Spontaneous combustion is burning that may occur when heat generated by chemical changes within a substance becomes trapped inside the substance. The substance combines with oxygen in a chemical reaction called oxidation. Normally, the atmosphere absorbs heat produced during oxidation. But a substance may retain this heat if stored in a place with poor air circulation. The temperature of the substance then rises and oxidation speeds up, producing even more heat. This cycle continues until the substance ignites. Spontaneous combustion occurs most readily in large piles of coal, heaps of oily rags, and damp hay. See also Combustion ; Fire (Kinds of fire) .
