IMDG Class 1: Explosives

Background

What Class 1 covers

The IMDG Code classifies dangerous goods for sea transport into nine principal classes. Class 1 (Explosives) is the first and one of the most strictly regulated. The class includes:

• Substances that are themselves explosive (TNT, RDX, HMX, PETN, picric acid, lead azide).
• Articles containing explosive substances (cartridges, detonators, fuses, ammunition, military rockets, signal flares, fireworks).
• Substances and articles intended to produce a practical explosive or pyrotechnic effect.

Excluded from Class 1 are substances with such a low risk of accidental ignition or propagation that the consequences are limited to the package; these may fall under other classes (e.g., desensitised explosives in Class 4.1 if wetted with water or alcohol). The dividing line is set by United Nations standardised classification tests (UN Manual of Tests and Criteria, Part I).

Why Class 1 has its own regime

Most dangerous goods classes regulate handling primarily to prevent release of the cargo: a corrosive that leaks corrodes the surrounding deck plating; a flammable that escapes ignites; a toxic that vaporises poisons the crew. The regulatory mitigation is containment.

Class 1 is different. The cargo can release catastrophic energy without first leaving the package. A magazine fire on a vessel carrying Division 1.1 explosives can mass-detonate the entire stowage in a single event, destroying the ship and any vessels or shore facilities within hundreds of metres. The mitigation must address the explosion event itself, not just the containment, which is why the regime imposes specific requirements on:

• Magazine construction (steel-plated, non-spark, secure access).
• Stowage location (away from heat, away from accommodation, away from cargo gear).
• Segregation distances from other classes (much larger than for non-Class-1 goods).
• Quantity limits per magazine and per ship.
• Documentation and pre-arrival notification.
• Crew training in explosives handling.

The result is that Class 1 is the most operationally constrained of all the dangerous goods classes. Most general cargo vessels do not carry Division 1.1 or 1.2 explosives at all; the trade is concentrated in dedicated explosives carriers, military vessels and a small number of heavy-lift carriers configured for the role.

The relationship to UN Recommendations

The IMDG Code Class 1 system mirrors the UN Recommendations on the Transport of Dangerous Goods (Model Regulations, the ‘Orange Book’). The same six divisions and thirteen compatibility groups appear in:

• ADR (European road transport).
• RID (European rail transport).
• ADN (European inland waterway transport).
• ICAO Technical Instructions and IATA DGR (air transport).
• 49 CFR (United States Department of Transportation).

A Class 1 product that ships internationally typically passes through several modal regimes; the common UN classification ensures the safety information travels with the consignment. The IMDG Code adds maritime-specific rules on top (stowage on deck or under deck, distance from accommodation, EmS Emergency Schedules) but does not change the fundamental classification.

The six hazard divisions

Division 1.1: Mass explosion hazard

Substances and articles that have a mass explosion hazard: the entire load can be expected to detonate substantially instantaneously. Detonation propagates through the entire mass of cargo within a fraction of a second.

Typical entries:

• UN 0004 Ammonium picrate, dry or wetted with less than 10 % water.
• UN 0027 Black powder (gunpowder).
• UN 0072 Cyclotrimethylenetrinitramine (RDX), wetted with not less than 15 % water.
• UN 0081 Explosive, blasting, type A.
• UN 0082 Explosive, blasting, type B.
• UN 0084 Explosive, blasting, type D.
• UN 0143 Nitroglycerin, desensitised with not less than 40 % water.
• UN 0144 Nitroglycerin solution in alcohol, more than 1 % but not more than 5 %.
• UN 0150 Pentaerythrite tetranitrate (PETN), wetted with not less than 25 % water.
• UN 0224 Barium azide, dry or wetted.

Division 1.1 is the most strictly regulated. The maximum net explosive quantity per ship is normally controlled by the flag state and the receiving port; many ports refuse Division 1.1 entirely or require specific anchorage areas.

Division 1.2: Projection hazard

Substances and articles that have a projection hazard but not a mass explosion hazard. The cargo may detonate or deflagrate, but the propagation is limited to one package or sub-stack at a time. The principal hazard is from projected fragments (artillery shells, bomb casings, mortar bombs).

