One of the main drawbacks of using regular tap water to humidify a room is there can be both minerals and impurities in that water which can not only gum up a machine over time, but also get emitted in the room, to possibly spread odours, in the short term, and "white dust" residues from the minerals as the air dries out, in the longer term. This 2-in-1 filter which goes into the humidifier's tank is designed to tackle both those problems: it includes active carbon to absorb the impurities and remove the sources of odour, and also provides significant reduction of water's hardness (demineralization). It can fit all BONECO & AIR-O-SWISS Ultrasonic Humidifiers, except for the Travel model (7146), and lasts up to 6 months (a full season of use, for most consumers).
