A respiration system, comprising: a regulator comprising an inlet for connection to a source of pressurized gas comprising oxygen, and an outlet for delivery of the gas to the user; and a respiration facepiece for use in connection with the regulator comprising: an interface to which the regulator is removably attachable; at least one seal system to form a sealing engagement with the face of a user to encompass the nose and mouth of a wearer; an inhalation port in fluid connection with the regulator interface and in fluid connection with the interior of the facepiece; at least one inhalation check valve in fluid connection between the inhalation port and the interior of the facepiece; an exhalation port in fluid connection with the facepiece; an exhalation check valve in fluid connection with the exhalation port; and an ambient port separate from the inhalation port and from the exhalation port, the ambient port being in fluid connection with the interior of the facepiece without an intervening check valve, characterized in that the ambient port is in fluid connection with the interface such that the regulator seals the ambient port from fluid connection with the outlet of the regulator upon connection of the regulator to the regulator interface. a regulator comprising an inlet for connection to a source of pressurized gas comprising oxygen, and an outlet for delivery of the gas to the user; and a respiration facepiece for use in connection with the regulator comprising: an interface to which the regulator is removably attachable; at least one seal system to form a sealing engagement with the face of a user to encompass the nose and mouth of a wearer; an inhalation port in fluid connection with the regulator interface and in fluid connection with the interior of the facepiece; at least one inhalation check valve in fluid connection between the inhalation port and the interior of the facepiece; an exhalation port in fluid connection with the facepiece; an exhalation check valve in fluid connection with the exhalation port; and an ambient port separate from the inhalation port and from the exhalation port, the ambient port being in fluid connection with the interior of the facepiece without an intervening check valve, characterized in that the ambient port is in fluid connection with the interface such that the regulator seals the ambient port from fluid connection with the outlet of the regulator upon connection of the regulator to the regulator interface. an interface to which the regulator is removably attachable; at least one seal system to form a sealing engagement with the face of a user to encompass the nose and mouth of a wearer; an inhalation port in fluid connection with the regulator interface and in fluid connection with the interior of the facepiece; at least one inhalation check valve in fluid connection between the inhalation port and the interior of the facepiece; an exhalation port in fluid connection with the facepiece; an exhalation check valve in fluid connection with the exhalation port; and an ambient port separate from the inhalation port and from the exhalation port, the ambient port being in fluid connection with the interior of the facepiece without an intervening check valve, characterized in that the ambient port is in fluid connection with the interface such that the regulator seals the ambient port from fluid connection with the outlet of the regulator upon connection of the regulator to the regulator interface.