Optimizing ventilation cycles to control airborne transmission risk of SARS-CoV2 in school classrooms

Open schools in winter in highly epidemic areas pose a controversial issue: ventilation of classrooms (an essential mitigation factor for airborne transmission) is expected to sensibly decrease due to outdoor temperatures getting colder and regulators going to allow less restrictive policies on windows closure. Fundamental questions to be addressed are therefore: to which extent can we contain airborne transmission risk in schools? what would be the optimal ventilation strategy during the cold season considering the fact that most schools are not provided with mechanical ventilation systems? To try answering these questions a risk model for airborne transmission of covid-19 in classrooms has been develped based on previous models for tubercolosis and influenza. The separate cases of infective student and infective teacher, as well as infective teacher with microphone are investigated. We explored 3500 different air ventilation cycles for different lesson+break times and carried out a numerical optimization of the risk function. Safety risk-zones for breaks and lessons durations were estimated combining the effect of surgical masks and optimal windows opening cycles.