A breathing apparatus for providing a respiratory gas to a crewmember in a cabin of an aircraft, said breathing apparatus comprising: - an air inlet for admission of ambient air in said breathing apparatus, - an additional gas inlet for admission of additional gas in said breathing apparatus, - mixing means for mixing the ambient air with the additional gas, - an outlet nozzle for feeding said crew member with the respiratory gas comprising said ambient air and/or additional gas, and - an air circuit (52, 62) for feeding said mixing means with ambient air, the air circuit (52, 62) comprising the air inlet and an air feed pipe, the pressure loss of said air circuit (52, 62) depending upon the cabin altitude, wherein the breathing apparatus further comprises neutralizing means (70, 141) for neutralizing the admission of said additional gas below a predefined cabin altitude (Z 1), the neutralizing means comprising: - the air circuit (52, 62), - a sealing element movable between a first position wherein the pressure loss of the air circuit (52, 62) is minimal and a second position wherein the pressure loss of said air circuit (52, 62) is increased, said sealing element being in its first position when the cabin altitude is below the predefined cabin altitude (Z 1), and said sealing element being in its second position when the cabin altitude is beyond said predefined altitude (Z 1), - a first altimetric device (141, 73, 80) adapted to move the sealing element into its second position when the cabin altitude is greater than the predefined altitude (Z 1), and characterized in that the neutralizing means (70, 141) also comprises - a second altimetric means provided to pilot the entry of ambient air into said apparatus as a function of the cabin altitude when the sealing element is in its second position. - an air inlet for admission of ambient air in said breathing apparatus, - an additional gas inlet for admission of additional gas in said breathing apparatus, - mixing means for mixing the ambient air with the additional gas, - an outlet nozzle for feeding said crew member with the respiratory gas comprising said ambient air and/or additional gas, and - an air circuit (52, 62) for feeding said mixing means with ambient air, the air circuit (52, 62) comprising the air inlet and an air feed pipe, the pressure loss of said air circuit (52, 62) depending upon the cabin altitude, wherein the breathing apparatus further comprises neutralizing means (70, 141) for neutralizing the admission of said additional gas below a predefined cabin altitude (Z 1), the neutralizing means comprising: - the air circuit (52, 62), - a sealing element movable between a first position wherein the pressure loss of the air circuit (52, 62) is minimal and a second position wherein the pressure loss of said air circuit (52, 62) is increased, said sealing element being in its first position when the cabin altitude is below the predefined cabin altitude (Z 1), and said sealing element being in its second position when the cabin altitude is beyond said predefined altitude (Z 1), - a first altimetric device (141, 73, 80) adapted to move the sealing element into its second position when the cabin altitude is greater than the predefined altitude (Z 1), and characterized in that the neutralizing means (70, 141) also comprises - a second altimetric means provided to pilot the entry of ambient air into said apparatus as a function of the cabin altitude when the sealing element is in its second position. - the air circuit (52, 62), - a sealing element movable between a first position wherein the pressure loss of the air circuit (52, 62) is minimal and a second position wherein the pressure loss of said air circuit (52, 62) is increased, said sealing element being in its first position when the cabin altitude is below the predefined cabin altitude (Z 1), and said sealing element being in its second position when the cabin altitude is beyond said predefined altitude (Z 1), - a first altimetric device (141, 73, 80) adapted to move the sealing element into its second position when the cabin altitude is greater than the predefined altitude (Z 1), and characterized in that the neutralizing means (70, 141) also comprises - a second altimetric means provided to pilot the entry of ambient air into said apparatus as a function of the cabin altitude when the sealing element is in its second position.