Typical entries:

• UN 0006 Cartridges for weapons, with bursting charge.
• UN 0009 Ammunition, incendiary with or without burster, expelling charge or propelling charge.
• UN 0073 Detonators for ammunition.
• UN 0181 Rockets, with bursting charge.

Division 1.2 is common in military shipments and is normally stowed in dedicated magazines on dedicated explosives carriers.

Division 1.3: Fire hazard

Substances and articles that have a fire hazard and either a minor blast hazard or a minor projection hazard or both, but no mass explosion hazard. The cargo burns vigorously and may produce localised blast, but does not detonate.

Typical entries:

• UN 0066 Cord, igniter.
• UN 0143 Nitroglycerin, desensitised.
• UN 0234 Sodium dinitro-o-cresolate, dry or wetted.
• UN 0335 Fireworks, with hazard division 1.3.
• UN 0336 Fireworks, with hazard division 1.4.

Many fireworks fall in Division 1.3. The 2020 fireworks shipment that caused the Beirut port explosion was not the originating cause (that was the ammonium nitrate stowed nearby), but the fireworks shipment ignited in the 2015 Tianjin port explosion, contributing to its severity.

Division 1.4: No significant hazard

Substances and articles that present no significant hazard: the explosive effect is largely confined to the package and no projection of fragments of appreciable size or range is to be expected. An external fire shall not cause virtually instantaneous explosion of almost the entire contents of the package.

Division 1.4 is further subdivided into compatibility group S (1.4S), which has even more relaxed handling rules because the effects of accidental ignition are confined to the package and there is no significant blast or projection hazard.

Typical entries:

• UN 0012 Cartridges for weapons, inert projectile or cartridges, small arms.
• UN 0014 Cartridges for weapons, blank.
• UN 0337 Fireworks, with hazard division 1.4S.
• UN 0349 Articles, explosive, n.o.s.

Division 1.4S is the only Class 1 cargo that may be carried on conventional general cargo vessels in mixed stowage (subject to segregation rules) and on passenger vessels in limited quantities.

Division 1.5: Very insensitive substances

Substances that have a mass explosion hazard but are so insensitive that there is very little probability of initiation or of transition from burning to detonation under normal conditions of transport. Used principally for blasting agents (ammonium-nitrate-based industrial explosives) that require a primary detonator to function.

Typical entries:

• UN 0331 Explosive, blasting, type B (agent blasting type B).
• UN 0332 Explosive, blasting, type E.

Division 1.5 is widely shipped in the mining, quarrying and construction industries. The insensitivity rating allows higher quantity limits per ship, but the goods are still treated as Class 1 for stowage and documentation purposes.

Division 1.6: Extremely insensitive articles

Articles that contain only extremely insensitive detonating substances and which demonstrate a negligible probability of accidental initiation or propagation. Reserved for the most modern military explosives (insensitive munitions).

Typical entries are highly application-specific and not commonly seen in commercial shipping. The division was introduced in the 1990s and is still small in volume.

The thirteen compatibility groups

Why compatibility groups exist

Within a single division, different explosive substances and articles can be incompatible with each other: a fire or detonation involving one item could initiate a more dangerous chain reaction with another. The IMDG Code defines thirteen compatibility groups (A through S, omitting I, M, O, Q and R) that classify each substance or article by what it can safely be stowed with.

Compatibility group is determined by the manufacturer based on the chemical composition, the construction of the article and the intended function. The group is part of the UN entry for each substance and is shown alongside the division (e.g., ‘Division 1.1D’ is Division 1.1 in compatibility group D; ‘Division 1.4S’ is Division 1.4 in compatibility group S).

