A gas washout vent for a patient interface system configured to maintain a therapy pressure in a range of about 4 cmH2O to about 30 cmH2O above ambient air pressure in use, throughout a patient's respiratory cycle, while the patient is sleeping, to ameliorate a respiratory or a sleep disordered breathing condition, the gas washout vent comprising: a housing comprising a first wall with one or more passages through the first wall, the one or more passages being configured to provide fluid communication with a portion of the patient interface system that is configured to be exposed to the therapy pressure, the one or more passages each including a respective first opening on a first surface of the first wall, the housing at least partially defining a second opening that is in communication with ambient atmosphere; and a diffusing material located at least partially within the housing to be adjacent the first surface, wherein the diffusing material is spaced away from the first wall by a gap that extends to provide fluid communication between all of the first openings, as well as between all of the first openings and the second opening, wherein the housing is configured so that air is prevented from flowing out of the housing at all areas directly opposite each of the first openings, characterized in that the second opening is spaced away from the first surface in a direction perpendicular to the first surface, wherein the diffusing material is a material allowing at least partial penetration by an air flow and providing a tortious path for the air flow, wherein a surface of the diffusing material facing the first surface is spaced away from the first surface by a first portion of the gap, wherein the gap comprises a second portion around a periphery of the diffusing material, so that air can flow into the surface of the diffusing material facing the first surface and out of a lateral surface of the diffusing material before flowing out through the second opening, and wherein air can also flow through the first portion of the gap and the second portion of the gap without passing through the diffusing material. a housing comprising a first wall with one or more passages through the first wall, the one or more passages being configured to provide fluid communication with a portion of the patient interface system that is configured to be exposed to the therapy pressure, the one or more passages each including a respective first opening on a first surface of the first wall, the housing at least partially defining a second opening that is in communication with ambient atmosphere; and a diffusing material located at least partially within the housing to be adjacent the first surface, wherein the diffusing material is spaced away from the first wall by a gap that extends to provide fluid communication between all of the first openings, as well as between all of the first openings and the second opening, wherein the housing is configured so that air is prevented from flowing out of the housing at all areas directly opposite each of the first openings, characterized in that the second opening is spaced away from the first surface in a direction perpendicular to the first surface, wherein the diffusing material is a material allowing at least partial penetration by an air flow and providing a tortious path for the air flow, wherein a surface of the diffusing material facing the first surface is spaced away from the first surface by a first portion of the gap, wherein the gap comprises a second portion around a periphery of the diffusing material, so that air can flow into the surface of the diffusing material facing the first surface and out of a lateral surface of the diffusing material before flowing out through the second opening, and wherein air can also flow through the first portion of the gap and the second portion of the gap without passing through the diffusing material.