Indoor CO2 concentrations exceeded the ASHRAE standard due to overcrowded classrooms and inadequate ventilation. Implementation of more breaks and recesses between classes might alleviate the high level of CO2. The occupancy of classrooms in Hong Kong often exceeded the maximum occupancy of the ASHRAE 62-1989 standard. Decreasing the number of students in each classroom can also lower the CO2 level. Increase the rate of ventilation could remove the accumulated CO2; for example, the use of ceiling fans, exhaust fans could increase the exchange of indoor air with the outdoor.