Spontaneous combustion is a type of combustion which occurs without an external ignition source. The element sodium is an example of a pyrophoric material which can undergo a kind of spontaneous (and potentially very violent) explosion when exposed to oxygen, water, or moisture in the air. Pyrophoric substances have an autoignition temperature below room temperature and often require mere contact with air or water in order to spontaneously ignite.

A characteristic of pyrophoric materials is also their large specific surface of contact with air. Raney nickel is pyrophoric because of the very fine size of its particles. In general practice, the threat of spontaneous combustion can be substantially reduced by placing the material in a tightly confined, self-closing disposal container (thus greatly limiting the supply of oxygen or water vapor), or submerging it in a fluid which smothers the reaction before it can begin.

Depending on the hazard, this protective fluid may be water, kerosene (especially for a reactive metal such as sodium, which ignites upon contact with water) or an inert gas such as nitrogen or argon.

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