A mask (10) for use with a system for supplying breathable gas pressurized above atmospheric pressure to a human or animal's airways. The mask (10) includes a mask shell (12) which is, in use, in fluid communication with a gas supply conduit and a gas washout vent assembly (20). The gas washout vent assembly (20) includes at least one gas washout orifice (22) extending from a first side of the vent assembly (20) positioned, in use, adjacent the human or animal's face and a second side positioned, in use, adjacent the atmosphere. The cross-sectional area of the orifice (22) at the first side is larger than the cross-sectional area of the orifice (22) at the second side.
