However, common cold infections were not significantly associated with indoor air temperature (p = 0.806) and relative humidity (p = 0.642). Figure 3 shows that the lowest quartile of out-to indoor air flow rates per person in both summer and winter were associated with an increased proportion of occupants with $6 common colds in the previous 12 months. The adjusted odds ratios of ventilation rates for common cold infections increased slightly across the quartiles. The critical ventilation rate, below which common cold incidence increases, is identified. When ventilation rate is below 6 L/s per person, the common cold incidence in dorm rooms with average 4 occupants increased from 10% to 12%. When ventilation rate is below 1 L/s per person, the common cold incidence increased from 10% to 15%.