- In the course of the COVID-19 pandemic, indoor ventilation came into focus. In Germany, some schools were retrofitted with decentralized ventilation systems. This monitoring study compares the influence of mechanical and natural ventilation in classroom settings on indoor air quality (IAQ). Three different concepts are investigated: exclusively natural ventilation in combination with carbon dioxide (CO2) measuring devices and mechanical ventilation with decentralized air handling units (AHUs) and with a central ventilation system. This study examines eight classrooms across three elementary schools, continuously measuring CO2 concentrations and various other parameters. The evaluation of the monitoring data shows that the correct setting of ventilation systems is not a matter of course and is an important factor.