A mouthpiece through which a person may breathe has a sealing flange for introduction into the mouth. The sealing flange has an upper portion for lying against the hard palate and a lower portion for fitting between the lower lip and the anterior surface about the lower teeth and gums. The juxtaposition in the mouth of the upper portion against the hard palate and the lower portion seated in the lower vestibule portion of the mouth results in the sealing flange forming an effective seal and the sealing flange being comfortably held in place in the mouth even when the mouth is open to the extent of the normal relaxed muscle tone of the mouth. To facilitate the seal when the mouthpiece is introduced into the mouth, a tooth tray is provided. Another aspect of the invention permits the mouthpiece to envelop the nostrils such that when the mouthpiece is in place all inspired and expired air must pass through the mouthpiece. In the best embodiment of the invention, the nostrils are enveloped by a naso-labial hood which envelops the entire nose forming a seal against the outline of the nose, thereby minimizing the non-respiratory tract dead air space inside the volume of the naso-labial hood.