Class 4 isn't only for solids

Class 4 includes liquids

The 9 hazmat classes are generalized as follows:

  • Class 1—Explosives
  • Class 2—Gases
  • Class 3—Flammable Liquids
  • Class 4—Flammable Solids
  • Class 5—Oxidizers
  • Class 6—Toxins and Poisons
  • Class 7—Radioactive Materials
  • Class 8—Corrosives
  • Class 9—Miscellaneous

Describing Class 4 as “Flammable Solids” is actually a misnomer. Each of the three Class 4 Divisions contain liquids as well as solids.

Class 4.1 - Flammable Solid

The full title of Class 4.1 as listed in the IMDG Code is Class 4.1: flammable solids, self-reactive substances and solid desensitized explosives (IMDG; paralleled in 49 CFR §173.124(a)). Class 4.1, therefore, is further divided into three groups:

  1. Flammable Solids—solids which, under conditions encountered in transport, are readily combustible or may cause or contribute to fire through friction
  2. Self-reactive Substances—solids and liquids which are liable to undergo a strongly exothermic reaction
  3. Solid Desensitized Explosives—solid desensitized explosives which may explode if not diluted sufficiently

(IMDG, 49 CFR §173.124(a))

Flammable Solids and Solid Desensitized Explosives are never liquid. Self-reactive Substances, however, can be either solid or liquid.

Self-reactive Substances

The definition for this sub-group of Class 4.1 is given in IMDG (also 49 CFR §173.124(a)(2)). To paraphrase, self-reactive substances:

  • are thermally unstable substances liable to catch fire even without participation of oxygen
  • do not meet classification criteria for classes 1, 5.1, or 5.2
  • will create enough energy (heat of decomposition greater than 300 J/g) and become flammable at a low enough temperature (self-accelerating decomposition for a 50 kg package starts at or below 75° C—167° F) to be dangerous

(IMDG, 49 CFR §173.124(a)(2))

Self-reactive substances meeting these criteria are divided into seven types according to the degree of danger they present. The types range from type A, which is forbidden for transport (if using the same packaging in which it is tested), to type G, which is not subject to the regulations for self-reactive substances of class 4.1. Assignment to types B through F is directly related to the maximum quantity allowed in one packaging.

Substances of types B through F are explicitly listed in a table in IMDG (also 49 CFR §173.224(b)). Those permitted for transport in IBCs are listed in packing instruction IBC520 in the IMDG Code, and those permitted for transport in portable tanks (which are necessarily liquids) are listed in portable tank instruction T23 in the IMDG Code. 49 CFR §173.225(f) and 49 CFR §173.225(h) list requirements for self-reactive substances in IBCs or bulk packages, respectively. IMDG and IMDG (also 49 CFR §173.224(c)) detail how to classify self-reactive substances which are not listed in the table, and require that any unlisted substances be assigned to a UN number by the competent authority (PHMSA in the US) on the basis of a test report.

Each entry in the self-reactive substances table specifies the type (B to F), physical state (liquid or solid), and if temperature control is required. The liquids in the table are:

  • UN3221, Self-Reactive Liquid Type B
  • UN3223, Self-Reactive Liquid Type C
  • UN3225, Self-Reactive Liquid Type D
  • UN3227, Self-Reactive Liquid Type E
  • UN3229, Self-Reactive Liquid Type F
  • UN3231, Self-Reactive Liquid Type B, Temperature Controlled
  • UN3233, Self-Reactive Liquid Type C, Temperature Controlled
  • UN3235, Self-Reactive Liquid Type D, Temperature Controlled
  • UN3237, Self-Reactive Liquid Type E, Temperature Controlled
  • UN3239, Self-Reactive Liquid Type F, Temperature Controlled

There are three other Class 4.1 substances which, in their natural state, are solids, but must be shipped under a separate UN number—and therefore shipped subject to slightly different requirements—when molten.

