A humidifier includes a humidifier tub including an air inlet and an air outlet. The air outlet may have an exit port positioned in a plane disposed below the air inlet. A base plate is provided to the bottom of the humidifier tub. The base plate and humidifier tub define a water chamber adapted to receive a volume of water. The humidifier tub may include a guidance structure adapted to direct air entering the humidifier tub via the air inlet to swirl downwardly and around within the humidifier tub reaching the water surface before exiting the exit port. The tub may include a structure to prevent the inadvertent entry of water from the tub to the flow generator.