In our study, old buildings had more dampness problems, while new buildings using modern construction technologies had smaller ventilation rates [21] . Dampness problems have been reported to be associated with an increased incidence of common cold infections [18] . In order to eliminate the influence of indoor environmental factors other than poor ventilation, the mean ventilation rates in newly constructed dorm buildings were calculated and related to the percentage of occupants with common cold infections more than 6 times annually. The ventilation rates in winter are less than those in summer, and may help nail down the critical ventilation rate, below which common cold incidence increases. Figure 4 shows that the infection rate of common colds in the ''tight'' buildings constructed after 1993 is, in winter, associated with mean ventilation rate. There were 7 buildings constructed after 1993. One building was not included in the analysis because measurements were performed in only 9 dorm rooms. On average, there were 1140 occupants in each dorm building. A mean ventilation rate of 5 L/ (sNperson) was associated with $6 common colds per year in 5% of occupants , compared to a 35% for 1 L/(sNperson). There were 6 buildings constructed before 1993, among which 4 buildings had ,10 dorm rooms measured in winter and were excluded from the analysis. Of the remaining 2 buildings, one had mean ventilation rate of 5.7 L/(sNperson) and a common cold infection rate of 23.8%, while the other had a mean ventilation rate of 6.4 L/ (sNperson) and a common cold infection rate of 7.1%.