Although existing standards typically prescribe fixed ventilation rates, a large portion of the building stock lacks mechanical ventilation systems. Such buildings obtain outdoor air through unintentional infiltration, open windows, and the operation of exhaust fans, which are mainly dependent on user awareness and behavior. This study investigates whether the presence of a simple CO2 meter display can alter user behavior and improve indoor air quality (IAQ) in dwellings. We conducted a two-week monitoring of CO2 concentration, air temperature, and relative humidity in 60 student dwellings (bedrooms and living rooms) in Denmark and Switzerland. During the first week, the CO2 display was concealed, and occupants adhered to their usual routines. In the second week, occupants used the display's visual feedback to enhance IAQ when necessary. Results revealed that over 95% of dwellings witnessed a reduction in median CO2 levels during the second week, both overall and in individual rooms. Furthermore, a descriptive analysis of CO2 concentration step changes exceeding the thresholds of "normal variation" showed an increase in the number of negative step changes, indicating a shift in user behavior. These findings underscore the efficacy ofa display interface providing information on the indoor environment in triggering behavior changes and improving IAQ in dwellings.