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