Abstract: A patient interface may include: a plenum chamber pressurisable to a therapeutic pressure; a seal-forming structure connected to the plenum chamber, the plenum chamber being constructed and arranged to seal with a region of the patients face; a positioning and stabilising structure configured to provide a force to hold the seal-forming structure in a therapeutically effective position on the patients head; a vent structure configured to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient; and a material configured to adsorb water vapour from gases exhaled by the patient and desorb water vapour into the flow of air at the therapeutic pressure, the material being positioned within the plenum chamber.