Where R A0 is the basic reproduction number; n is the number of occupants; f is the re-breathed fraction; and I is the number of infectors ( = 1). q is the quantum generation rate by an infected person (quanta/h), where a quantum is the amount of infectious material needed to produce infection in 63% of uniformly exposed animals, and is therefore 1.25 times the median infectious dose, 1.256ID 50 . t is the exposure time (h); f = (C-C 0 )/C a , where C a is the volume fraction of CO 2 added to exhaled breath, C is the volume fraction of CO 2 in indoor air, and C 0 is the volume fraction of CO 2 in outdoor air. The incidences (,6 times; 6-10 times; .10 times) and durations (,2 weeks; 2-4 weeks; .4 weeks) of common colds in the previous 12 months for different occupancy levels (6-people; 4people; 3-people per dorm) were self-reported by occupants. The mean duration of a common cold is 7-10 days [1] . For this study we assumed that the duration of a common cold was 9 days. Although many viruses can produce symptoms of common cold, rhinovirus is the most frequent cause of the common cold [24] . Riley and Nardell suggested that q for rhinovirus is in the range of 1-10/h [25] . Here we inferred q = 9/h. We assumed that the infector remained in the dorm room 8 hours per day. The average CO 2 concentrations in each dorm room from 1:00 a.m. to 8:00 a.m. were calculated. The estimated and self-reported number of common colds in each day in winter is compared (Table 3) . These two numbers fit very well indicating the validity of this CO 2 -based risk model in predicting infection rate of infectious disease like common cold.