An apparatus for humidification of air to be delivered to a patient's airways may include a reservoir, and a humidifier chamber. The humidifier chamber may include a humidifier wick and a heating element for heating the humidifier chamber. The humidifier wick may comprise a fibrous sheet material. The humidifier chamber and wick may be vertically oriented in use such that a first end of the wick is above the second end of the wick. A deioniser may be provided to deionise the liquid prior to the humidifier wick. The apparatus may pasteurise liquid to be delivered to the humidification wick.