IMDG Class 4: Flammable Solids, Self-Heating, Water-Reactive

IMDG Class 4 is the class of the International Maritime Dangerous Goods Code for solids and related substances that burn, ignite or release flammable gas without being explosives or flammable liquids. It has three divisions: 4.1 flammable solids, self-reactive substances, solid desensitized explosives and polymerizing substances; 4.2 substances liable to spontaneous combustion; and 4.3 substances which, in contact with water, emit flammable gases. The text in force is chapter 2.4 of Amendment 42-24, adopted by IMO resolution MSC.556(108) on 23 May 2024 and mandatory from 1 January 2026 under SOLAS chapter VII .

A Division 4.1 solid needs an outside ignition source. A Division 4.2 substance supplies its own heat from reaction with air. A Division 4.3 substance makes its fuel from water, so the default fire-fighting agent at sea is the one that feeds it. Every stowage, segregation and emergency rule below follows from those three mechanisms.

Class 4 divisions at a glance

The table sets out the three divisions against the Amendment 42-24 text. Packing groups, test references and examples come from IMDG chapter 2.4 and the Dangerous Goods List in the annex to MSC.556(108).

Division 4.2 also contains liquids. The pyrophoric organometallics UN 3392 and UN 3394 are liquids, which is why the class title “flammable solids” understates its scope.

Regulatory framework for Class 4

Class 4 carriage in packaged form is mandatory under SOLAS chapter VII, part A, which makes the IMDG Code binding on every ship to which the SOLAS Convention applies. IMDG 1.1.1.3 extends the Code to all ships, irrespective of type and size, carrying marine pollutants, which brings MARPOL Annex III into the same text.

SOLAS chapter VII and the IMDG Code

The Code was adopted as a mandatory instrument by resolution MSC.122(75) and has applied as such from 1 January 2004. It is amended every two years by a resolution that replaces the complete text, so the amendment number, not the edition year on a book cover, identifies what is in force. MSC.556(108) states in its annex that “the complete text of the IMDG Code is replaced” by Amendment 42-24.

Not every column of the Code is mandatory. IMDG 1.1.1.5 lists the recommendatory parts, among them columns 15 and 17 of the Dangerous Goods List (EmS references and properties and observations), chapter 7.8 and the training paragraphs 1.3.1.4 to 1.3.1.7. Classification, packing, marking, documentation, stowage and segregation are mandatory.

UN Model Regulations and the Manual of Tests and Criteria

The class definitions, packing-group criteria and generic entries are taken from the UN Model Regulations , the Recommendations on the Transport of Dangerous Goods. The tests are in the UN Manual of Tests and Criteria . Amendment 42-24 references the Manual in revision ST/SG/AC.10/11/Rev.7 with Amendment 1, and chapter 2.4 points to its sections directly.

The IMDG Code applies the results; it does not reproduce the apparatus. A shipper or a competent authority classifying a new substance works from the Manual, then reads the outcome against the IMDG criteria.

Amendment cycle and the edition applied here

Three amendments matter to a ship in service in 2026.

Amendment 42-24 is the mandatory text throughout 2026 and 2027. The class-level changes most visible in Class 4 under 42-24 are the charcoal rules in special provision 978 and the separate activated-carbon provision 979, covered below.

Division 4.1: flammable solids, self-reactive substances, desensitized explosives and polymerizing substances

Division 4.1 is defined in IMDG 2.4.2.1 as four types of substance grouped together because they burn or decompose readily, not because they behave alike. The four types carry different tests, different generic entries and, for two of them, temperature control.

Readily combustible solids and the burning-rate test

A readily combustible solid is a powder, granule or paste that can be ignited easily by brief contact with an ignition source such as a burning match, and in which the flame spreads rapidly. IMDG 2.4.2.2.2.1 classifies it as Division 4.1 when, in test N.1, the burning time is less than 45 seconds or the burning rate exceeds 2.2 mm/s.

