The evolution of the building sector, with increasing airtightness and the reduction of leakages in the envelopes, has made the presence of mechanical ventilation systems in indoor spaces growing in importance and becoming essential for fulfilling indoor air quality requirements. With traditional ventilation systems, decentralized solutions have recently appeared in the market as they can fulfil the requirements at cheaper costs because of their compactness and lower installation costs. Therefore, they can be a good solution in building refurbishment as well as in new constructions. The concept of these systems deals with the air change in the room where they are installed, with air intake and outlet near each other without the presence of ducting, in contrast to traditional centralized systems where supply and extract air vents are placed in different rooms to enhance uniform air change within the building. The proximity of the air vents is one of the most debated points concerning these systems, as possible air short-circuits can reduce the ventilation efficiency in removing contaminants. In this paper, insight into the topic of ventilation effectiveness is provided. The existing literature on this topic is analysed, and its findings are then used to study the effectiveness of a decentralized ventilation system. A measurement campaign was carried out in a real office building based on the generation and detection of the CO2 concentration in different room positions, aimed at investigating whether the decentralized ventilation system is suitable for fulfilling a good indoor air quality level in similar rooms.