The invention relates to a method for operating a ventilation system for indoor air in an indoor space, in particular a motor vehicle. The method comprises the steps of: providing a fresh air portion of supplied fresh air and a recirculated air volume flow according to a predetermined or selected operating strategy; determining settings for a recirculated air actuator for adjusting a fresh air portion, in particular a recirculated air flap or a recirculated air valve, and a fan power of a fan for adjusting a mixed air volume flow (QM) of supplied recirculated and fresh air; increasing the fresh air portion and/or the mixed air volume flow (QM) as a function of a pathogen filtration efficiency of a cabin filter and a predetermined contamination prevention volume flow as a pathogenic indoor concentration limit value (CP) for a maximum concentration of pathogenic aerosols.