The classrooms in Wing A selected for the present experiments were occupied by pupils from the 4th and the 5th grade, i.e. 10e11 years old pupils. The ventilation in these classrooms is by automatically operated windows, each classroom being equipped with three automatically operated windows at the upper level (Fig. 2a).An exhaust fan is mounted in the middle of the ceiling in each classroom to provide additional ventilation when natural ventilation is inadequate. Each classroom has two windows in the lower part of a window section and one garden door. They can be opened manually by children and teachers if needed and face schoolyard.The operation of the automatic windows and the exhaust fan is controlled with a specially developed system. The prime control is provided by sensors located centrally in the classroom, which measure classroom temperature and CO2 concentration. The feedback from the sensors controls the degree of window opening and the fan speed.