Various embodiments are described for humidifying an air stream in a duct are described. In one form the duct is part of a humidification system in a building such as a residence or commercial structure while in another form the duct is part of a standalone unit. The described embodiments use ultrasonic elements for generating a mist composed of droplets and fog and the fog is drawn or pushed by an air stream towards a duct. The duct utilizes a dispersal structure formed of one or a plurality of tubes with openings and distributed at least partly across the duct so that the air stream in the duct can entrain the fog and distribute this in manner whereby the fog can quickly dissipate and evaporate for humidification of the air stream.