The out-to indoor air flow rate required by the Indoor Air Quality Standard of China is 8.3 L/s per person [28] . In the present study, 90% of the dorm rooms measured during winter had night-time ventilation rates less than this value. CO 2 concentration in corridors was not measured, so that the fresh out-to indoor air flow rate may have been even lower than the calculated value in cases when corridor windows were closed.