The IMDG Code, short for the International Maritime Dangerous Goods Code, is the International Maritime Organization’s mandatory rulebook for shipping packaged hazardous materials by sea. It covers how dangerous goods are classified, packaged, marked, documented, loaded, and responded to in an emergency, and it applies to every country that has ratified the SOLAS convention, which is nearly every maritime nation. The current edition incorporates Amendment 42-24 and became mandatory on January 1, 2026.1International Maritime Organization. IMDG Code Anyone who packs, documents, or carries hazardous cargo across an ocean has to work within it, because violations reach into criminal prosecution, vessel detention, and civil penalties above $100,000 per infraction.
Where the Code Gets Its Legal Force
The code is not a voluntary industry standard. It binds through two international treaties. The first is the International Convention for the Safety of Life at Sea (SOLAS). Chapter VII, Part A of SOLAS requires that all dangerous goods shipped in packaged form comply with the IMDG Code.2Marine Department. SOLAS Chapter VII – Carriage of Dangerous Goods Any vessel flying the flag of a contracting state is legally obligated to follow it.
The second treaty is MARPOL, the International Convention for the Prevention of Pollution from Ships. Annex III addresses harmful substances carried in packaged form and defines them by reference to the IMDG Code’s marine pollutant classifications.3International Maritime Organization. International Convention for the Prevention of Pollution from Ships (MARPOL) Between them, SOLAS handles safety of life and MARPOL handles protection of the marine environment, and the IMDG Code carries out both.
The Nine Hazard Classes
Every dangerous substance falls into one of nine classes based on its primary hazard. Several classes split into divisions that separate meaningfully different risks within the same category:
- Class 1, Explosives — six divisions, from mass-explosion hazards (1.1) down to extremely insensitive articles (1.6).
- Class 2, Gases — flammable (2.1), non-flammable and non-toxic (2.2), and toxic (2.3).
- Class 3, Flammable liquids — substances with flash points low enough to ignite during transport.
- Class 4, Flammable solids and reactive substances — ordinary flammable solids (4.1), spontaneously combustible materials (4.2), and materials that emit flammable gases on contact with water (4.3).
- Class 5, Oxidizers (5.1) and organic peroxides (5.2).
- Class 6, Toxic substances (6.1) and infectious substances (6.2).
- Class 7, Radioactive material.
- Class 8, Corrosives.
- Class 9, Miscellaneous dangerous goods, including lithium batteries and environmentally hazardous substances.
Every classified substance is assigned a four-digit UN Number and a Proper Shipping Name that identify it universally. Gasoline is UN 1203 in every port and every language. That common identity is what allows the code to work across borders, and it is why getting the class wrong is not just paperwork: an oxidizer stowed next to a flammable solid because someone mislabeled the hazard can start a fire at sea.
Packing Groups and Packaging
Dangerous goods must travel in UN-certified packaging built to withstand ocean transport. Most substances are assigned to one of three packing groups based on the degree of danger they present:4United Nations Economic Commission for Europe. UN Recommendations on the Transport of Dangerous Goods – Part 2
- Packing Group I — high danger.
- Packing Group II — medium danger.
- Packing Group III — low danger.
The packing group sets the performance standard the container must meet. Group I packaging is tested at higher drop heights and greater internal pressures than Groups II and III. Intermediate bulk containers and portable tanks have their own technical specifications calibrated to what they carry. Approved containers pass drop tests, internal pressure tests, and stacking tests that simulate the weight of containers piled above during a voyage.
When several packages are consolidated into a single outer container for handling, that container becomes an “overpack” and takes on its own requirements. The overpack must show the proper shipping name, UN number, and hazard labels for each substance inside, unless the original markings remain visible from the outside. The word “OVERPACK” must appear in letters at least 12 mm high when specification packaging is required. Packing Group I corrosives and Packing Group I oxidizers cannot be overpacked with other materials at all.5eCFR. 49 CFR 173.25 – Authorized Packagings and Overpacks
Marking and Labeling
The code’s visual system uses diamond-shaped hazard labels on individual packages and larger placards on cargo transport units. Each class has assigned colors and symbols: red for flammables, yellow for oxidizers, white with a skull for toxic substances. The Proper Shipping Name and UN Number appear on the exterior of the cargo so dock workers, ship crews, and emergency responders can identify what they’re handling. Marks must sit on contrasting backgrounds and remain legible after three months of seawater immersion, a standard designed so identification survives an accident at sea.
Marine pollutants carry an extra marking beyond the standard hazard labels. The marine pollutant mark is a square-on-point symbol showing a fish and tree in black on a white background, at least 100 mm per side for non-bulk packages. Bulk packages of 3,785 liters or more need the mark on each side and each end at a 250 mm minimum, and freight containers carrying marine pollutants must display it on all four exterior faces. Single packages containing 5 liters or less of liquid or 5 kg or less of solid are exempt.6eCFR. 49 CFR 172.322 – Marine Pollutants
Shipping Documentation
Every hazardous shipment needs a completed Multimodal Dangerous Goods Form. It is the legal declaration for the cargo and must list the UN Number, Proper Shipping Name, hazard class, and packing group, entered in that specific sequence.7International Maritime Organization. Multimodal Dangerous Goods Form The shipper must also supply a 24-hour emergency contact number staffed by someone with technical knowledge of the substance. The details must match the entries in the code’s Dangerous Goods List exactly; small discrepancies between the paperwork and the physical cargo create real problems at inspection.
