Our multi-room apartment in Redmond, WA, has an always running intake fan in the master bathroom advertised as a way to keep air fresh. I always wonder how effective that really is (at apartment scale) and if it's really worth the 24/7 power draw.
Almost certainly designed to ASHRAE 62.2's specs so that you get enough ventilation through a combination of infiltration (leakage) and additional mechanical ventilation. It's certainly worth it and if designed as a continuous ventilation fan should be one of the lower (~15W) draw units. Not sure how to really measure effectiveness other than by trusting in the experts who came up with the spec and air quality as an indirect measure.
It's to keep positive pressure in your unit for safety purposes. If there is a fire the fresh air will help keep smoke out of your unit and as a bonus it keeps other building odors out. Generally it is a large fan on the roof that just sucks air down a stack pumping it into every unit. You could put a filter over the register and filter that air.
Sorry, by intake I meant it's sucking air in. That is, air is leaving my apartment. I think you're on the right track with the safety angle here though. Will do more research.