Metal powders have their own criterion. A metal or metal-alloy powder is Division 4.1 when it can be ignited and the reaction spreads over the whole length of the sample in 10 minutes or less.

Packing groups follow IMDG 2.4.2.2.3:

There is no packing group I for a readily combustible solid. Practitioners who read a PG I against a Division 4.1 entry are looking at a desensitized explosive, not a flammable solid.

Coated and uncoated aluminium powder show how the division boundary works. UN 1309 ALUMINIUM POWDER, COATED is Division 4.1, packing group II or III. UN 1396 ALUMINIUM POWDER, UNCOATED is Division 4.3, because the bare metal surface reacts with water to give hydrogen.

Self-reactive substances, Types A to G

A self-reactive substance is thermally unstable and liable to undergo a strongly exothermic decomposition even without participation of oxygen. IMDG 2.4.2.3.1.1 excludes from the definition any substance that is an explosive, an oxidizer or an organic peroxide , or whose heat of decomposition is less than 300 J/g, or whose self-accelerating decomposition temperature (SADT) exceeds 75 °C for a 50 kg package.

The type decides whether and how the substance may be shipped. IMDG 2.4.2.3.3.2 sets the packaging limits:

Self-reactive substances are carried under twenty generic entries, UN 3221 to UN 3240, split by type, by solid or liquid, and by whether temperature control applies. UN 3223 is SELF-REACTIVE LIQUID TYPE C and UN 3224 is SELF-REACTIVE SOLID TYPE C, both EmS F-J, S-G and stowage category D. A Type C solid that needs refrigeration travels as UN 3234 SELF-REACTIVE SOLID TYPE C, TEMPERATURE CONTROLLED, with EmS F-F, S-K and stowage codes SW1 and SW3.

Temperature control is required where the SADT is 55 °C or less (IMDG 2.4.2.3.4.1). The derivation of the control and emergency temperatures is covered in the temperature-control section below, because the same table governs polymerizing substances.

Solid desensitized explosives

A desensitized explosive is an explosive substance wetted with water or alcohol, or diluted with other substances, to form a homogeneous solid mixture in which the explosive properties are suppressed. IMDG 2.4.2.4.1.1 lists the Division 4.1 entries, and 2.4.2.4.2 adds UN 2956, UN 3241, UN 3242 and UN 3251.

The Dangerous Goods List entries make that dependency explicit in their proper shipping names:

A desensitized explosive that dries out is no longer the substance its declaration describes. The Tianjin explosions of 2015, covered below, began in exactly this way.

Polymerizing substances

Polymerizing substances are the fourth Division 4.1 type, defined in IMDG 2.4.2.5. A substance falls in it when, without stabilization, it is liable to undergo a strongly exothermic reaction producing larger molecules or polymers, its self-accelerating polymerization temperature (SAPT) is 75 °C or less, and its heat of reaction exceeds 300 J/g.

They are carried under four generic entries, UN 3531 to UN 3534, for solids and liquids, stabilized and temperature controlled. UN 3531 POLYMERIZING SUBSTANCE, SOLID, STABILIZED, N.O.S. is packing group III, EmS F-J, S-G, stowage category D.

Temperature control applies when the SAPT is 50 °C or less in packagings and IBCs, or 45 °C or less in portable tanks (IMDG 7.3.7.2.3). Under 7.3.7.2.4, a substance shipped as STABILIZED may still need temperature control if the transport temperature could exceed 55 °C.

Common Division 4.1 entries

Sulphur is a Division 4.1 cargo shipped in packaged form and in bulk. The sulphur schedule of the IMSBC Code covers crushed lump and coarse-grained UN 1350 as a Group B cargo; molten sulphur is a separate entry, UN 2448 SULPHUR, MOLTEN, Division 4.1, packing group III, stowage category C.