Group definitions

The thirteen groups are:

• A: Primary explosive substances. Highly sensitive initiating compounds (lead azide, mercury fulminate). Not normally carried at sea outside specialised consignments.
• B: Articles containing a primary explosive substance and not containing two or more effective protective features. Detonators, blasting caps.
• C: Propellant explosive substance or other deflagrating explosive substance, or article containing such substance. Smokeless powder, rocket propellant.
• D: Secondary detonating explosive substance, or black powder, or article containing a secondary detonating explosive substance. TNT, RDX, dynamite.
• E: Article containing a secondary detonating explosive substance, without means of initiation, with a propelling charge. Tank gun ammunition without primer.
• F: Article containing a secondary detonating explosive substance, with its means of initiation, with or without a propelling charge. Complete artillery rounds with fuse.
• G: Pyrotechnic substance, or article containing a pyrotechnic substance, or article containing both an explosive substance and an illuminating, incendiary, tear-producing or smoke-producing substance. Flares, signal cartridges, illumination ammunition.
• H: Article containing both an explosive substance and white phosphorus.
• J: Article containing both an explosive substance and flammable liquid or gel.
• K: Article containing both an explosive substance and a toxic chemical agent.
• L: Explosive substance or article containing an explosive substance and presenting a special risk (water-activated devices, hypergolic propellants).
• N: Articles containing only extremely insensitive detonating substances. The Division 1.6 group.
• S: Substance or article so packed or designed that any hazardous effects arising from accidental functioning are confined within the package, unless the package is degraded by fire. The Division 1.4S group, with the most relaxed handling rules.

Compatibility table

The IMDG Code Table 7.2.6.3.1 specifies which compatibility groups may be stowed together in the same compartment. The rules are dense; the broad pattern is:

• Group A is incompatible with everything except itself.
• Group B can be stowed with Group D (with some restrictions).
• Group C, D and E are mutually compatible in some configurations.
• Group F is incompatible with most other groups due to the integrated initiation risk.
• Group G must be separated from other classes due to the pyrotechnic flash hazard.
• Group H, J, K must be separated due to the additional hazards (phosphorus, flammable liquid, toxic agent).
• Group L must be in a separate magazine.
• Group N is mutually compatible only with itself.
• Group S is the most permissive and may be stowed with most other Class 1 groups.

The IMDG segregation calculator implements the full table for any pair of UN entries, returning a stowage decision that satisfies both the inter-class and intra-class rules.

Stowage and segregation

Magazine types

Class 1 cargo is stowed in a magazine: a defined cargo space constructed and equipped for the purpose. The IMDG Code Chapter 7.1 defines four magazine types:

• Type A: a closed cargo space surrounded by steel plating, with no openings other than secured access doors. The most secure. Required for most Division 1.1 cargoes.
• Type B: a closed cargo space surrounded by steel plating but with the possibility of openings to the deck for ventilation. Suitable for Division 1.2 and Division 1.3.
• Type C: a closed cargo space without the full integrity of Type A or Type B. Suitable for some Division 1.4 cargoes.
• Type D: a portable magazine: a freight container or vehicle constructed to magazine standards and treated as a temporary magazine. Suitable for Division 1.4S and limited 1.4 quantities.

The required magazine type for a given cargo is set out in the IMDG Code Schedule for that UN entry. A vessel without the required magazine type cannot accept the cargo.

On-deck vs under-deck stowage

The IMDG Code Schedule for each Class 1 UN entry specifies whether the cargo may be stowed:

• On deck only: many Division 1.4 and 1.5 cargoes, particularly in containers. The on-deck stowage allows fast jettison in emergency.
• On deck or under deck: most Division 1.1, 1.2, 1.3 cargoes when stowed in approved magazines.
• Under deck only: some hazardous-fume cargoes that must be confined.

The choice between on-deck and under-deck affects the response to fire (on-deck cargoes can be jettisoned; under-deck cargoes must be fought in place) and the structural protection (under-deck cargoes are protected from impact, on-deck cargoes are exposed).

Segregation from other classes

The IMDG Code Chapter 7.2 segregation rules require minimum separation distances between Class 1 and other classes:

• From Class 1: between Class 1 cargoes themselves, the requirement is governed by the compatibility group table (above).
• From Class 2 (gases): ‘separated from’ (different cargo holds, different compartments above deck).
• From Class 3 (flammable liquids): ‘separated longitudinally by an intervening complete compartment or hold from’.
• From Class 4 (flammable solids, self-reactive substances): ‘separated longitudinally by an intervening complete compartment or hold from’.
• From Class 5.1 (oxidising substances): ‘separated longitudinally by an intervening complete compartment or hold from’ for most divisions; strictly prohibited in some configurations involving Division 1.1 and ammonium nitrate.
• From Class 6.1 (toxic): ‘separated from’.
• From Class 8 (corrosives): ‘separated from’.

