Abstract: A nasal mask for use with a system for supplying air with a therapeutic gas to the airways of a patient includes a mask shell which is adapted to be put over the nose of the patient using the nasal mask; an air inlet port through which air is supplied from an air source; an exhaust port, provided in the mask shell, for discharging the exhalation gas to atmosphere; and a therapeutic gas inlet port, provided on the mask shell adjacent to the exhaust port. The therapeutic gas port is fluidly connected to a therapeutic gas source which supplies a therapeutic gas at a constant flow rate. The therapeutic gas port is oriented toward a portion of the inside volume of the nasal mask over the nose of the face of the patient using the nasal mask.