The GeneralAire 45-3 Evaporator Sleeve is designed to function as a sponge for model 45 humidifers. The Evaporator Sleeve absorbs water as the motor spins and creates moisture for the dry air in your home. Dry air in the home can cause allergies, dry nose, and itchy skin. This Evaporator Sleeve is constructed out of aluminum that's durable enough to last over the course of the heating season. This Evaporator Sleeve has to be replaced each year.