Disclosed is a respiratory treatment apparatus (100) for generating a controlled supply of ionized breathable gas. The apparatus comprises a flow generator (105), an ioniser (101-A) and a controller. The flow generator (105) generates a flow of breathable gas at a pressure above atmospheric pressure. The flow generator is adapted for connection with a patient respiratory interface. The ioniser (101-A) ionises the flow of breathable gas at a level of ionisation. The controller includes a processor which is coupled with the ioniser (101-A) and the flow generator (105). The controller is configured to control the ioniser (101-A) to programmatically change the level of ionisation of the pressurised flow of breathable gas to set the level to a plurality of different ionisation levels.
