In Phase II, air temperature, relative humidity and CO 2 concentration in dorm rooms were measured by indoor air quality monitor PS 31 (http://www.sensotron.pl) for 24 hours. Air quality monitors were calibrated at the International Center for Indoor Environment and Energy, Technical University of Denmark prior to measurements. Dorm occupants reported opening status of doors and windows at day and at night during measurement The out-to indoor air flow rate at night was calculated from an analysis of the build-up period of metabolic CO 2 produced by sleeping occupants (1:00 a.m.-8:00 a.m.) [19] . Calculation details are described in Information S1. CO 2 concentrations of dorm rooms were measured both in the summer (May-Jul., 2006) and in the winter (Dec., 2006-Apr., 2007) [20] . The average indoor air temperature and relative humidity at night were calculated (1:00 a.m.-8:00 a.m.). Outdoor CO 2 concentration and meteorological parameters were also measured on campus during the same time.