Abstract: A breathing system includes a facepiece and a regulator to deliver breathing gas to the facepiece. The regulator includes a sensor system including at least one sensor responsive to respiration of a user. The breathing system further includes a processor system in operative connection with the at least one sensor, a memory system in operative connection with the processor system and at least one algorithm stored in the memory system and executable by the processor system. The at least one algorithm is adapted, configured or programmed to determine at least one of a rate of respiration and a respiration volume from data from the at least one sensor. The algorithm is further adapted, configured or programmed to relate at least one of the rate of respiration and the respiration volume to a physiological state of the user.