"INERGEN® is a mixture of 52% Nitrogen, 40% Argon and 8% CO2. However, in the event of a fire, when INERGEN® is discharged, it mixes with the air present in the room to create a mixture that comprises of 67.3% Nitrogen, 12.5% Oxygen, 17% Argon and 3.2% Carbon Dioxide.
This discharge mixture makes INERGEN® singularly unique. No other inert gas has the unique ability to rapidly extinguish a fire yet at the same time provide a safe environment for any person within the occupied area by actually decreasing cardiac distress and maintaining arterial blood oxygenation and mental performance in low oxygen levels. "
Not many substances will combust at 12.5% O2. The CO2 is added to increase respiration rate to compensate for the lower oxygen levels.
One of the hazards of cockpit fires was a breach in the oxygen system.