D i is the assumed number of common cold infections in winter under different self-reported incidence rate, times. i indicates common cold incidence. i = 1, 2, 3. 1common cold less than 6 times in the previous 12 months; 2-common cold 6-10 times; 3-comon cold more than 10 times. We assume D 1 = 3; D 2 = 6; D 3 = 8. O j is the occupancy level, person/room. j indicates occupancy level. j = 3, 4, 6. 3-three people per dorm room; 4-four people per dorm room; 6-six people per dorm room. D i,j is the proportion of students with different self-reported common cold incidences, %. M is the duration of a common cold, days. We assume M = 9 days [1] . T is days in winter season, 120 days. C j is the average CO 2 concentration from 1:00 a.m. to 8:00 a.m. in rooms with different occupancy levels, ppm. f j is the re-breathed fraction of indoor air in rooms with different occupancy levels. f j = (C j -C 0 )/C a . C a is the volume fraction of CO 2 added to exhaled breath, 37000 ppm. C 0 is the volume fraction of CO 2 in outdoor air, 300 ppm. q is the quantum generation rate by an infected person, quanta/h. We assume q = 9 quanta/h [25] . t is the time a infector remaining in the dorm room, hour/day. We assume t = 8 hours per day. doi:10.1371/journal.pone.0027140.t003 dorm buildings. In summer, the mean outdoor air temperature was 29.6uC, ranging from 22.5uC to 35.2uC. The median air change rate was 4.42 h 21 and 4.67 h 21 when outdoor air temperature is below and above 29.6uC. There was no significant difference of air change rate for different temperatures in summer (p = 0.319). Therefore, it is reasonable to assume that, the air change rates measured in summer and winter are representative for respective season, without influence from small climate changes within each period.