Class 4.1 Flammable solids
Solids which, under conditions encountered in transport, are readily combustible or may cause or contribute to fire through friction; self-reactive substances (solids and liquids) which are liable to undergo a strongly exothermic reaction; solid desensitized explosives which may explode if not diluted sufficiently;
Class 4.2 Substances liable to spontaneous combustion
Substances (solids and liquids) which are liable to spontaneous heating under normal conditions encountered in transport, or to heating up in contact with air, and being then liable to catch fire;
Class 4.3 Substances which, in contact with water, emit flammable gases
Substances (solids and liquids) which, by interaction with water, are liable to become spontaneously flammable or to give off flammable gases in dangerous quantities.
.1 Flammable solids (class 4.1);
.2 Self-reactive substances (class 4.1);
.3 Pyrophoric solids (class 4.2);
.4 Pyrophoric liquids (class 4.2);
.5 Self-heating substances (class 4.2); and
.6 Substances which, in contact with water, emit flammable gases (class 4.3).
Test methods and criteria for self-reactive substances are given in Part II of the United Nations Manual of Tests and Criteria, and test methods and criteria for the other types of substances of class 4 are given in the United Nations Manual of Tests and Criteria, Part III, chapter 33.