Division 4.2: substances liable to spontaneous combustion

Division 4.2 covers substances that heat up and ignite through reaction with air, with no external flame. IMDG 2.4.3.1.1 divides them into pyrophoric substances, which ignite within five minutes of contact with air even in small quantities, and self-heating substances, which ignite only in large amounts and after long periods.

Pyrophoric substances

A pyrophoric solid is tested by N.2 and a pyrophoric liquid by N.3. Every pyrophoric substance is packing group I (IMDG 2.4.3.3). The five-minute criterion means a leaking package can ignite before a damaged container has been landed ashore.

White phosphorus takes F-A, the general fire schedule. The organometallics carrying a 4.3 subsidiary hazard, and pyrophoric calcium, take F-G, the water-reactive schedule, so two pyrophoric entries in the same division call for opposite first actions.

Self-heating substances and the cube test

A self-heating substance is classified by test N.4, in which a sample cube is held in an oven at 140 °C. The result and the package size together decide the packing group under IMDG 2.4.3.3:

The volume conditions in IMDG 2.4.3.2.3 reflect the physics of self-heating : heat generation scales with volume and heat loss with surface area, so a material exempt in packages of 450 litres or less is not exempt in larger ones. No self-heating substance is packing group I.

Self-heating cargoes in trade

Many Division 4.2 entries are organic materials that oxidize slowly. The Amendment 42-24 Dangerous Goods List entries below cover the cargoes a ship planner meets most often.

Three boundaries in that list catch shippers. Stabilized fish meal is UN 2216 and is Class 9 , not Division 4.2. Hydrated sodium sulphide with not less than 30% water is UN 1849 and is Class 8 , packing group II. Both seed cake entries are packing group III only.

Charcoal under special provision 978

Charcoal is UN 1361, packing group II or III, and Amendment 42-24 rewrote its conditions through special provision 978. The provision removes the route by which a shipper used a negative N.4 result to declare charcoal outside the Code: the self-heating test can no longer be used for exemption, and untested material is assigned at least packing group III.

SP 978 then sets conditions on the product itself. The charcoal must be weathered for at least 14 days, or steamed, cooled and inerted, before packing. It must be packed at 40 °C or less, and the cargo transport unit must leave 30 cm of headspace above the packages. Under IMDG 5.4.1.5.18 the transport document carries the production date, the packing date and the packing temperature, so the carrier can see whether weathering time was respected. The details are in charcoal under special provision 978 .

Activated carbon is a separate entry. UN 1362 CARBON, ACTIVATED is packing group III under special provision 979, which turns on certification that the material was steam-activated or tested negative in N.4.

Division 4.3: substances which in contact with water emit flammable gases

Division 4.3 substances react with water to release a flammable gas, and the heat of reaction can ignite it. IMDG 2.4.4.2.1 classifies a substance as Division 4.3 when, in test N.5, spontaneous ignition takes place at any step, or flammable gas evolves at more than 1 litre per kilogram per hour.

Gases evolved and why water is excluded

The gas depends on the chemistry, and it decides the second hazard the crew faces once the fire is out.

Phosphide and ferrosilicon cargoes therefore carry a toxic hazard on top of the fire hazard. That is why UN 1397, UN 1714, UN 2011 and UN 1408 all carry subsidiary hazard 6.1. The toxic-gas side in bulk cargoes is covered in arsine and phosphine in solid bulk cargoes .

Packing groups from the gas-evolution rate

IMDG 2.4.4.3 assigns the packing group on the N.5 result:

The rates are measured per kilogram of substance under test conditions. They are not a prediction of gas volume in a flooded hold, which also depends on surface area, temperature and the amount of water.

Common Division 4.3 entries

UN 1418 is easy to mis-describe from its name. It is Division 4.3 primary with a Division 4.2 subsidiary hazard, in all three packing groups; it is not a Division 4.1 flammable solid.

Stowage of Class 4 cargo

Stowage of a Class 4 entry is read from columns 16a and 16b of the Dangerous Goods List, not from the class. Column 16a gives the stowage category and column 16b the stowage and handling codes defined in IMDG 7.1.5.

