2021 No https://onlinelibrary.wiley.com/doi/10.1111/ina.12725 Che, Wenwei Asia Review Elementary (K-5) Middle/Jr High (6-8) High School (9-12) Fine particulate matter (PM 2.5) Indoor air quality Nitrogen dioxide (NO2) Ozone (O3) Respirable particulate matter (PM 10) School exposure Ten factors were significantly associated with indoor microenvironmental concentrations, together accounting for 74%, 61%, 46%, and 38% of variations observed for PM 2.5, PM10, O3, and NO2 microenvironmental concentrations, respectively. Outdoor concentration is the single largest predictor for indoor concentrations. Infiltrated outdoor air pollution contributes to 90%, 70%, 75%, and 50% of PM 2.5, PM 10, O3, and NO2 microenvironmental concentrations, respectively, in classrooms during school hours.