Water panel (also known as media filter, evaporator pad or humidifier pad) with AgION® antimicrobial coating which contains silver to naturally inhibit bacterial growth. (Silver is non-toxic.) Water panel is composed of layers of aluminum mesh with porous coating, which allows water absorption but not beading that would inhibit evaporation. Replace once/year or, in case of very hard water, twice/year. Coating starts to crumble when in need of replacement. Hard water may lead to calcium deposits that can clog furnace filter.