Stowage categories A to E

IMDG 7.1.3.2 sets five categories for Classes 2 to 9. The table is for a cargo ship or a passenger ship carrying a limited number of passengers, against other passenger ships. The full scheme is in IMDG stowage categories and codes .

Category E is common in Class 4. Nitrocellulose with water, white phosphorus, the phosphides, lithium and calcium all carry it, so a cargo ship may stow them under deck while a passenger ship may not carry them at all.

Stowage and handling codes

A container stowed on deck against a heated bunker tank bulkhead does not meet SW1, and a container of ferrosilicon stowed under deck in a hold without mechanical ventilation does not meet SW5. Both are planning errors that a stowage check against column 16b catches before loading.

Segregation of Class 4 cargo

Segregation between classes is read from IMDG table 7.2.4, and any SG code in column 16b of the Dangerous Goods List takes precedence over it. The table uses four numbered terms: 1 “away from”, 2 “separated from”, 3 “separated by a complete compartment or hold from”, and 4 “separated longitudinally by an intervening complete compartment or hold from”. X means no general segregation, subject to the entry. The IMDG segregation table article covers the terms in container stowage.

Division 4.2 must be separated from Class 3 flammable liquids , from flammable and toxic Class 2 gases and from Class 5.1, because a self-heating source next to any of them supplies the ignition energy. Division 4.1 needs only “away from” Class 5.1, but “separated from” Class 5.2.

The entry overrides the table where it says so. UN 1350 sulphur carries SG17 in column 16b, an entry-specific requirement read from the Code’s list of segregation codes. A planner who checks only the class table misses it.

Temperature control for self-reactive and polymerizing substances

IMDG section 7.3.7 governs temperature-controlled carriage of self-reactive substances, organic peroxides and polymerizing substances. The control temperature is the maximum at which the substance can be safely carried; the emergency temperature is the point at which the emergency procedures begin.

Control and emergency temperatures

IMDG table 7.3.7.2.6 derives both from the SADT, or the SAPT for polymerizing substances:

The emergency temperature is derived from the SADT, not from the control temperature. A substance with an SADT of 30 °C in a packaging has a control temperature of 15 °C and an emergency temperature of 20 °C.

Equipment and monitoring on board

The Code sets equipment rules for the refrigerated cargo transport unit carrying these substances:

• Insulation. The heat transfer coefficient is not more than 0.4 W/(m2 K) for a cargo transport unit and 0.6 W/(m2 K) for a tank (7.3.7.4.1).
• Two thermometers. Two independent temperature-measuring devices, read every 4 to 6 hours, with the readings logged (7.3.7.4.3).
• Alarm. An alarm warning of refrigeration failure where the control temperature is below +25 °C (7.3.7.4.4).
• Electrical fittings. Plugs rated IP 55 under deck and IP 56 on deck under IEC 60529, temperature class T4 and gas group IIB (7.3.7.4.5).
• Dual refrigeration. UN 3231 and UN 3232, the temperature-controlled Type B self-reactives, require dual or combined refrigeration methods under 7.3.7.5.1, subject to the stated exception.

IMDG 7.3.7.3.4 requires operating instructions, procedures for loss of temperature control and spare parts to be on board. The Code does not set a fixed jettison distance; the action on reaching the emergency temperature comes from those procedures and the EmS schedule for the entry.

Fire response and the EmS Guide

The EmS Guide gives each Dangerous Goods List entry a fire schedule (F-A to F-J) and a spillage schedule (S-A to S-Z) in column 15. Column 15 is recommendatory under IMDG 1.1.1.5, but it is the reference every master and every port fire service uses on a dangerous goods casualty.

Fire schedules by division

F-G is the schedule that excludes water. It covers Division 4.3 and the Division 4.2 entries that react with water, so the label on the container is not on its own a reliable guide to the fire schedule. The crew reads column 15, or the transport document, for the actual entry.