  • UN1334, Naphthalene, Crude and UN1334, Naphthalene, Refined become UN2304, Naphthalene, Molten
  • UN1350, Sulphur becomes UN2448, Sulphur, Molten
  • UN1325, Flammable Solid, Organic, N.O.S. becomes UN3176, Flammable Solid, Organic, Molten, N.O.S.

There are 119 UN numbers assigned to Class 4.1, and the 15 above are liquids.

Class 4.2 - Spontaneously Combustible Material

Spontaneously Combustible Materials are substances which are liable to spontaneous heating in contact with air, which then become liable to catch fire. This happens through a process where the substance reacts with the oxygen in air to generate heat. If more heat is generated than is lost, then the temperature of the substance will rise and, after some time, may lead to self-ignition and combustion.

Class 4.2 is divided into two groups:

  • Pyrophoric substances—liquids or solids which, even in small quantities and without an external ignition source, can ignite within five minutes after coming in contact with air. These substances are the most liable to spontaneous combustion
  • Self-heating substances—substances other than pyrophoric substances which, in contact with air and without any external energy supply, are liable to self-heating. These substances will ignite only when in large amounts (kilograms) and after long periods of time (hours or days). Specifically, these materials heat to temperatures in excess of 200° C (392° F) or to the point of ignition within 24 hours.

(IMDG, 49 CFR §173.124(b))

16 of the 75 UN numbers assigned to Class 4.2 are liquids.

UN1380, Pentaborane

Pentaborane is a colorless liquid which ignites spontaneously in air. In contact with water it breaks down and gives off hydrogen, which is a highly flammable gas. Pentaborane is toxic if swallowed, inhaled, or in contact with skin, so it has a subsidiary risk of Class 6.1 (Toxic).

UN1381, Phosphorus, White (or Yellow), in Solution or UN2447, Phosphorus, White, Molten

Phosphorus is produced primarily for use in fertilizers. When exposed to air, white phosphorus will ignite in air at 30° C (86° F) and will emit a green glow as it slowly oxidizes. To prevent oxidation and ignition, phosphorus is submerged in water. Sunlight will cause white phosphorus to turn yellow and finally to a more stable red phosphorus which ignites only at much higher temperatures.

Phosphorus is also toxic if swallowed, inhaled, or in contact with skin, so it has a subsidiary risk of Class 6.1 (Toxic).

Because phosphorus “in solution” is only just phosphorus submerged in water, the phosphorus may breach the surface of the water and ignite spontaneously in air.

White phosphorus may also be shipped as a molten liquid. In molten form, white phosphorus still ignites spontaneously in air and is toxic. It’s shipped above its melting point of 44° C (111° F) to keep it molten in transport.

UN2870, Aluminum Borohydride

Aluminum Borohydride (or Aluminium Borohydride in International English) is a liquid used as rocket fuel, an additive in jet fuel, and in laboratories as a reducing agent. It reacts with water or moisture in the air to produce highly flammable hydrogen gas and plenty of heat.

Because Aluminum Borohydride reacts violently with both water and air, it has a subsidiary risk of 4.3 (Dangerous When Wet). Also, packages containing Aluminum Borohydride are required to be hermetically sealed (airtight) so that no air can enter the package and react with the Aluminum Borohydride.

UN3254, Tributylphosphane

Tributylphosphane (widely referred to as Tributylphosphine) is a colorless yellowish liquid that does not mix with water. It has a nauseating odor like garlic or rotting fish. It reacts with oxygen in the air to produce phosphine gas, which is flammable and highly toxic. The heat from this reaction accelerates the release of phosphine gas, which can ignite spontaneously. Also, if Tributylphosphane is involved in a fire, the heat from the fire will cause the same condition.

UN3255, Tert-Butyl Hypochlorite

Tert-Butyl Hypochlorite is a volatile, slightly yellow liquid with a pungent smell. It does not mix with water and boils at about 77° to 79° C (170° to 174° F). It is quite flammable as an ignitable vapor forms on the surface of the liquid at temperatures above -15° C (5° F). Exposure to light causes the liquid to break down and rapidly release heat, which may ignite the flammable vapor.

