Quantifying ventilation rates in heterogeneous rooms based on point measurements of carbon dioxide

Indoor air quality (IAQ) is increasingly accepted as a leading factor in human health, and the ventilation of our indoor spaces is a key modifier of IAQ as the principal means by which indoor pollutants are diluted. Knowledge of the ventilation rate is essential for understanding and modelling our indoor environment, yet quantifying the ventilation rate for regular operational spaces remains a challenge. Measurements of carbon dioxide (CO2) concentrations indoors are established as a reasonable general indicator of IAQ in occupied spaces and are the basis of many regulations and standards, e.g. ASHRAE (2022). We present a robust method to better exploit point CO2 measurements to estimate the daily mean per-person ventilation rate, requiring very limited other contextual information.  
Current widely applied methods rely on special-case solutions to the CO2 mass balance equation. These methods include: the decay rate, for which the space must be unoccupied, and the ventilation rate is assumed constant during the decay; and the steady-state method, for which the balance between the source and ventilation must remain constant for a sufficiently long time so concentration becomes constant. Both methods further assume the room to be well-mixed. The present method overcomes some limitations of these methods since it makes no assumptions regarding the ventilation provision throughout the day, nor requires the room to be in a steady state, nor the air within to be well-mixed. Importantly, the method facilitates ventilation estimates in operational spaces during normal use, not requiring the room to be unoccupied. This is significant since ventilation provision can significantly differ during occupied and unoccupied periods.