The four IMDG segregation distances (‘away from’, ‘separated from’, ‘separated by a complete compartment or hold from’, ‘separated longitudinally by an intervening complete compartment or hold from’) are defined in Code Chapter 7.2 and apply both to packaged goods in containers and to bulk cargoes interacting with packaged goods.

Quantity limits

Many flag states impose maximum net explosive quantity (NEQ) per ship limits on Class 1 cargoes:

• Division 1.1 may be limited to 50 tonnes NEQ per ship for general cargo vessels (higher for dedicated explosives carriers).
• Division 1.2 limits are typically higher, in the 200-500 tonnes range.
• Division 1.4 limits are usually only operational (fitting in available magazine space).

Port states add their own NEQ limits, often expressed as maximum quantity per anchorage or per berth. The shipowner’s chartering and operations team must verify both flag and port limits before accepting a fixture for Class 1 cargo.

EmS Emergency Schedules

The EmS system

The Emergency Response Procedures for Ships Carrying Dangerous Goods (EmS) is a companion publication to the IMDG Code, published jointly by IMO. EmS provides standardised emergency response procedures that the ship’s crew can follow without prior chemistry knowledge.

Each UN entry in the IMDG Code is assigned two EmS schedules:

• A Fire schedule (F-A through F-Z, with subscripts).
• A Spillage schedule (S-A through S-Z).

The Class 1 fire schedules are concentrated in the F-A through F-E range:

• F-A: General fire schedule (applies to Division 1.4S and minor Class 1 entries).
• F-B: Class 1 explosives, general fire (applies to most Division 1.1, 1.2, 1.3 entries).
• F-C: Class 1, fire on deck only (some Division 1.4 entries with on-deck stowage requirement).
• F-D: Class 1 with secondary fire risks (combustible packaging).
• F-E: Class 1 with toxic combustion products.

Spillage schedules for Class 1 are less critical because the principal hazard is fire and explosion rather than spillage; S-A (general) and S-B (small spillage requiring containment) cover most cases.

The IMDG EmS lookup calculator returns the F-x and S-y schedules for any UN entry and links to the full schedule text from the IMO publication.

What the schedules tell the master

Each F-x or S-y schedule provides:

• Initial response steps (sound general alarm, muster crew, assess scene).
• Fire-fighting agent (water, foam, dry powder, CO₂, none).
• Boundary cooling requirements.
• Jettison guidance (when permitted, when prohibited).
• Personal protective equipment requirements.
• Special precautions (e.g., ‘do not fight fire in cargo, evacuate vessel’).

For Division 1.1 cargoes (F-B), the standard guidance is to abandon ship if fire reaches the magazine. The mass detonation hazard makes any attempt to fight a developed Class 1 fire futile and lethal. The boundary cooling and crew evacuation focus determines that all crew survive even if the cargo is lost.

Documentation

The Multimodal Dangerous Goods Form (MDGF)

The shipper of any Class 1 consignment must provide the carrier with a Multimodal Dangerous Goods Form (MDGF) that contains:

• UN number and proper shipping name.
• Class (1) and division (e.g., 1.1).
• Compatibility group (e.g., D).
• Packing group (Class 1 generally has no packing group assigned; the field is left blank).
• Number and type of packages.
• Net explosive quantity (NEQ) in kilograms.
• Marine pollutant indicator if applicable.
• EmS reference.
• 24-hour emergency contact number.

The MDGF is the foundation document. The carrier uses it to construct the cargo manifest, the dangerous goods declaration to port authorities and the stowage plan.

