Improving indoor air quality has been proposed as a strategy to reduce respiratory illness transmission, particularly in settings with young children. This study evaluated whether portable air filtration was associated with changes in reported respiratory illness prevalence at the UWEC Children’s Nature Academy. HEPA air filters and a Corsi-Rosenthal box were installed in classrooms following ventilation assessment using CO2 monitoring. Filter efficacy was tested by measuring PM2.5 air concentration reduction. Anonymized illness reports from three academic years (2022–2025) were used to calculate monthly respiratory illness prevalence before and after filter installation in November 2023. Interrupted time series (ITS) regression and ANOVA were used to evaluate changes in illness trends. While filtration reduced PM2.5 concentration and ventilation levels were adequate, no significant reduction in reported respiratory illness prevalence was observed. Illness patterns closely followed seasonal community respiratory disease trends.
