Genuine Fujitsu 9315212016 Indoor ION Deodorant Filter (2 pcs) is a pair of air cleaning filters that sit behind the indoor unit's main filter and treat the air on its way through, before it goes back out into the room. Treating the air is all these filters do, so once they are dirty, torn or past their useful life the air passing them goes back into the room untreated. A fresh pair puts that stage of cleaning back to work.