Mitigating aerosol infection risk in school buildings: the role of natural ventilation, volume, occupancy and CO2 monitoring

Issues linked to aerosol physics within school buildings and related infection risk still lack a proper recognition in school safety regulations. Limited spaces and limited available window-surfaces require to precisely investigate the seasonal airing factors and the occupancy/volume ratios in each classroom in order to assess the specific risk levels from viral loads of potentially infective sources. Moreover, most schools are still not provided with mechanical HVAC systems nor with air quality sensors. Fundamental questions are therefore: how the specific classroom volume and the specific airing cycle affects the long-range contagion risk in a given classroom? is linear social distancing the right way to assess a volumetric risk problem? We present here the results of an extended quantitative analysis based on the GN-Riley infection risk model applied to a real classroom scenario. The study discusses seasonality of the airing flow and the effectiveness of single and combined mitigation interventions, such as limiting student groups, equipping teachers with microphones, increasing classroom volumes, and equipping classrooms with CO2 sensors to safely drive airing intervals. Moreover, we show experimental CO2 concentrations as well as occupancy and airing factors monitored in real time in a real classroom scenario. In agreement with recent literature, the results emphasize the need for a dynamic evaluation of the complex risk function over the whole exposure time (and not just the monitoring of the istanteneous CO2 concentration) in order to correctly control the infection risk from aerosolization.