The effectiveness of a positive pressure fresh air system (PPS) versus a standalone air purifier was compared in a 32m2 room. The PPS takes air from outside, filters it and then pumps it into the room. The comparison was made between similar specifications on both the systems, with the purifier being slightly more powerful in terms of airflow. When the PPS was switched off, polluted air entered the room through small air gaps, which increased until it reached equilibrium with the purifier. The standalone purifier was able to reduce the ambient PM 2.5 values by about 75%. The PPS system had a permanent effect of keeping PM2.5 values at zero and preventing dirty air from entering the room.
https://www.airgradient.com/open-airgradient/blog/air-purifier-vs-positive-pressure-fresh-air-system-an-unfair-battle/