Fixed systems and the cargo space

A ship carrying dangerous goods is subject to the special requirements of SOLAS chapter II-2 , regulation 19, for fire protection of the spaces in which they are carried. Under regulation II-2/19.4 the flag state issues a Document of Compliance listing the classes permitted in each cargo space, and the Class 4 divisions appear on it separately.

Where a Division 4.3 fire is under deck, the hold’s fixed gas fire-extinguishing system avoids the water that water spray and drenching would add to the reaction. Opening a hold after a CO2 release brings in oxygen and a carbon dioxide hazard in the cargo space at the same time, so re-entry follows the gas measurements, not the clock.

First aid for exposure to phosphine, acetylene combustion products or phosphorus burns is in the Medical First Aid Guide , the companion to the EmS Guide.

Marine pollutants and subsidiary hazards

A Class 4 substance is a marine pollutant when column 4 of the Dangerous Goods List carries a P or when the shipper’s classification under IMDG 2.10 says it is one. In the Amendment 42-24 list, UN 1381 white phosphorus and UN 1334 naphthalene carry the P. UN 1397 aluminium phosphide and UN 2011 magnesium phosphide do not.

Single or inner packagings of 5 litres or 5 kg or less are relieved of the marine pollutant provisions under IMDG 2.10.2.7. Above that, the transport document carries the words MARINE POLLUTANT, which may be supplemented by ENVIRONMENTALLY HAZARDOUS (5.4.1.4.3.6). MARPOL Annex III is the treaty behind this layer; the IMDG Code is its implementing text for packaged goods.

Subsidiary hazards are common in Class 4 and change every downstream rule. UN 1381 is 4.2 with subsidiary 6.1; UN 1418 is 4.3 with subsidiary 4.2; UN 3393 and UN 3394 are 4.2 with subsidiary 4.3; the phosphides and ferrosilicon are 4.3 with subsidiary 6.1. The subsidiary hazard is shown in parentheses after the class on the transport document under 5.4.1.4.1, and it carries its own label under the rules in IMDG marking, labelling and placarding .

Class 4 cargoes in bulk and the IMSBC Code

A Class 4 substance shipped as a solid in bulk is carried under the IMSBC Code , not under the IMDG packaging rules. SOLAS regulation VII/7 defines dangerous goods in solid form in bulk by reference to the IMDG Code, so a bulk cargo that carries a UN number and a Class 4 division is a Group B cargo that triggers the Document of Compliance under IMSBC section 4.8.3.

Direct reduced iron is the case that confuses the two Codes. It self-heats and evolves hydrogen in contact with water, so it behaves like a Division 4.2 and 4.3 substance, but it has no IMDG entry in bulk and is classified under the MHB criteria instead. Wood pellets sit on the same side of that line, with no UN number, although the IMSBC Code requires them to be segregated as for Division 4.1 materials. Ammonium nitrate based fertilizer UN 2067 is Class 5.1, not Class 4, and its separation from Class 4 cargo in bulk follows the IMSBC segregation requirements .

Documentation for Class 4 consignments

The shipper’s dangerous goods transport document is the legal description of the cargo. IMDG 5.4.1.4.1 requires the UN number, the proper shipping name with the technical name where the entry is N.O.S., the class, any subsidiary hazard in parentheses, and the packing group.

Class 4 adds entry-specific statements to that core:

• Temperature-controlled substances. “Control temperature … °C, Emergency temperature … °C” under 5.4.1.5.5.
• Charcoal, UN 1361. Production date, packing date and packing temperature under 5.4.1.5.18 and special provision 978.
• Marine pollutants. The words MARINE POLLUTANT under 5.4.1.4.3.6.

Where the goods are packed into a container or vehicle, the person responsible for packing signs a container or vehicle packing certificate . IMDG 5.4.2.1 lists nine conditions: the unit is clean, dry and fit; segregation has been applied; packages are inspected and undamaged; drums are stowed upright and braced; bulk material is evenly distributed; Class 1 units are structurally serviceable; the unit and packages are marked, labelled and placarded; any asphyxiant coolant is marked; and the transport document has been received. The certificate is not required for portable tanks.

