Abstract: A robust integral method to estimate the daily mean per-person ventilation rate based on routine carbon dioxide concentration measurements in operational spaces, and limited other data, will be presented. The method 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. The findings demonstrate that several integral parameters remain reliably close to a value of unity, over large variations in room conditions and operation. Evaluating the likely distributions of integral parameters provides a method to quantify the uncertainty bounds and therefore assess the reliability of these ventilation estimates. The method deployed to UK classrooms will be presented and interpretation of the CO2 data and the inferred ventilation rate will be discussed, including statistical identification of school-wide factors which meaningfully correlate with classroom ventilation rates.