Class 4.2 - Substances Liable to Spontaneous Combustion

Two types of substances are included in Class 4.2. The first type of substance ignites within five minutes of coming into contact with air, even in small quantities. These are referred to as pyrophoric substances, and they are the most liable to spontaneous combustion. The second type is self-heating substances. These substances will heat up without an energy supply through a gradual reaction with oxygen. Self-heating substances ignite slowly and only when large quantities are present. A number of tests are used to determine whether solids or liquids must be classified under these designations, and three packing groups, designated I, II, or III, are specified for this class based on the results of these tests.

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