Container/Vehicle Packing Certificate

For Class 1 cargo packed in a freight container or vehicle, the packer must complete a Container/Vehicle Packing Certificate (per Section 5.4.2 of the IMDG Code) confirming:

• The container or vehicle was clean, dry and apparently fit before loading.
• Packages were securely loaded and secured against movement.
• Compatibility groups within the unit are mutually compatible.
• Marking and placarding are correct on all four sides plus top and bottom of the unit.
• The MDGF is enclosed.

The certificate is normally signed by the consolidator at the inland packing facility. Without it, the carrier should refuse to load the container.

Marking and placarding

Class 1 packages and freight containers carry the standard Class 1 placard: orange diamond with the explosion symbol, the division number (1.1 through 1.6), the compatibility group letter and ‘EXPLOSIVES’ written across the bottom band. The placard must be visible on all four sides plus the roof and the floor (or four sides plus top and bottom for a freight container).

Compatibility group letter must match the cargo (e.g., a 1.4S placard for fireworks UN 0337). A 1.4 placard without the S suffix indicates the more restrictive 1.4 (non-S) treatment.

Notification to port authorities

Most ports require pre-arrival notification for Class 1 cargo, typically 24-72 hours before arrival, with the manifest, the MDGF and the stowage plan. Some ports require an explosives anchorage (a designated anchor position outside the main port area) where the vessel waits for berth allocation, sometimes with port-supplied tugs or fire-fighting craft on standby.

The vessel’s agent normally coordinates the notification. Failure to notify can result in the cargo being refused, the vessel being held at anchorage at the owner’s expense, and in serious cases prosecution under the port’s hazardous cargo regulations.

Notable casualties

SS Mont-Blanc, Halifax 1917

The Halifax Explosion of 6 December 1917 was caused when the French munitions ship SS Mont-Blanc, loaded with approximately 2,653 tonnes of explosives (TNT, picric acid, gun cotton, benzol), collided with the Norwegian SS Imo in Halifax Harbour. A fire on the Mont-Blanc spread to the cargo and detonated the entire load in a single event approximately 20 minutes after the initial collision. The explosion killed approximately 2,000 people, injured 9,000 and destroyed most of the north end of Halifax. It was the largest man-made explosion before the Trinity test of 1945.

The disaster drove substantial reform in the regulation of explosives at sea, including the requirement for trained explosives officers on board explosives carriers and the standardisation of stowage rules.

SS Grandcamp, Texas City 1947

The Texas City Disaster of 16 April 1947 was caused when the French Liberty ship SS Grandcamp, loaded with approximately 2,300 tonnes of ammonium nitrate fertiliser, caught fire at the dock. The fire spread to the cargo and the ammonium nitrate detonated in a mass explosion. The blast killed approximately 580 people, sank the SS High Flyer (also loaded with ammonium nitrate, which detonated 16 hours later in a secondary explosion) and destroyed the Texas City refining and chemical complex.

Texas City established that ammonium nitrate fertiliser, although not classified as Class 1, can mass-detonate under conditions of fire and confinement. The lesson drove the modern classification of ammonium nitrate as either a Class 1.5 explosive (when so manufactured) or a Class 5.1 oxidiser with mandatory segregation from organic combustibles.

Tianjin port explosion 2015

The Tianjin port explosion of 12 August 2015 occurred at a hazardous goods storage facility in the Port of Tianjin, China. A fire involving approximately 800 tonnes of ammonium nitrate, 500 tonnes of potassium nitrate, 700 tonnes of sodium cyanide and various Class 1 fireworks led to two large explosions estimated at 21 tonnes and 240 tonnes of TNT equivalent. 173 people were killed, including 99 first responders, and 798 were injured.

Tianjin highlighted the dangers of co-storage of incompatible classes (oxidisers, toxic substances, explosives) and inadequate enforcement of separation distances. The Chinese authorities subsequently revised the hazardous storage regulations and increased the separation distances between hazardous storage facilities and residential areas.

Beirut port explosion 2020

The Beirut port explosion of 4 August 2020 occurred when approximately 2,750 tonnes of ammonium nitrate stored in Hangar 12 at the Port of Beirut detonated. The cargo had been confiscated from the MV Rhosus in 2014 and stored without proper segregation or security for six years. A fire of unclear origin (possibly involving fireworks stored nearby in the same hangar) ignited the ammonium nitrate, producing one of the largest non-nuclear explosions in history.

