measurement points. While poor ventilation facilitated the dispersion of infectious respiratory particles (IRPs) and increased infection risk, closing doors suppressed IRP dispersion and markedly reduced infection risk. Therefore, checking equipment conditions during normal operation and performing maintenance as needed, together with door operation according to the risk of nosocomial infection, are recommended for infection control in wards.

Household airborne transmission can be promoted when infectious and susceptible occupants share indoor air for long periods, yet practical infection-risk models often require pathogen-specific parameters that are uncertain. This study proposes a measurement-informed multizone/HVAC-network workflow that identifies inter-room airflow rates (q) from CO₂ tracer time series and estimates an effective first-order non-ventilation aerosol loss rate (λ) by fitting PM_2.5 concentration decay dynamics; the identified parameters are then reused within the same whole-house recirculating network model (vtsim) to compute a steady-state exhaled-air tracer concentration index for scenario comparison. The workflow is demonstrated in a high-insulation, airtight detached house equipped with a duct-type whole-house air-conditioning system with return-air recirculation. The results indicate measurable cross-room dispersion under baseline operation and show that a return-side filtration scenario reduces the steady-state index in non-source rooms relative to baseline under the tested operating assumptions. These findings illustrate how measurement-informed identification can support rapid, threshold-free relative comparison of ventilation/HVAC operation or mitigation scenarios within a specific house, rather than estimating absolute infection probability. Limitations include potential non-uniqueness in inverse identification, simplified treatment of leakage and pressure-drop-induced airflow changes, and the use of a steady-state index for inherently transient residential exposures; further validation across additional houses and HVAC topologies is warranted.