Division 4.1 - Flammable Solids


This division is comprised of flammable solids, self-reactive and related substances, solid desensitized explosives,

and polymerizing (the chemical process that causes the bonding of two or more monomers to form a polymer)



Division 4.1 substances are assigned to UN Packing Groups II or III. 

  • Flammable solids are solids that, under conditions encountered in transport, are readily combustible or may cause or contribute to fire through friction.  They can also be a powder or paste.  An added danger can be from toxic combustion products.

  • Self-reactive and related substances are substances that are liable to undergo a strong exothermic reaction (heat release.) Decomposition can be initiated by heat, contact with catalytic impurities (e.g. acids, heavy metal compounds, bases), friction or impact. The rate of decomposition increases with temperature and varies with the substance. Decomposition, particularly if no ignition occurs, may result in the evolution of toxic gases or vapours. Some self-reactive substances may decompose explosively, particularly if confined. Some may burn vigorously. All self-reactive substances are assigned to one of twenty generic UN entries.

  • Solid desensitized explosives are Class 1 explosives which are wetted with sufficient water, alcohol, or plasticizer to suppress their explosive properties. Desensitized explosives may explode if not diluted sufficiently.

  • Polymerizing Substances are substances which, without stabilization, are liable to undergo a strongly exothermic reaction resulting in the formation of larger molecules or resulting in the formation of polymers under conditions normally encountered in transport.

Division 4.2 - Substances Liable To Spontaneous Combustion

This division is comprised of substances that are liable to spontaneous heating under normal transport conditions or

heating up in contact with air and are consequently liable to catch fire due to such heating. They include pyrophoric

substances and self-heating substances.


Division 4.2 substances can be in UN Packing Groups I, II, or III.

  • Pyrophoric substances are substances, including mixtures and solutions (liquid or solid), which even in small quantities ignite within five minutes of coming in contact with air. These Division 4.2 substances are the most liable to spontaneous combustion.

  • Self-heating substances are substances, other than pyrophoric substances, which in contact with air without energy supply are liable to self heating. These substances will ignite only when in large amounts (kilograms) and after long periods of time (hours or days).

Division 4.3 - Substances Which, In Contact With Water, Emit Flammable Gases

This division is comprised of substances which when in contact with water may emit flammable gases that can form

explosive mixtures with air. Such mixtures are easily ignited by all ordinary sources of ignition, for example naked

lights, sparking hand tools or unprotected lamps. The resulting blast wave and flames may endanger people and the

environment. In transportation they must be kept in watertight containers, hermetically sealed to avoid the entry of

moisture or water vapour.


Division 4.3 substances can be in UN Packing Groups I, II, or III.