Criteria

The heading of Class 4.2 covers:
- Pyrophoric substances which are substances, including mixtures and solutions (liquid or solid),
which even in small quantities ignite on contact with air within five minutes. These are the
Class 4.2 substances the most liable to spontaneous combustion; and
- Self-heating substances and articles which are substances and articles, including mixtures and
solutions, which, on contact with air, without energy supply, are liable to self-heating. These
substances will ignite only in large amounts (kilograms) and after long periods of time (hours
or days).

The substances and articles of Class 4.2 are subdivided as follows:
S Substances liable to spontaneous combustion, without subsidiary risk:
S1 Organic, liquid;
S2 Organic, solid;
S3 Inorganic, liquid;
S4 Inorganic, solid;
S5 Organometallic;
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.

Properties

Self-heating of a substance is a process where the gradual reaction of that substance with oxygen (in
air) generates heat. If the rate of heat production exceeds the rate of heat loss, then the temperature of
the substance will rise which, after an induction time, may lead to self-ignition and combustion.

Classification

Substances and articles classified in Class 4.2 are listed in Table A of Chapter 3.2. The assignment of
substances and articles not mentioned by name in Table A of Chapter 3.2 to the relevant specific
N.O.S. entry of 2.2.42.3 in accordance with the provisions of Chapter 2.1 can be based on experience
or the results of the test procedures in accordance with the Manual of Tests and Criteria, Part III,
Section 33.3. Assignment to general N.O.S. entries of Class 4.2 shall be based on the results of the test
procedures in accordance with the Manual of Tests and Criteria, Part III, section 33.3; experience shall
also be taken into account when it leads to a more stringent assignment.

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 risks, 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 risk 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.

Assignment of packing groups

to packing groups I, II or III on the basis of test procedures of the Manual of Tests and Criteria, Part
III, section 33.3, in accordance with the following criteria:
(a) Substances liable to spontaneous combustion (pyrophoric) shall be assigned to packing
group I;
(b) Self-heating substances and articles in which, in a 2.5 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 packing group II;
Substances with a temperature of spontaneous combustion higher than 50 °C for a volume
of 450 litres are not to be assigned to packing group II;
(c) Slightly self-heating substances in which, in a 2.5 cm sample cube, the phenomena referred to
under (b) are not observed, in the given conditions, but 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 packing group III.

Substances not accepted for carriage

The following substances shall not be accepted for carriage:
- UN No. 3255 tert-BUTYL HYPOCHLORITE; and
- Self-heating solids, oxidizing, assigned to UN No. 3127 unless they meet the requirements for
Class 1 (see 2.1.3.7).

List of collective entries

a Dust and powder of metals, non toxic in a non-spontaneous combustible form which nevertheless, in contact with
water, emit flammable gases, are substances of Class 4.3.

Class 4.3 Substances which, in contact with water, emit flammable gases

Criteria

The heading of Class 4.3 covers substances which react with water to emit flammable gases liable to
form explosive mixtures with air, and articles containing such substances.

Substances and articles of Class 4.3 are subdivided as follows:
W Substances which, in contact with water, emit flammable gases, without subsidiary risk, and
articles containing such substances:
W1 Liquid;
W2 Solid;
W3 Articles;
WF1 Substances which, in contact with water, emit flammable gases, liquid, flammable;
WF2 Substances which, in contact with water, emit flammable gases, solid, flammable;
WS Substances which, in contact with water, emit flammable gases, solid, self-heating;
WO Substances which, in contact with water, emit flammable gases, oxidizing, solid;
WT Substances which, in contact with water, emit flammable gases, toxic:
WT1 Liquid;
WT2 Solid;
WC Substances which, in contact with water, emit flammable gases, corrosive:
WC1 Liquid;
WC2 Solid;
WFC Substances which, in contact with water, emit flammable gases, flammable, corrosive.
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