Application strategies and effectiveness of CO2 signal lights for improving indoor air quality in classrooms

Improving air quality in existing classrooms can be difficult if retrofitting a mechanical ventilation system is considered too expensive or cannot be implemented due to other reasons, e.g., heritage protection. Especially in the cold winter months, window airing initiated by pupils or teachers is often not sufficient. To address this issue, a possible approach involves the incorporation of CO₂ feedback devices (or CO₂ traffic lights) into classrooms, which serve to visually and sometimes acoustically alert occupants when CO₂ concentrations get too high, signalling the need for a higher outdoor air change rate. Different types of CO₂ feedback devices and strategies are investigated, looking at threshold values, the CO₂ sensors used, and their precision within existing studies, standards, and commercially available devices. In the scope of an ongoing Citizen Science project in ten Austrian schools, a typical CO₂ sensor light application strategy is tested in a subset of the investigated classrooms and evaluated under real-world conditions by monitoring CO₂ concentrations before and after its implementation, as well as a comparison to a control group. Analysis is done by comparing CO₂ levels at different outside temperatures and applying statistical tests such as the Welch t-Test and Mann-Whitney-U-Test. Covariances are investigated using the ANCOVA test. The comparison of CO₂ levels under different outdoor temperatures shows improvements after implementation of the visual feedback at lower ambient temperatures, suggesting that CO₂ traffic lights can be a valid intervention for classrooms. This is partly supported by the comparison to the control group, which showed unchanged air quality up to 9°C outdoor temperature when looking at a hypothetical intervention. The study concludes that the visual feedback system appears to be effective in reducing CO₂ levels in classrooms during colder periods, although long-term effects and potential behavioural changes require further investigation, including the effect on thermal comfort and energy use. Limitations include potential habituation to the equipment and the late intervention date, which affect interpretation due to seasonal differences in weather or behaviour. Future research could explore the removal of the CO₂ traffic lights after prolonged use to observe changes in behaviour in a reverse manner.