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Flammable solids

Hazard Knowledge

Flammable solids are amongst the most common of all flammable materials, but relatively few are included in the GHS or UN hazardous substances classifications. Those that are included can exhibit special risks and this section highlights some of the more important problems associated with these hazardous materials.

A number of hazardous properties are covered within this broad hazard classification, including:

UN Class 4.1 Flammable solids, self-reactive substances and desensitised explosives

  • Flammable solids – powdered, granular or paste-like substances that are readily combustible (easily ignited by brief contact with an ignition source such as a burning match) and where the flame then spreads rapidly
  • Self-reactive substances and mixtures – thermally unstable liquid or solid substances or mixtures liable to undergo a strong exothermic decomposition even without the presence of oxygen.
  • Desensitised explosives – explosives that, when dry, are Class 1 explosives, which are kept wetted with sufficient water or alcohol to keep them stable and safer in long term storage

These solids rarely produce sufficient vapours at ordinary temperatures to be ignitable; naphthalene and camphor being notable exceptions. However, as solids are heated, vapour production will increase as they melt and possibly boil (e.g. sulphur). Some solids, normally organic in nature, will decompose as they are heated to produce small combustible molecules that when mixed with air, may eventually ignite (e.g. wood, plastics, paper).

For naturally occurring flammable solids the water content of the material can determine its ignition temperature. For example, it is relatively easy to ignite dry grass but rather more difficult to ignite damp grass.

The shape and surface area of solids also affects the ease of ignition.

UN Class 4.2 Substances liable to spontaneous combustion


Pyrophoric liquids and solids – substances liable to ignite within five minutes of coming into contact with air

Self-heating substances and mixtures – solid or liquid substances or mixtures, other than a pyrophoric liquid or solid, that can react with air and are liable to self-heat

UN Class 4.3 Substances which in contact with water emit flammable gases


Substances and mixtures that, in contact with water, are liable to become spontaneously flammable or give off flammable gas in dangerous quantities.

Special care should be taken at incidents involving these substances to manage the risk of reactions with water. Water inside a ruptured or leaking container of water-reactive material may cause an explosion.