Tert-Butyl Hypochlorite also has a subsidiary risk of Class 8 (Corrosive) as it causes burns to skin, eyes and mucous membranes. When it burns, it releases Hydrogen Chloride gas which combines with moisture in the air or in tissue to form Hydrochloric Acid.

Because of the dangerous nature of Tert-Butyl Hypochlorite, it is not allowed to be transported except with special authorization granted by the competent authority of the country concerned (PHMSA in the US).

NOS Entries

In addition to the six UN numbers listed above for specific spontaneously combustible liquids, another ten UN numbers are listed for NOS entries used for spontaneously combustible liquids:

  • UN2845, Pyrophoric Liquid, Organic, N.O.S.
  • UN3194, Pyrophoric Liquid, Inorganic, N.O.S.
  • UN3183, Self-Heating Liquid, Organic, N.O.S.
  • UN3184, Self-Heating Liquid, Toxic, Organic, N.O.S.
  • UN3185, Self-Heating Liquid, Corrosive, Organic, N.O.S.
  • UN3186, Self-Heating Liquid, Inorganic, N.O.S.
  • UN3187, Self-Heating Liquid, Toxic, Inorganic, N.O.S.
  • UN3188, Self-Heating Liquid, Corrosive, Inorganic, N.O.S.
  • UN3392, Organometallic Substance, Liquid, Pyrophoric
  • UN3394, Organometallic Substance, Liquid, Pyrophoric, Water-Reactive

Class 4.3 - Dangerous When Wet Material

While Class 4.3 is labelled “Dangerous When Wet Material” in 49 CFR, the IMDG Code uses a longer title: “Substances which, in contact with water, emit flammable gases.” Both titles describe the same risk, as defined in IMDG and 49 CFR §173.124(c).

Class 4.3 materials are liquids or solids which, when in contact with water, give off flammable gases in dangerous quantities (greater than 1 L of gas per kilogram of the material, per hour). They are spontaneously flammable because the chemical reaction with water produces both a highly flammable gas (usually Hydrogen gas) and a lot of heat. Some Class 4.3 liquids produce corrosive gases when reacting with water.

19 of the 87 UN numbers assigned to Class 4.3 are liquid. Most have a subsidiary risk of Class 3 (Flammable Liquid), and 5 of the 19 also have a tertiary risk of Class 8 (Corrosive). They are:

  • UN1183, Ethyldichlorosilane
  • UN1242, Methyldichlorosilane
  • UN1295, Trichlorosilane
  • UN1389, Alkali Metal Amalgam, Liquid
  • UN1391, Alkali Metal Dispersion or UN1391, Alkaline Earth Metal Dispersion
  • UN1392, Alkaline Earth Metal Amalgam, Liquid
  • UN1411, Lithium Aluminium Hydride, Ethereal
  • UN1420, Potassium Metal Alloys, Liquid
  • UN1421, Alkali Metal Alloy, Liquid, N.O.S.
  • UN1422, Potassium Sodium Alloys, Liquid
  • UN1928, Methylmagnesium Bromide in Ethyl Ether
  • UN2965, Boron Trifluoride Dimethyl Etherate
  • UN2988, Chlorosilanes, Water-Reactive, Flammable, Corrosive, N.O.S.
  • UN3129, Water-Reactive Liquid, Corrosive, N.O.S.
  • UN3130, Water-Reactive Liquid, Toxic, N.O.S.
  • UN3148, Water-Reactive Liquid, N.O.S.
  • UN3398, Organometallic Substance, Liquid, Water-Reactive
  • UN3399, Organometallic Substance, Liquid, Water-Reactive, Flammable
  • UN3482, Alkali Metal Dispersion, Flammable or UN3482, Alkaline Earth Metal Dispersion, Flammable


Almost 1 in 5 UN numbers in Class 4 are liquid (52 liquid of 281 total). So, the next time you hear Class 4 described as “Flammable Solids,” set the record straight.