The present invention provides for a breathing device comprising a reservoir bag, oxygen source, scrubber, and activation device. The oxygen source and an exit from the scrubber may be fluidly connected to the reservoir bag. The reservoir bag may be attached to a mouthpiece to provide inhalation air. Expired air may be exhaled through the mouthpiece and directed to an inlet for the scrubber. As a user breaths normally, expired air is scrubbed of undesired components, such as excess CO2. The scrubbed expired air is then mixed with generated oxygen, and delivered back to the user for inhalation. Additionally, the oxygen source and the scrubber may be replaced and / or replenished without interrupting or compromising a breathing cycle.