The Vornado Evap40 4-gallon Evaporative Humidifier delivers a perfectly humidified environment for people to breathe easier and avoid the negative effects of dry air. Efficient blades pull in large volumes of air, then the air is filtered and moisturized air is released through your entire room. The whole-room humidification system works in rooms up to 1,000 square feet. The 4-gallon capacity makes it only necessary to refill the reservoir every 24 hours.