Devices and systems provide methods of controlling breathable gas generation such as for a respiratory treatment and/or for controlling ionization of the gas. In an example, a controller of a respiratory treatment apparatus controls generation of a supply of ionized air. The apparatus may include a flow generator to generate a flow of pressurized breathable gas. The flow generator may be adapted for connection with a respiratory interface. The apparatus may also include an ionizer to ionize the flow of gas. The controller may be coupled with the ionizer and the flow generator and be configured to control the ionizer to programmatically change levels of ionization of the gas. Such ionized gas treatments may be suitable for helping users to sleep or improving respiratory oxygen absorption, and may be for patients with, for example, sleep disordered breathing or chronic obstructive pulmonary disease.