The chemically unstable substances of Class 4.1 shall not be accepted for carriage unless the necessary steps have been taken to prevent their dangerous decomposition or polymerization during carriage. To this end, it shall in particular be ensured that receptacles and tanks do not contain any substance liable to promote these reactions.

Flammable solids, oxidizing, assigned to UN No. 3097 shall not be accepted for carriage unless they meet the requirements for Class 1 (see also 2.1.3.7).

The following substances shall not be accepted for carriage:

- Self-reactive substances of type A (see Manual of Tests and Criteria, Part II, paragraph 20.4.2 (a));

- Phosphorus sulphides which are not free from yellow and white phosphorus;

- Solid densitized explosives other than those listed in Table A of Chapter 3.2;

- Inorganic flammable substances in the molten form other than UN No. 2448 SULPHUR,MOLTEN.

The heading of Class 4.2 covers:

-Pyrophoric substanceswhich are substances, including mixtures and solutions (liquid or solid),which even in small quantities ignite on contact with air within five minutes. These are theClass4.2 substances the most liable to spontaneous combustion; and

-Self-heating substances and articleswhich are substances and articles, including mixtures andsolutions, which, on contact with air, without energy supply, are liable to self-heating. Thesesubstances will ignite only in large amounts (kilograms) and after long periods of time (hoursor days).

The substances and articles of Class 4.2 are subdivided as follows:

S Substances liable to spontaneous combustion, without subsidiary hazard:

S1 Organic, liquid;

S2 Organic, solid;

S3 Inorganic, liquid;

S4 Inorganic, solid;

S5 Organometallic;

S6 Articles

SW Substances liable to spontaneous combustion, which, in contact with water, emit flammable gases;

SO Substances liable to spontaneous combustion, oxidizing;

ST Substances liable to spontaneous combustion, toxic:

ST1 Organic, toxic, liquid;

ST2 Organic, toxic, solid;

ST3 Inorganic, toxic, liquid;

ST4 Inorganic, toxic, solid;

SC Substances liable to spontaneous combustion, corrosive:

SC1 Organic, corrosive, liquid;

SC2 Organic, corrosive, solid;

SC3 Inorganic, corrosive, liquid;

SC4 Inorganic, corrosive, solid.

When substances or articles not mentioned by name are assigned to one of the entries listed in 2.2.42.3 on the basis of the test procedures in accordance with the Manual of Tests and Criteria, Part III, section 33.3, the following criteria shall apply:

(a) Solids liable to spontaneous combustion (pyrophoric) shall be assigned to Class 4.2 when they ignite on falling from a height of 1 m or within five minutes;

(b) Liquids liable to spontaneous combustion (pyrophoric) shall be assigned to Class 4.2 when:

(i) on being poured on an inert carrier, they ignite within five minutes, or

(ii) in the event of a negative result of the test according to (i), when poured on a dry, indented filter paper (Whatman No. 3 filter), they ignite or carbonize it within five minutes;

(c) Substances in which, in a 10 cm sample cube, at 140 °C test temperature, spontaneous combustion or a rise in temperature to over 200 °C is observed within 24 hours shall be assigned to Class 4.2. This criterion is based on the temperature of the spontaneous combustion of charcoal, which is at 50 °C for a sample cube of 27 m3. Substances with a temperature of spontaneous combustion higher than 50 °C for a volume of 27 m3 are not to be assigned to Class 4.2.

NOTE 1: Substances carried in packages with a volume of not more than 3 m3 are exempted from Class 4.2 if, tested with a 10 cm sample cube at 120 °C, no spontaneous combustion nor a rise in temperature to over 180 °C is observed within 24 hours.

NOTE 2: Substances carried in packages with a volume of not more than 450 litres are exempted from Class 4.2 if, tested with a 10 cm sample cube at 100 °C, no spontaneous combustion nor a rise in temperature to over 160 °C is observed within 24 hours.

NOTE 3: Since organometallic substances can be classified in Class 4.2 or 4.3 with additional subsidiary hazards, depending on their properties, a specific classification flow chart for these substances is given in 2.3.5.

If substances of Class 4.2, as a result of admixtures, come into different categories of hazard from those to which the substances mentioned by name in Table A of Chapter 3.2 belong, these mixtures shall be assigned to the entries to which they belong on the basis of their actual degree of danger. 

NOTE: For the classification of solutions and mixtures (such as preparations and wastes), see also 2.1.3.

On the basis of the test procedure in the Manual of Tests and Criteria, Part III, section 33.3 and the criteria set out in 2.2.42.1.5, it may also be determined whether the nature of a substance mentioned by name is such that the substance is not subject to the provisions for this Class.