<p>Indoor portable air cleaners effectively reduce indoor particle concentrations and mitigate associated health risks in built environments. Moreover, research on the influence of outdoor contaminants on human health, especially from wildfires, is becoming increasingly important. Nonetheless, only a limited number of studies have conducted the impact of outdoor particle infiltration on removal performance of portable air cleaners. Previous studies have highlighted that the efficiency of portable air cleaners in removing indoor particles depends on various factors, including placement. Thus, this study numerically evaluated the impact of airtightness and the placement of a portable air cleaner on particle removal performance and occupants’ exposure in an elementary school classroom. We conducted transient simulations to analyze the spatiotemporal distributions of indoor contaminant concentration while varying the classroom’s airtightness levels and the placements of a portable air cleaner. The results reveal that the breathing zone concentrations in the airtight classrooms with a portable air cleaner in operation are up to 63.4% lower than those in leaky classrooms. In addition, air cleaner effectiveness varies by approximately 10% depending on the placements of the portable air cleaner, indicating that placement does not strongly affect the overall removal performance for the entire classroom. However, a properly positioned portable air cleaner with enhanced airtightness can lead to a reduction of up to 40.3% in 30-min integrated occupants’ exposures, resulting in more uniform and lower indoor contaminant concentration distributions. Consequently, our results emphasize that improving airtightness contributes to enhancing stable portable air cleaner performance in classrooms. </p> Graphical Abstract <p></p>