Complex sentence: Typically fires are extinguished when the oxygen concentration falls below 15% so given that the atmospheric oxygen concentration is about 20% this requires that about 1/3 of the air is replaced with inert gas to smother the fire.
Split into simpler sentences: Typically fires are extinguished when the oxygen concentration falls below 15%. Given that the atmospheric oxygen concentration is about 20% this requires that about 1/3 of the air is replaced with inert gas to smother the fire.