Spontaneous combustion can occur when a substance with a relatively low ignition temperature (hay, straw, peat, etc.) begins to release heat. This may occur in several ways, either by oxidation in the presence of moisture and air, or bacterial fermentation, which generates heat.

The combustion in which substances suddenly burst into flames, without the application of any apparent cause is called spontaneous combustion. The combustion in which sudden reactions take place on ignition of some substances to produce heat, light, and sound is called explosion.

The combination of energy decrease and entropy increase dictates that combustion reactions are spontaneous reactions. Bonfire. A nonspontaneous reaction is a reaction that does not favor the formation of products at the given set of conditions.

As the temperature rises above 130°F (55°C), a chemical reaction occurs and may sustain itself. This reaction does not require oxygen, but the flammable gases produced are at a temperature above their ignition point.

Descriptions of spontaneous human combustion date back to the 17th century, with a large number of cases recorded in the 19th century and a handful in the 20th and 21st centuries. Altogether, there are a few hundred recorded possible cases.

Under a perfect storm of conditions — wet weather and warm temperatures — those leaves could start to heat up, and even cause a fire. They can spontaneously combust, creating a safety hazard for homeowners. But it can happen — especially when a big pile of leaves starts to compost.

Rags and towels soaked with oils, including cooking oils; hot laundry left in piles; large compost, mulch, manure, and leaf piles; and moist baled hay can spontaneously combust in the right conditions.