The scientific community has been aware of the importance of indoor air quality (IAQ) for many decades, but the COVID-19 pandemic has brought a significantly higher level of attention from the general public and governmental entities to this theme. However, IAQ comprises hundreds of other parameters besides infectious pathogens, many of which can equally impact the health, comfort and well-being of occupants. In this context, an intervention study was conducted in Flanders (Belgium) with the aim of investigating the potential impact of ventilation and air cleaning on the IAQ, comfort and infection risk control in Flemish public spaces. This paper describes part of this study, focusing on the IAQ assessments carried out in four daycare facilities for infants in the province of Antwerp. The two first facilities were assessed simultaneously in March 2022, while the two last ones were assessed simultaneously in September 2022. At each facility, CO₂ concentration, different size fractions of particulate matter (PMx) concentration, temperature and relative humidity (RH) were continuously monitored in selected indoor spaces and one outdoor site for 2 consecutive weeks. Average ventilation rates were measured in each facility under different airing scenarios. Biological air samples were also collected 2 days per week, in the same spaces at each facility, for in-lab qPCR analysis of over 20 genetic markers of respiratory pathogens. Results generally highlighted the positive impact of efficient ventilation on IAQ, while the effects of air cleaning were not as prominent in each room. CO₂ concentrations up to 4200 ppm were measured in the facilities without mechanical ventilation, while they remained consistently below 800 ppm in the facility with the most effective mechanical ventilation system. SARS-CoV-2 was detected more frequently and in larger quantities in the facilities with lower ventilation rates. The variety of other pathogens was also higher in these less-ventilated facilities. The effectiveness of air cleaners ...