While it is possible that some of the self-reported common colds were influenza, the infection rate of flu among adults is approximately once per year in this part of China. Therefore, this possible error would not change our results. Moreover, common colds and influenza are spread in a similar way; the present study could have been titled ''airways infections''. In each dorm room, CO 2 concentrations were measured for 24 hours in both summer and winter. As measurements were made over a long period, i.e. summer measurements between May and July and winter measurements between December and April, and for 238 rooms, the mean values of ventilation rates should be valid for rooms with different occupancies and opening status of windows/ doors, and for changes in the outdoor climate.