Abstract: A patient interface (1) is for aerosol treatment, having a base (2) to surround the patients mouth and nose and engage the skin with a resilient seal, and with a strap (8) to attach to a patients head. There is a support (3) on and across the base for supporting an aerosol delivery head with prongs (4, 5). An enclosed volume is formed in the interface by attachment of a shell (10), which has an extraction port (11) for attachment of an extraction system (20) to extract gas from this volume. An HFNT system includes a patient interface surrounding the nose and mouth and an aerosol delivery apparatus (4, 6), an extraction apparatus (20), and a controller (100) to control delivery of aerosol and/or gas to the interface and to extract gases from a volume enclosed by the interface. Because of the fully sealed volume within the interface there are a wide range of control scenarios possible, using pressure sensing in the volume, bypass valves (201), dynamically-controllable nebulizer (203).