A separate container/vehicle packing certificate is signed by the person responsible for loading the cargo transport unit, confirming the goods were packed in accordance with the regulations. Port state control officers verify these documents against the vessel’s manifest before allowing entry into a harbor, and intentional failures in documentation can lead to criminal prosecution.
Under U.S. federal regulations, shippers must retain copies of hazardous materials shipping papers for at least two years after the initial carrier accepts the material. For hazardous waste, the period is three years. Records must be available at the shipper’s principal place of business and produced for inspection on request, and each retained copy must show the date the initial carrier accepted the shipment.8eCFR. 49 CFR 172.201 – Preparation and Retention of Shipping Papers
Stowage and Segregation
Once the paperwork clears, the vessel’s master decides where each container is stowed. Some goods are restricted to on-deck stowage; others may go below. The harder question is segregation: keeping incompatible substances far enough apart that an incident involving one does not trigger a reaction with another. The code defines four segregation levels, each with progressively greater separation:
- “Away from” — at least 3 meters horizontal separation, but the goods may share the same compartment.
- “Separated from” — different compartments below deck, or at least 6 meters apart on deck.
- “Separated by a complete compartment or hold from” — a full compartment between the goods below deck, or at least 12 meters on deck.
- “Separated longitudinally by an intervening complete compartment or hold from” — the strictest, requiring at least 24 meters of longitudinal separation on deck with a complete compartment in between.
The required distance between any two classes comes from a master segregation table that maps every class against every other class with a numbered code. Class 1 explosives paired with Class 3 flammable liquids call for maximum separation. After loading, the crew verifies the stowage plan and confirms every container is secured against movement and correctly separated.
Training Requirements
The code requires all shore-based personnel involved in dangerous goods shipping to be trained before they take on any responsibility. Anyone who has not yet completed the training may work only under the direct supervision of someone who has. The code calls for periodic refresher training to keep pace with regulatory changes, without setting a rigid schedule.
U.S. federal regulations spell the components out more specifically. Hazardous materials employees must complete general awareness training, function-specific training on the tasks they actually perform, safety training on emergency response and hazard avoidance, and security awareness training on recognizing threats. Employees at companies that maintain a security plan for hazardous materials must also receive in-depth security training on the plan’s objectives, procedures, and breach protocols. All training must be repeated at least every three years, and new employees must receive security awareness training within 90 days of starting.9eCFR. 49 CFR 172.704 – Training Requirements Professional IMDG compliance courses typically run between $189 and $625, depending on the provider and scope.
Emergency Response at Sea
Each entry in the code is assigned an Emergency Schedule, or EmS, with specific guidance for fires and spills involving that substance. The EmS Guide runs two response tracks, one for firefighting and one for spillage, and these are referenced in the shipping documentation so the crew can find the correct procedure quickly if something goes wrong.10International Maritime Organization. Emergency Response Procedures for Ships Carrying Dangerous Goods
Under the International Safety Management Code, every vessel must maintain a Safety Management System that includes procedures for responding to potential emergencies. The IMO intends the EmS Guide to form the foundation of the dangerous goods portion of those procedures, built into the ship’s contingency plan.
Amendment 42-24 and the Two-Year Cycle
The code operates on a two-year amendment cycle, with each new edition incorporating updated UN recommendations, new entries, and revised special provisions.11International Maritime Organization. IMDG Code Amendment Cycle Amendment 42-24, the 2024 Edition, became mandatory on January 1, 2026. Through 2025, both 41-22 and 42-24 were accepted, giving shippers a transition window that has now closed.
Amendment 42-24 added new UN numbers for sodium-ion batteries (UN 3551 and 3552), lithium battery-powered vehicles (UN 3556 and 3557), and fire suppressant devices. It revised classification definitions for certain explosives and updated special provisions for marine pollutant packaging exemptions. Documentation and procedures should now reflect the 42-24 edition; port inspectors are no longer accepting references to the previous amendment.
Enforcement and Penalties
The code itself does not impose fines or criminal sentences. Each contracting state enforces it through domestic law, so the severity of penalties varies by country. In practice, port state control inspections are the main enforcement mechanism worldwide. Inspectors can detain a vessel for the absence of a dangerous cargo manifest, stowage or segregation violations, damaged or leaking packages, or crew unfamiliarity with their cargo-related duties.
In the United States, civil penalties for hazardous materials transportation violations are adjusted annually for inflation. The current maximum civil penalty is $102,348 per violation, rising to $238,809 if the violation results in death, serious injury, or substantial property destruction.12Federal Register. Revisions to Civil Penalty Amounts, 2025 Criminal penalties are steeper. Knowingly violating federal hazardous materials transportation law carries up to five years in prison, and if the violation involves a release that causes death or bodily injury, the maximum imprisonment jumps to ten years.13eCFR. 49 CFR Part 209 Subpart B – Hazardous Materials Penalties
Discrepancies between the physical cargo and the manifest carry their own consequences. When goods on board don’t match the filed documentation, the master or person responsible is subject to penalties under customs law, and the cargo can be seized. Intentional failure to file a manifest exposes the responsible officer to criminal prosecution and forfeiture of the vessel used in the offense.14eCFR. 19 CFR 123.9 – Explanation of a Discrepancy in a Manifest