Oxygen:
This is simply a gas found in air. The air we breath contains about 21% of oxygen. In fact, only 16% is all that is needed to produce fire. When fuel burns, it reacts with oxygen from the surrounding air. This chemical reaction releases heat and other products such as gases, smoke and particles. This process is known as oxidation. This is why some smoke can be very dangerous because depending on what is burning, the gases produced can be very deadly.