Fresh air systems, or HVAC systems that have a fresh air intake can help improve indoor air quality in certain circumstances. When indoor air has high levels of VOCs or gas build up, for example from recent remodelling, smoking, cooking etc. then fresh air systems can help by bringing in new, cleaner air from outside to dilute the pollutants.
However, fresh air systems or fresh air intakes may have a negative effect when outdoor air is more polluted than indoor air. This is common in highly polluted areas, or for buildings close to major pollution sources (such as roads, factories etc.). In this situation, using a fresh air system may in fact bring polluted air into the room, causing the indoor air quality levels to deteriorate. In this situation, a filtered fresh air system, that first filters particulate (PM2.5, PM10 etc.) from the air can be used.