Mitigating indoor risk of airborne infections using CO2 and PM measurements in university classroom: the MIRAI project

Background: Indoor air quality (IAQ) impacts well-being and the spread of airborne diseases, particularly in multi-occupancy environments like classrooms. Carbon dioxide (CO2) and particulate matter (PM) levels have been used in continuous monitoring as indicators of indoor air quality. Methods: We used cost-effective sensors to assess levels of CO2 and PM in two Italian university classrooms (Unit 1 and Unit 2), differing in size and ventilation, from March 2022 to May 2023. For CO2 and PM monitoring we used respectively ARANET4 sensors and Optical Particle Counters (OPC) assessing aerosol diameter size distribution between 350 nm and 40 μm in order to evaluate trends in IAQ and the influence of environmental factors including ventilation, occupancy, and season. Results: The naturally ventilated classroom (Unit 1) exhibited higher CO2 and PM concentrations with greater variability, whereas the HVAC-supported one (Unit 2) maintained more consistent air quality but faced occasional spikes. Occupancy also affected CO2 and PM levels, with higher variability in Unit 1 characterized by lower size and generally full occupancy. Seasonal trends highlighted increased PM levels during colder months due to reduced ventilation in both units. Conclusions: In this feasibility study, variations in building design, ventilation strategies, and occupancy dynamics were associated with corresponding patterns in IAQ, supporting the use of low-cost, user-friendly sensors as practical tools to characterize ventilation performance and to inform interventions aimed at safer learning spaces. Implementation of CO2 and PM measurements as proxy indicators of ventilation adequacy and of potential airborne infection risk, along with outreach programs and guidelines to educate teachers and students about relevance of IAQ assessment is recommended to promote occupant health and mitigate risk of airborne disease transmission.