Abstract: This invention provides a self-contained user-actuated breathing device of the type used by industry and firefighters to protect against toxic atmospheres, which includes means for reducing user respiratory fatigue during periods of heavy physical exertion or stress, while maintaining facepiece pressure in a preselected narrow range of positive pressures relative to atmospheric to protect the user from ingress of toxic outside atmospheres. The invention includes a feedback mechanism responsive to facepiece pressure which actuates a supplemental second inlet air flow path to the facepiece during periods of high user demand. Also provided is a novel nonlinear expiration valve mechanism which is responsive to facepiece pressure to prevent facepiece pressure from exceeding a preselected positive level, thereby minimizing user expiratory fatigue which is commonly associated with such devices.