What reactive element is necessary for a fire to sustain combustion?

Oxygen is a critical reactive element necessary for sustaining combustion and is one of the three components of the fire triangle, which includes heat, fuel, and oxygen. When a fire starts, the fuel (such as wood, paper, or other materials) requires an adequate supply of oxygen to continue burning. During combustion, oxygen reacts with the fuel to produce heat and light, which is the visible manifestation of fire.

Without sufficient oxygen, a fire cannot sustain itself; it will either smolder or extinguish entirely. This is why oxygen is essential for combustion, and it is commonly present in the atmosphere at about 21% concentration, making it readily available for fires to ignite and continue burning. Other gases like carbon dioxide and nitrogen do not support combustion in the way oxygen does; in fact, carbon dioxide is often used in fire extinguishers precisely because it can displace oxygen and help to smother fires.