Flammable solids are any materials in the solid phase of matter that can readily undergo combustion in the presence of a source of ignition under standard circumstances, i.e. without:

Division 4.1: Flammable solid

Flammable solids are any of the following four types of materials:

Division 4.2: Spontaneously combustible

Spontaneously combustible material is:

Division 4.3: Dangerous when wet

Dangerous when wet material is material that, by contact with water, is liable to become spontaneously flammable or to give off flammable or toxic gas at a rate greater than 1 liter per kilogram of the material, per hour, when tested in accordance with the UN Manual of Tests and Criteria. Pure alkali metals are known examples of this.