On board, IMDG 5.4.3.1 requires a special list, manifest or detailed stowage plan under SOLAS regulation VII/4.2 and MARPOL Annex III regulation 5.2. Emergency response information under 5.4.3.4 may be given in the transport documents, in a safety data sheet, or by reference to the EmS Guide and MFAG. The IMDG Code has no transport emergency card requirement.

For a single lookup of an entry’s class, packing group, EmS and stowage columns, the IMDG dangerous goods finder on ShipCalculators.com returns the Dangerous Goods List row.

Inspection and enforcement

Class 4 compliance is checked at three points: by the carrier at booking, by the ship before loading, and by port state control after. On board, the comparison of the transport document against the stowage plan, the Document of Compliance and the segregation table is assigned through the cargo procedures of the safety management system required by the ISM Code .

A Division 4.3 consignment misdeclared as a non-dangerous chemical receives no H1 handling, no SW5 ventilation and no F-G fire plan. The X-Press Pearl fire off Sri Lanka in May 2021 is a container-ship loss that began in dangerous goods cargo.

Incident record: Tianjin, 12 August 2015

The fire and explosions at the Ruihai International Logistics hazardous goods warehouse at Tianjin Port on 12 August 2015 are a Class 4 casualty with a published state investigation. The Tianjin port explosion was investigated by a State Council investigation team, whose report was published in February 2016.

The report found that nitrocellulose in a container in the south of the arrival area had lost its wetting agent and dried locally; under high summer temperatures it decomposed, accumulated heat and ignited. The fire spread to ammonium nitrate and other dangerous chemicals stored in the arrival area, about 800 tonnes of ammonium nitrate on the day, stored in breach of the rules.

Two explosions followed, 31 seconds apart, at 23:34:06 and 23:34:37, estimated at about 15 tonnes and 430 tonnes of TNT equivalent. The report records 165 dead, 8 missing and 798 injured admitted to hospital, and a direct economic loss of CNY 6.866 billion.

Wetted nitrocellulose is UN 2555 or UN 2556, a Division 4.1 desensitized explosive, only while the water or alcohol content in its proper shipping name holds; dry, it is a Class 1.1D explosive. The report does not name calcium carbide or water applied to it as a cause, and accounts that attribute the explosions to that mechanism do not match the investigation.

Limitations

This article states the Amendment 42-24 text in force from 1 January 2026. The Dangerous Goods List rows were read from the annex to MSC.556(108); a blank cell in the tables above means the column was not stated here, not that the entry has no value, and the full row in the Code governs. Amendment 43-26, mandatory from 1 January 2028, is described here only by its dates; its Class 4 rows are to be read against the 43-26 text before that date.

The test criteria are those stated in IMDG chapter 2.4. The apparatus, sample preparation and procedure for tests N.1 to N.5 are in the UN Manual of Tests and Criteria and are not reproduced. Classification of a new substance is the shipper’s responsibility, and in many cases the competent authority’s.

National rules for domestic legs are not covered. The United States applies 49 CFR to Class 4 hazardous materials and 46 CFR part 148 to bulk solids, and other administrations apply their own overlays; a consignment moving by road or rail before or after the sea leg must meet the modal rules for that leg. The EmS schedules named here are references only; the response on board follows the EmS Guide text, the ship’s procedures and the master’s judgement.

Related Articles

• IMDG Code
• SOLAS Chapter VII: carriage of dangerous goods
• IMDG Class 1: explosives
• IMDG Class 5: oxidizers and organic peroxides
• IMDG Class 6: toxic and infectious substances
• IMSBC Group B cargoes
• Self-heating and spontaneous combustion of solid bulk cargoes
• Self-accelerating decomposition temperature
• Temperature-controlled dangerous goods transport