Hazard Class 4

Class 4 - Flammable solids, spontaneously combustible material, and dangerous when wet material includes solid materials that may ignite or combust, such as matches, and solids that may react with water, such as sodium and potassium. These materials are covered in §173.124.

Division 4.1 - Flammable solids includes solid material that is readily combustible or that may cause or contribute to a fire through friction. This division also includes self-reactive materials that are liable to undergo a strongly exothermic, or heat-producing, reaction and desensitized explosives that can explode if not sufficiently diluted.

A self-reactive material is thermally unstable and is liable to undergo a strongly exothermic decomposition even without the presence of oxygen. The decomposition of self-reactive materials is initiated by heat, friction, impact, or contact with catalytic impurities such as acids and bases. The rate of decomposition is dependent on temperature and the type of material. Because of this, self-reactive materials must be protected from direct sunlight and all sources of heat. They must also be placed in well-ventilated areas.

Desensitized explosives are explosives that have been wetted with alcohol or water or diluted with other substances to form a homogeneous solid mixture in order to suppress their explosive properties.

Examples of Division 4.1 dangerous goods include matches, sulfur, and straw.

Division 4.2 - Spontaneous combustible material includes those substances that are liable to spontaneous heating under normal conditions of transport, or those substances that heat up and have a potential to catch fire upon contact with air. These substances are classified as either pyrophoric or self-heating substances.

Pyrophoric materials and mixtures ignite within five minutes of coming in contact with air. Even small quantities of these substances are liable to spontaneously combust. Division 4.2 includes pyrophoric solids and liquids, and they are all assigned to Packing Group I.

Self-heating materials undergo a gradual reaction with oxygen that generates heat. If the rate of heat production exceeds the rate of heat loss, then the temperature of the material will rise. Over time, that can lead to self-ignition and combustion. Self-heating materials do not require any additional energy source. Unlike pyrophoric materials, self-heating materials will ignite only in large quantities and after long periods of time.

Examples of Division 4.2 hazardous materials include activated carbon, oily rags, and seed cake.

Note that due to PHMSA's latest ruling, Division 4.2 materials may no longer be assigned to PGII, effective July 5, 2016.

Division 4.3 - Dangerous when wet material includes materials that, by contact with water, are liable to become spontaneously flammable or to give off flammable or toxic gas in dangerous quantities. Mixtures of these materials are easily ignited by all ordinary sources of ignition, such as sparks. Examples of Division 4.3 dangerous goods include calcium carbide, sodium, and alkaline earth metal alloys.

To learn more about DOT HAZMAT Awareness visit our DOT HAZMAT General Awareness Online Training web page.