Inert Gases are naturally occurring gases. We are surrounded by inert gases. The air we breathe is filled will them. These are not man made or fossil gases.
The air we breathe has approximately 21% of Oxygen. Oxygen is the key factor in sustaining a fire and the key factor in keep us alive too!.
By removing the oxygen, we will certainly extinguish a fire, but that comes with obvious problems. How do we sustain life at the same time?
Fires need more than 15% Oxygen to combust. Anything below this level of oxygen will not be enough for a fire to sustain combustion.
Luckily, we only need 12% plus of oxygen to survive and this is where the answer lies. To extinguish a fire and sustain life, we need to reduce oxygen from 21% (assuming were at sea level) to below 15%, but not less than 12%. This will extinguish a fire and sustain life at the same time.