Size of opened window and resultant air change rate were measured in occupied classrooms of elementary schools in Kanto area. It was found that 2.2 air change rate requirement in the School Environment Hygienic Standard could not be met in case that doors and windows are closed in winter season and with air-conditioner operated in summer season. Opening of windows at corridor was found effective in improving air quality in corridor and classroom with keeping indoor thermal environment comfortable in winter season. Air change rate was well correlated with the size of opening, although it was nearly independent of outside air velocity and inside-outside temperature difference.
