A humidifier comprising: an inlet; an outlet configured to be coupled to an air delivery conduit; and a valve arrangement coupled to the outlet, wherein the humidifier is configured to, in use: receive a flow of pressurized gas through the inlet; humidify said received flow of pressurized gas; and provide the humidified flow of pressurized gas through the outlet, the valve arrangement comprising: a first side adapted to couple to the humidifier outlet; a second side adapted to couple to the air delivery conduit; a blocking member that is located between the first side and the second side and is configured to move between a first position and a second position; and an aperture to atmosphere on the second side of the valve arrangement, wherein the blocking member is adapted to, in the absence of a force being applied through the valve arrangement, be in the first position, obstructing access from the first side to the second side, and to, in the presence of a force being applied through the valve arrangement, to move to the second position to allow access from the first side to the second side, such that the valve arrangement is adapted to adjustably obstruct the outlet of the humidifier to prevent humidified gas from leaving the humidifier when there is no pressurized flow through the humidifier, and wherein the aperture is configured to be open to atmosphere when the blocking member is in the first position and closed when the blocking member is in the second position. an inlet; an outlet configured to be coupled to an air delivery conduit; and a valve arrangement coupled to the outlet, wherein the humidifier is configured to, in use: receive a flow of pressurized gas through the inlet; humidify said received flow of pressurized gas; and provide the humidified flow of pressurized gas through the outlet, the valve arrangement comprising: a first side adapted to couple to the humidifier outlet; a second side adapted to couple to the air delivery conduit; a blocking member that is located between the first side and the second side and is configured to move between a first position and a second position; and an aperture to atmosphere on the second side of the valve arrangement, receive a flow of pressurized gas through the inlet; humidify said received flow of pressurized gas; and provide the humidified flow of pressurized gas through the outlet, the valve arrangement comprising: a first side adapted to couple to the humidifier outlet; a second side adapted to couple to the air delivery conduit; a blocking member that is located between the first side and the second side and is configured to move between a first position and a second position; and an aperture to atmosphere on the second side of the valve arrangement, a first side adapted to couple to the humidifier outlet; a second side adapted to couple to the air delivery conduit; a blocking member that is located between the first side and the second side and is configured to move between a first position and a second position; and an aperture to atmosphere on the second side of the valve arrangement, wherein the blocking member is adapted to, in the absence of a force being applied through the valve arrangement, be in the first position, obstructing access from the first side to the second side, and to, in the presence of a force being applied through the valve arrangement, to move to the second position to allow access from the first side to the second side, such that the valve arrangement is adapted to adjustably obstruct the outlet of the humidifier to prevent humidified gas from leaving the humidifier when there is no pressurized flow through the humidifier, and wherein the aperture is configured to be open to atmosphere when the blocking member is in the first position and closed when the blocking member is in the second position. <EOS>