The Honeywell WF-813 is the same as the HAC-506 Humidifier Filter.
The Honeywell WF-813 Humidifier Filter for Honeywell HCM-525 Cool Moisture Humidifiers (it's the same filter as the HAC-506) effectively removes minerals and pollutants from water, helping keep your humidifier clean. It features Agion Antimicrobial Treatment to help prevent the growth of mold, fungus, algae and bacteria on the filter.