Class 4 Hazards

Class 4 Hazard includes Solid materials that can cause serious risk of fire in uncontrolled conditions in the presence of a source of ignition under standard circumstances i.e., without:

  • Artificially changing variables such as pressure or density; or
  • Adding accelerants to speed up a chemical process

The hazard class is divided into three categories:

  1. Flammable Solids
  2. Spontaneously Combustible 
  3. Water Reactive (also called dangerous when wet)

4.1 Flammable Solids are readily combustible or may contribute to a fire through friction. These include:

  • Flammable solids
  • Self-reactive substances
  • Solid desensitized explosives

E.g.:  Metallic sodium and potassium, oily fabrics, matches, and nitrocellulose products

4.2 Spontaneously Combustible materials are also called substances liable to spontaneous combustion which occurs by self-heating (increase in temperature due to exothermic internal reactions), followed by thermal runaway (self heating which rapidly accelerates to high temperatures) and finally, autoignition. These include:

  • Pyrophoric substances (highly reactive chemicals that spontaneously ignite when exposed to air). Examples – potassium hydride (KH) and white phosphorus
  • Self-heating substances

E.g: Oil-seed products, linseed oil rags, coal dust, hay, wood chips, manure, and latex

4.3 Water Reactive (also called dangerous when wet) substances emit a flammable gas when in contact with water.

E.g.: Alkali metals (e.g., Sodium, Potassium) and metal hydrides (e.g., Lithium aluminum hydride, Sodium hydride) 

Reactivity series of Metals:


Order of Reactivity


Reactions with water or Steam

Most reactive 

Potassium (K)

Very vigorous reaction with cold water

Second most reactive

Sodium (Na)

Vigorous reaction with cold water

Third most reactive

Calcium (Ca)

Less vigorous reaction with cold water

Least reactive 

Magnesium (Mg)

Slow reaction with cold water, vigorous with hot water