Outdoor air is usually considered as a source of clean air in building ventilation principles. Although outdoor air quality has already improved in our cities, this principle may be challenged. Particulate matter remains especially problematic. This simulation study investigates the role that the mechanical ventilation system, with or without filtration, plays in the penetration of outdoor air pollutants, which may have adverse effects on indoor air quality and occupant health. Based on the Brussels PM2.5 pollutant data, several configurations were examined using the CONTAM software. The influence of the airtightness of the building, the filter efficiency and the airflow rate of the mechanical ventilation system on the indoor air pollutant concentration were highlighted for a simple model, i.e. a single room housing, and afterwards extended to a complex model, i.e. a semi-detached house with several rooms. This influence was evaluated in terms of a total efficiency, taking into account filter effect and building envelope, as well as in terms of the occupant exposure to outdoor pollutants. Ventilation strategies based on outdoor pollutant concentration were implemented and assessed through exceedance days linked to World Health Organization (WHO) guidelines. On this basis, recommendations for managing the ventilation system in case of high concentrations of outdoor air pollutants can be discussed.