Investigating and understanding CO2 levels in school classrooms

 It is a reported fact that a high CO2 concentration is a problem in school classrooms. However, the mere reporting of such results stops short of investigating causes; understanding is often missing. Steady-state results are often used in situations where changes occur frequently, such as varying student numbers, opening and closing classroom doors and windows and changing weather conditions. We revisit the mass balance model commonly used to predict or track CO2 concentrations in enclosed spaces as these factors change over time under varying conditions. This has prompted the study in several classrooms of actual air exchange rates, student exhalation rates, room volumes and ventilation design. In these cases, student numbers, room ventilation conditions (open and closed doors), room volume and the CO2 concentration have been recorded throughout the day. By fitting the model equation to the data, unknown parameters such as actual air change rates and CO2 exhalation rates per student can be determined. Having verified that the data can be modelled, we can predict behaviour in other cases such as a realistic rate of CO2 increase. This allows designers to size classrooms and ventilation systems to achieve a desired CO2 characteristic for known usages while saving energy.