Assessing classroom ventilation rates using CO2 data from a nationwide study of UK schools and identifying school-wide correlation factors

Per-person ventilation rates were assessed from data measured in 322 UK schools throughout the Autumn term, 2023. It was found that the overall mean ventilation rate was 5.3 L s−1 p−1; rising to 6.8 L s−1 p−1 during warmer weather and falling to 3.8 L s−1 p−1 during colder weather. Daily mean CO2 levels recorded in the majority of schools consistently fell below the 1500 ppm threshold within government school ventilation guidance but the contribution of relatively few very high CO2 levels in some classrooms shows the need for targeted improvement and highlights the value of large-scale datasets. Exceedance of the 1500 ppm threshold rose to approximately 20% when the daily mean outdoor temperature fell to around 5 °C, indicating a barrier to ventilation. Controlling for outdoor temperature as a covariate, analysis of the CO2 measurements indicated schools in more-deprived regions, state-funded schools (relative to private), and secondary schools (relative to primary) exhibited statistically-significant higher CO2 concentration. Reassuringly, these findings remained unchanged when the analysis was deployed on estimated per-person ventilation rates; except that this enhanced analysis accounted for the greater CO2 production associated with older children and demonstrated that ventilation rates were not significantly different between primary and secondary schools.