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, a surface of the diffusing material facing the first surface being spaced away from the first surface 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 further comprises a third opening in communication with ambient atmosphere; the third opening does not overlap with an area of an outlet of any of the passages that is projected along a central axis of the respective passages, and is located so that at least part of the diffusing material is between each first opening and the third opening; 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; and the diffusing material is fibrous material, open-celled foam, moisture wicking material, sintered plastic, or hydrophobic 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, a surface of the diffusing material facing the first surface being spaced away from the first surface 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 further comprises a third opening in communication with ambient atmosphere; the third opening does not overlap with an area of an outlet of any of the passages that is projected along a central axis of the respective passages, and is located so that at least part of the diffusing material is between each first opening and the third opening; 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; and the diffusing material is fibrous material, open-celled foam, moisture wicking material, sintered plastic, or hydrophobic material.