A patient interface comprising: a plenum chamber pressurisable to a therapeutic pressure of at least 4 cmH2O above ambient air pressure by a flow of air at the therapeutic pressure for breathing by a patient, the plenum chamber further comprising two plenum chamber connectors positioned on corresponding lateral sides of the plenum chamber and configured to receive the flow of air at the therapeutic pressure; a seal-forming structure connected to the plenum chamber, the seal-forming structure being constructed and arranged to seal with a region of the patient's face at least partly surrounding an entrance to the patient's airways, said seal-forming structure having a hole therein such that the flow of air at said therapeutic pressure is delivered to at least the patient's nares, the seal-forming structure being constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient's respiratory cycle in use; a positioning and stabilising structure configured to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient's head, the positioning and stabilising structure n further comprising two conduits, each of the conduits being configured to be positioned on a corresponding lateral side of the patient's head in use and configured to connect to a corresponding one of the plenum chamber connectors; a vent structure configured to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient, said vent structure being sized and shaped to maintain the therapeutic pressure in the plenum chamber in use; 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 such that at least a portion of the flow of air at the therapeutic pressure entering the plenum chamber from the plenum chamber connectors passes through the material before entering the patient's airways; wherein the patient interface is configured to leave the patient's mouth uncovered, or if the seal-forming structure is configured to seal around the patient's nose and mouth, the patient interface is configured to allow the patient to breath from ambient in the absence of a flow of pressurised air; characterized in that the plenum chamber further comprises a main frame and a removable frame that is removable from the main frame and the material is attached to the removable frame such that the removable frame and the material are removable from the main frame together. a plenum chamber pressurisable to a therapeutic pressure of at least 4 cmH2O above ambient air pressure by a flow of air at the therapeutic pressure for breathing by a patient, the plenum chamber further comprising two plenum chamber connectors positioned on corresponding lateral sides of the plenum chamber and configured to receive the flow of air at the therapeutic pressure; a seal-forming structure connected to the plenum chamber, the seal-forming structure being constructed and arranged to seal with a region of the patient's face at least partly surrounding an entrance to the patient's airways, said seal-forming structure having a hole therein such that the flow of air at said therapeutic pressure is delivered to at least the patient's nares, the seal-forming structure being constructed and arranged to maintain said therapeutic pressure in the plenum chamber throughout the patient's respiratory cycle in use; a positioning and stabilising structure configured to provide a force to hold the seal-forming structure in a therapeutically effective position on the patient's head, the positioning and stabilising structure n further comprising two conduits, each of the conduits being configured to be positioned on a corresponding lateral side of the patient's head in use and configured to connect to a corresponding one of the plenum chamber connectors; a vent structure configured to allow a continuous flow of gases exhaled by the patient from an interior of the plenum chamber to ambient, said vent structure being sized and shaped to maintain the therapeutic pressure in the plenum chamber in use; 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 such that at least a portion of the flow of air at the therapeutic pressure entering the plenum chamber from the plenum chamber connectors passes through the material before entering the patient's airways; wherein the patient interface is configured to leave the patient's mouth uncovered, or if the seal-forming structure is configured to seal around the patient's nose and mouth, the patient interface is configured to allow the patient to breath from ambient in the absence of a flow of pressurised air; characterized in that the plenum chamber further comprises a main frame and a removable frame that is removable from the main frame and the material is attached to the removable frame such that the removable frame and the material are removable from the main frame together.