A gas-washout vent includes a housing with a first wall with one or more passages through the wall configured to provide fluid communication with a portion of the patient interface system exposed to a therapy pressure. The passages include respective first openings on a first surface of the first wall. The housing at least partially defines a second opening in communication with ambient atmosphere. A diffusing material is located at least partially within the housing to be adjacent the first surface. A surface of the diffusing material facing the first surface is spaced away from the first surface by a gap that extends to provide a fluid communication between all of the first openings and between all of the first openings and the second 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.
