A respirator (10) having an oral-nasal unit (12) with a peripheral seal that seals around the nose and mouth of a wearer's face and having an inlet aperture (40) connectable to a supply of breathable air, and an outlet aperture (42). The unit having a rigid insert (106) with an exterior profile adapted to seat against a correspondingly shaped interior surface of the unit (12); an outlet conduit (112) with a tube adapted to extend through the exhale aperture (40) of the unit (12); and a wing portion (110) with a through aperture (122) registering with the unit's inlet aperture (40). The outlet conduit (112) having a bayonet connector (130, 132) that enables the unit to interchangeably and detachably connect to, a connector formed as part of an exhale aperture of a full-face mask, or of a harness assembly (102). The bayonet connector forms an airtight seal between the two.