A breathing system incorporating a phased-dilution demand oxygen regulator , comprising: at least one pressurized oxygen source ; a breathing mask configured to be worn by a user and configured to cover a nose and a mouth of the user , the breathing mask coupled to the pressurized oxygen source by at least one supply line; a dilution valve coupled to the supply line, the dilution valve configured to provide a supply of ambient air to the breathing mask when open; a demand valve coupled to the at least one oxygen source via the supply line; and the breathing regulator coupled to the at least one supply line and to the at least one oxygen source , the breathing regulator associated with a control volume pressure within the breathing regulator and comprising at least one main valve , the breathing regulator configured to: 1) receive at least one pressure signal from the breathing mask, the pressure signal corresponding to a) a start of an inhalation cycle, b) an initial demand phase of the inhalation cycle, and c) an opening of the demand valve ; 2) provide a pure oxygen supply to the breathing mask via the open demand valve throughout the initial demand phase; 3) when the control volume pressure deceeds a lower threshold pressure, cutting off the pure oxygen supply by closing the main valve ; 4) when the main valve closes, provide the ambient air supply to the breathing mask via the dilution valve ; and 5) when the control volume pressure exceeds an upper threshold, reopen the main valve in preparation for a next inhalation cycle. at least one pressurized oxygen source ; a breathing mask configured to be worn by a user and configured to cover a nose and a mouth of the user , the breathing mask coupled to the pressurized oxygen source by at least one supply line; a dilution valve coupled to the supply line, the dilution valve configured to provide a supply of ambient air to the breathing mask when open; a demand valve coupled to the at least one oxygen source via the supply line; and the breathing regulator coupled to the at least one supply line and to the at least one oxygen source , the breathing regulator associated with a control volume pressure within the breathing regulator and comprising at least one main valve , the breathing regulator configured to: 1) receive at least one pressure signal from the breathing mask, the pressure signal corresponding to a) a start of an inhalation cycle, b) an initial demand phase of the inhalation cycle, and c) an opening of the demand valve ; 2) provide a pure oxygen supply to the breathing mask via the open demand valve throughout the initial demand phase; 3) when the control volume pressure deceeds a lower threshold pressure, cutting off the pure oxygen supply by closing the main valve ; 4) when the main valve closes, provide the ambient air supply to the breathing mask via the dilution valve ; and 5) when the control volume pressure exceeds an upper threshold, reopen the main valve in preparation for a next inhalation cycle. 1) receive at least one pressure signal from the breathing mask, the pressure signal corresponding to a) a start of an inhalation cycle, b) an initial demand phase of the inhalation cycle, and c) an opening of the demand valve ; 2) provide a pure oxygen supply to the breathing mask via the open demand valve throughout the initial demand phase; 3) when the control volume pressure deceeds a lower threshold pressure, cutting off the pure oxygen supply by closing the main valve ; 4) when the main valve closes, provide the ambient air supply to the breathing mask via the dilution valve ; and 5) when the control volume pressure exceeds an upper threshold, reopen the main valve in preparation for a next inhalation cycle. <EOS>