Influence of airflow and the location of infected individuals on occupant exposure in classrooms without mechanical ventilation during the winter

Winter poses a high risk for spreading infectious respiratory diseases, particularly in classrooms, which are known hotspots for cross-infection. The health hazards in classrooms lacking mechanical ventilation systems often go unnoticed. To address this issue, we studied the risk of respiratory infection transmission in such environments during winter by assessing the spread of contaminants using computational fluid dynamics (CFD). We evaluated four common airflow setups (split-type air conditioner, open door, open window, and both door and window open) in classrooms without mechanical ventilation. Our findings indicate that while split air conditioners provide optimal thermal comfort, they significantly increase exposure risk. Conversely, simply opening a door can effectively balance thermal comfort with a reduced exposure risk—particularly when the infected individual is near the door, leading to a minimal 0.01% average intake fraction. Furthermore, under varying ventilation scenarios, the sensitivity of exposure risk to location changes of infected individuals differs significantly. Specifically, when a split-type air conditioner is used, occupant exposure is largely unaffected by changes in the location of the infected individual. However, the exposure risk becomes highly sensitive to location changes of infected individuals when a door or window is used for ventilation. Strategic positioning of the infected individual can decrease indoor exposure risk by up to 94% with the door open and 67% with the window open. Additionally, when the door and window are both open, the dependency of occupant exposure on the location of the infected individual decreases. In this case, the exposure risk for indoor occupants is low, regardless of the position of the infected individual.