I live in Tokyo, a city said to have reasonably clean air.

My apartment constantly pulls in outside air through a filter, circulates it through all of the rooms, and exhausts it out.

This is a very good thing (and legally mandated here)! It prevents mildew and buildup of questionable gases, and it gives me an opportunity to filter the air that I breathe.

This is what my filters look like after just two weeks of use at this time of year.

BTW, they're not washable.

Our air isn't clean enough.