2021 No https://doi.org/10.1016/j.buildenv.2021.108450 Choe, Youngtae Asia Experimental Exposure Study Students Elementary (K-5) Middle/Jr High (6-8) High School (9-12) Carbon Dioxide (CO2) Particulate Matter (PM) Cognative function Stress/Anxiety School particulate matter Carbon dioxide (CO2) Indoor air quality Perceived air quality Air purifier Measured PM10, PM2.5, CO2 in Korean classrooms using IoT sensors before/after air purifier use. PM2.5 fell ~30%, but CO2 exceeded 1,000 ppm, showing poor ventilation. Students reported stronger effects from CO2 than PM. Purifiers alone are inadequate for IAQ.