The associations among gender, age, whether family member ever had asthma and allergy, environmental tobacco smoke, cleaning routine, window opening frequency, occupancy levels, and self-reported common cold incidence and duration were analyzed by Chi-square tests. Adjusted odds ratios of crowdedness and air flow rate for common cold infections were evaluated in multiple logistic regression models. A carbon dioxide-based risk equation [21] was used to calculate the basic reproductive number of common colds which was compared to the self-reported infection rate.