Analysis of IAQ in classrooms during COVID-19 pandemic and the effect of window ventilation and air cleaners depending on season

• no demonstrable reduction in CO2 concentration in classrooms due to the pandemic. • causes for high CO2 concentrations were identified. • specific decay rates of PNC were determined for various ventilation scenarios. • additional use of air cleaner reduces inhaled particles for 90 min lesson by >50 %. • impact of air cleaner during window ventilation phase higher in summer than in winter. The first part of this study presents CO 2 measurements in classrooms without HVAC systems during the final phase of the COVID-19 pandemic. CO 2 concentration is a widely used proxy for indoor air quality (IAQ) and ventilation efficiency, and serves as an indicator of the potential viral aerosol load. Measurements taken during the cold season (03/2023) and warm season (06–07/2022) reveal significant variability, with CO 2 levels exceeding recommended thresholds in certain scenarios. The study identifies specific factors contributing to elevated CO 2 concentrations, which are generally associated with an increased presence of exhaled particles in the air. The second part of the study evaluates strategies to reduce particle number concentration (PNC) in classrooms, including different window ventilation approaches, the use of a large air cleaner (AP) with varying flow rates, and their combination. The effectiveness of these interventions in mitigating PNC is assessed through the introduction of DEHS particles and subsequent PNC measurements using aerosol spectrometers. The observed decay rates of PNC under various ventilation conditions demonstrate that combining AP usage with window ventilation leads to further reductions in particle concentration, irrespective of seasonal variations. The interaction between the AP and window ventilation is found to be influenced by external conditions and ventilation type, rather than by variations in the ventilation area. Furthermore, the study discusses the potential inhalation of virus-laden particles during a simulated 90 min school lesson, comparing exposure levels across different ventilation scenarios.