Indoor air quality in schools in Beijing: Field tests, problems and recommendations

Abstract This study investigated indoor CO2 and PM2.5 real-time concentrations together with air temperature, humidity and air-cleaning effectiveness of 33 classrooms in 21 schools in Beijing, China by natural ventilation with air cleaners (NV system) compared to mechanical ventilation with fresh air ventilators (MV system). We found that: (1) The proportions of PM2.5 concentrations >37.5 μ g / m 3 when the classrooms were occupied were 28 % and 31 % in MV and NV classrooms. (2) During periods of severe ambient pollution, the averaged PM2.5 I/O ratio was 0.47. (3) The proportions of CO2 concentrations >1000 ppm when the classrooms were occupied were 40 % and 60 % in MV and NV classrooms, although we note that the maximum CO2 concentration in NV classrooms occasionally exceeded 5000 ppm. (4)The mean ventilation rate was only 1.9 L / ( s ⋅ person ) during class in NV classrooms, and the air exchange rates (AERs) for NV and MV classrooms were smaller by 58 % and 4 % during class in times of severe ambient pollution than during those days that ambient PM2.5 ≤35 μ g / m 3 . (5) During the days close to the beginning or end of central space heating, both MV and NV classrooms had indoor air temperatures lower than the 18 °C mandated by the Chinese Thermal Comfort Standard (GB/T 17225-2017). (6) Although MV systems showed lower energy efficiency than NV systems in removing PM2.5 in classrooms, the benefits of CO2 reduction and ventilation rates increase afforded by MV systems are important. These findings can help to identify or develop a suitable ventilation system (NV or MV system) and an operating strategy in classrooms in severe ambient pollution regions.