Improving indoor air quality in schools: Evidence from an air purifier intervention with low-cost sensors

Poor air quality disproportionately affects children's health, causing both clinical and subclinical effects including respiratory infections, asthma, allergies, absenteeism, and cognitive impairment. This challenge is particularly acute in developing countries and urban areas, where air pollution frequently exceeds the World Health Organization's global air quality guidelines. Leveraging innovative low-cost sensor technology, we designed a cluster randomized control trial to evaluate the effectiveness and economic feasibility of classroom portable air purification systems, while simultaneously developing a framework for continuous air quality monitoring in educational settings. We randomly assigned 95 classrooms (~2000 students) across five primary schools in Milan, Italy, to treatment and control groups, implementing a comprehensive monitoring system that integrated indoor and outdoor air quality sensors with health outcome data. Our sensor network collected continuous measurements of air quality parameters while enabling real-time data analysis and integration with survey data on health symptoms and air quality perception. Results demonstrate that air purifiers reduced indoor air pollution by 28%, corresponding to an 11% reduction in student absences. The impact was most pronounced among vulnerable students with higher pre-treatment absences and those of non-Italian nationality. Notably, the purifiers' effectiveness showed an inverse relationship with outdoor pollution levels, suggesting limitations in their ability to maintain healthy indoor air quality during severe pollution events. Our intervention also revealed improved self-reported respiratory health, enhanced awareness of air quality issues, and increased support for urban air quality policies among treated students. This study not only demonstrates the cost-effectiveness of school-based air purifiers but also establishes a replicable framework for implementing and evaluating air quality interventions in resource-constrained educational settings using affordable sensor technologies.