During the COVID-19 pandemic, we surveyed some classroom environments of three elementary and junior high schools in Sendai. The CO2 concentrations were maintained much lower than pre-pandemic levels by opening windows; however in two schools, they exceeded 1000 ppm for more than 50% of school hours in January. Since the installation of new ventilation fans in one of the schools, the percentage of CO2 concentrations exceeding 1000 ppm has been reduced to 8.5%. Because of the ventilation path formed through the hall and increased heating usage, the temperature in that school remained nearly constant.
