Although it is widely held that people in crowded spaces have more airways infections [15, 29] , there are few studies on this. Our study is among the first published suggesting a relationship between occupancy levels, ventilation rates, and respiratory infections. With 6 occupants instead of 3 in a 20 m 2 dorm room, the proportion of occupants with incidence of more than 6 common colds in the previous 12 months doubled. When crowdedness is adjusted for, a lower ventilation rate is associated with an increased risk of common cold. This finding is consistent with Shendell's study in schools, which showed that a 1000 ppm increase in dCO 2 (difference between indoor and outdoor CO 2 levels) was associated with a 0.5%-0.9% decrease in annual average daily attendance [30] . For office buildings, Milton found that short-term sick leave was reduced by 35% at 24 L/s per person compared to 12 L/s per person outdoor air flow [13] .