The blast killed approximately 218 people, injured over 7,000 and rendered approximately 300,000 homeless. The 2.75 kT TNT-equivalent yield was approximately one-fifth of the Hiroshima atomic bomb. Beirut reinforced the lessons of Texas City and Tianjin: ammonium nitrate stored in bulk requires the strictest segregation and the strictest fire prevention controls.

Regulatory framework summary

Primary instruments

• SOLAS Chapter VII (Carriage of Dangerous Goods) makes IMDG Code compliance mandatory for vessels engaged in international voyages.
• MARPOL Annex III (Prevention of Pollution by Harmful Substances Carried by Sea in Packaged Form) addresses the pollution aspects.
• IMDG Code (the operational rule book, updated every two years; the current edition at the time of writing is IMDG 41-22, with IMDG 42-24 entering into force in 2026).
• EmS Guide (companion to IMDG, fire and spillage schedules).
• MFAG (Medical First Aid Guide), companion publication for crew exposure response.
• CSS Code (Cargo Stowage and Securing) for the lashing and securing of Class 1 freight containers and packages.

Flag state and port state

• Flag state: ratifies SOLAS and MARPOL, makes IMDG Code mandatory through national legislation, may add national requirements (US 49 CFR adds further restrictions in US waters).
• Port state: enforces local regulations on hazardous cargo handling, sets NEQ limits per anchorage and berth, designates explosives anchorages, requires pre-arrival notification.
• Coastal state: may require special routeing or pilotage for vessels carrying Class 1 in coastal waters (e.g., the Strait of Dover dangerous goods reporting requirements).

Related Calculators

• IMDG Segregation Distance Calculator
• IMDG Packing Group (PG I/II/III) Calculator
• IMDG, Container Hazard Class Display Calculator
• IMDG EmS, Emergency Schedule Calculator
• IMDG Limited Quantity, Mark Check Calculator
• Tank Container, T-Code Selection Calculator
• Imo Imdg Calculator
• IMDG, Packing Group Screening Calculator

See also

• IMSBC Code - the parallel framework for solid bulk dangerous cargoes.
• IMDG segregation calculator - automated segregation decision for any pair of UN entries.
• IMDG packing group calculator - packing group lookup.
• Container IMDG class lookup - container-level class verification.
• IMDG EmS lookup - Fire and Spillage schedule lookup.
• IMDG limited quantity calculator - limited quantity exemption check.
• IMDG tank container calculator - tank container compatibility.
• IMDG packing group from flash point calculator - packing group derivation from flash point and boiling point.
• Calculator catalogue - full listing of computational tools.
• ShipCalculators.com home - return to the home page.

References

• IMO, International Maritime Dangerous Goods Code, 2022 Edition (IMDG 41-22), International Maritime Organization, 2022.
• IMO, EmS Guide: Revised Emergency Response Procedures for Ships Carrying Dangerous Goods, International Maritime Organization, current edition.
• IMO, Medical First Aid Guide for Use in Accidents Involving Dangerous Goods (MFAG), International Maritime Organization, current edition.
• IMO, Code of Safe Practice for Cargo Stowage and Securing (CSS Code), International Maritime Organization, current edition.
• United Nations, Recommendations on the Transport of Dangerous Goods, Model Regulations, 22nd revised edition, United Nations, 2021.
• United Nations, Manual of Tests and Criteria, Part I, 7th revised edition, United Nations, 2019.
• John G Bartlett, The Halifax Explosion: An Analysis of the Mont-Blanc Detonation, Naval Institute Press, 1979.
• Hugh W Stephens, The Texas City Disaster, 1947, University of Texas Press, 1997.
• State Council of the People’s Republic of China, Report on the Tianjin Port 8.12 Major Fire and Explosion Accident Investigation, 2016.
• Lebanese Army, Investigation Report on the Beirut Port Explosion of 4 August 2020, 2021.