The invention relates to a breathing apparatus for providing a respiratory gas to a crewmember in a cabin of an aircraft, said breathable apparatus comprising an air inlet (5) for admission of ambient air in said breathing apparatus, an additional gas inlet (2) for admission of additional gas in said breathing apparatus, an outlet nozzle (4) for feeding said crew member with the respiratory gas comprising said ambient air and/or additional gas, said breathing apparatus further comprising neutralizing means (70, 141) for neutralizing at least partially the admission of said additional gas below a predefined cabin altitude (Z1).