There is provided a mask for delivery of a medical gas to a patient comprising a mask body having a rim for contacting the patients face and an interior space configured to surround and enclose the patients nose and mouth. The mask includes a gas diffuser structure projecting through said mask body into said interior space. The gas diffuser structure includes a support, a gas delivery tube within said support having an internal bore the axis of which is generally vertical when said mask body is upright, an inlet in said gas tube for communication with a source of pressurized gas, and an outlet from said gas tube within said interior space. The structure includes a gas flow disrupter or baffle located over the outlet of said bore in the path of gas exiting said bore configured to diffuse and/or redirect said gas radially outwardly. A flow of gas entering the mask forms a plume of enriched gas at the patients nose and mouth region, thereby permitting a relatively high oxygen content to be delivered to the patient at a lower flow rate.
