Five classrooms in Hong Kong (HK), air-conditioned or ceiling fans ventilated, were chosen for investigation of
indoor and outdoor air quality. Parameters such as temperature, relative humidity (RH), carbon dioxide (CO2), sulphur
dioxide (SO2), nitric oxide (NO), nitrogen dioxide (NO2), respirable particulate matter (PM10), formaldehyde (HCHO),
and total bacteria counts were monitored indoors and outdoors simultaneously. The average respirable particulate
matter concentrations were higher than the HK Objective, and the maximum indoor PM10 level exceeded 1000 lg/m3.
Indoor CO2 concentrations often exceeded 1000 ll/l in air-conditioning and ceiling fan classrooms, indicating inadequate
ventilation. Maximum indoor CO2 level reached 5900 ll/l during class at the classroom with cooling tower
ventilation. Increasing the rate of ventilation or implementation of breaks between classes is recommended to alleviate
the high CO2 level. Other pollution parameters measured in this study complied with the standards. The two most
important classroom air quality problems in Hong Kong were PM10 and CO2 levels.