During the corona-19 pandemic many activities inside buildings could no longer take place to prevent virus transmission. In order to allow cultural and recreational sectors to reopen in a safe way, a ventilation task force of the Belgian government prepared recommendations for the implementation and monitoring of indoor air quality in the context of COVID-19. This plan served as an instrument for building owners or facility managers to evaluate whether existing ventilation, in combination with other measures such as opening of windows or air cleaning, would provide sufficient ventilation to allow a certain number of occupants in a room. In preparation of the resumption of indoor sports activities, a research consortium investigated the consequences of the federal guidelines for sports infrastructures. To this end, sports federations organized a number of test events in June 2021 in fitness centres, a climbing gym and a sports hall, for varying numbers of athletes and audiences. In preparation of the events, the ventilation systems were inspected and ventilation flow rates measured. During the test events, CO2 measurements were carried out, and the concentrations were permanently logged. This paper discusses the results of the inspections, IAQ monitoring and subsequent analysis. By applying the recommendations of the implementation plan to the test events in sport, the paper further discusses the feasibility of implementing the plan in practice, what the consequences are for the maximum occupation in sports halls, and provides guidelines on how the ventilation in existing infrastructure can be improved.