It may be from bacteria and matter that react with oxygen.

Some compounds react with oxygen in such a way that there is a lot of "extra heat energy" left over.

Some things like to react with oxygen to make oxides.

This reacts quickly with oxygen, in the presence of water to form "rust".

The reagent may also react with oxygen present in the atmosphere.

But even if there is no water around, iron can react directly with oxygen, which generates heat.

When burned, hydrogen reacts with oxygen to produce energy and water only.

For goods that might react with oxygen in the compressed air, nitrogen is used instead.

These two chemicals react with oxygen, when available, and actually generate light.

If the temperature gets too high, the metal may begin to react with oxygen and ignite.