Dangerous goods by sea: the IMDG Code in plain English
Paint, perfume, lithium batteries, aerosols, resins, cleaning chemicals and many machine spares are all "dangerous goods" (DG) when they go to sea. The rules come from the International Maritime Dangerous Goods (IMDG) Code, which is mandatory under SOLAS. Here's how it works.
The 9 classes
| Class | What it covers | Everyday examples |
|---|---|---|
| 1 | Explosives (divisions 1.1 to 1.6) | Fireworks, flares, ammunition, airbag inflators |
| 2 | Gases: 2.1 flammable, 2.2 non-flammable and non-toxic, 2.3 toxic | Aerosols, gas cylinders, fire extinguishers |
| 3 | Flammable liquids | Paint, solvents, fuel, perfume, adhesives |
| 4 | 4.1 flammable solids, 4.2 spontaneously combustible, 4.3 dangerous when wet | Matches, some metal powders, calcium carbide |
| 5 | 5.1 oxidizers, 5.2 organic peroxides | Pool chlorine, ammonium nitrate, resin hardeners |
| 6 | 6.1 toxic, 6.2 infectious | Pesticides, some dyes, medical samples |
| 7 | Radioactive material | Medical isotopes, some gauges |
| 8 | Corrosives | Acids, caustic soda, wet-cell batteries |
| 9 | Miscellaneous, including environmentally hazardous substances | Lithium batteries, dry ice, vehicles, asbestos, airbag modules |
Many substances also carry a subsidiary hazard, for example a flammable liquid that is also toxic. Many are also marine pollutants and need the marine pollutant mark.
UN number, proper shipping name and packing group
- UN number: a four-digit ID, for example UN 1263 for paint. It's found in section 14 of the safety data sheet (SDS).
- Proper shipping name (PSN): the exact name from the IMDG Dangerous Goods List, for example "PAINT". Trade names aren't allowed on their own. Some "N.O.S." (not otherwise specified) entries also need the technical name in brackets.
- Packing group (PG): PG I is high danger, PG II is medium and PG III is low. It decides how strong the packaging must be, and the quantity limits.
- The Dangerous Goods List also gives the EmS (emergency schedule), the stowage and segregation codes, special provisions, and the limited and excepted quantity limits.
Packaging and marking
- Use UN-approved packaging marked with the UN symbol and a code, for example
4G/Y25/S/26/GB/…. The letter shows the packing groups it's approved for: X covers PG I to III, Y covers PG II and III, and Z covers PG III only. Follow the manufacturer's closing instructions exactly. - Mark each package with "UN" plus the number and the proper shipping name. Add the class hazard label(s), at least 100 × 100 mm, and the marine pollutant mark if it applies. Add orientation arrows for liquids in combination packagings.
Paperwork: the DG declaration and the container packing certificate
- Dangerous goods transport document (IMDG 5.4.1): the shipper's declaration. It must show the UN number, PSN, class (and any subsidiary hazard), packing group, marine pollutant status, flashpoint (if 60 °C or below), number and type of packages, and quantity. It's signed by the shipper's trained person.
- Container or vehicle packing certificate (IMDG 5.4.2): signed by whoever packed the container. It confirms the unit was clean, dry and fit, the goods were segregated, packed and secured properly, and the container is correctly marked and placarded.
- Multimodal Dangerous Goods Form (IMDG 5.4.5): one form that combines both. In the UK, it's usually called the DGN (Dangerous Goods Note) and is used by ports and carriers.
- Pack the container in line with the IMO/ILO/UNECE CTU Code. Securing and blocking matters just as much as the paperwork.
Placards and container marking
- Put class placards (at least 250 × 250 mm) on all four sides of the container, meaning both sides and both ends. Do the same with the marine pollutant mark where it applies.
- Some loads also need the UN number shown on the container, for example more than 4,000 kg gross of one UN number, with no other DG in the container. Fumigated units need the fumigation warning sign.
- Take the placards off once the container has been emptied and cleaned.
Segregation basics
Incompatible goods must be kept apart, for example acids and alkalis, or oxidizers and flammables. The IMDG Code uses four levels of segregation: away from, separated from, separated by a complete compartment or hold from, and separated longitudinally by an intervening complete compartment or hold from. It also uses segregation groups, such as acids, ammonium compounds and cyanides. Dangerous goods that need any segregation from each other, including "away from", must not be packed in the same container, unless the competent authority approves it for "away from" goods (IMDG Code 7.3.4.1). Your carrier's DG desk will check segregation on board.
Limited and excepted quantities
- Limited quantities (LQ): small inner packagings (the limit per substance is in the Dangerous Goods List) in strong outer packaging, normally up to 30 kg gross per package. Use the black-and-white diamond LQ mark instead of hazard labels. The container needs the LQ mark rather than class placards. You still declare it to the carrier, and it still needs DG documentation.
- Excepted quantities (EQ): very small amounts (codes E1 to E5), such as samples. They use the EQ mark and have very light requirements, but they're still DG.
Booking and carrier acceptance
- Declare the DG when you book. Never ship DG as general cargo. Undeclared DG is a major cause of ship fires.
- The carrier's DG desk must approve the booking before the container goes to the terminal. Some carriers refuse certain classes or UN numbers, or have their own extra rules. Expect a DG surcharge and an earlier documentation cut-off.
- Ports and terminals need advance notice too. Some vessels and routes (and transhipment ports) don't accept particular classes.
Who must be trained?
Under IMDG Chapter 1.3, every shore-side person whose job involves DG must be trained for their role, and the employer must keep training records. That includes whoever classifies, packs, marks, labels, placards, signs the documents, books or handles the goods. Training covers:
- general awareness: classes, labels, placards, documents and segregation
- function-specific training: the exact rules for that person's tasks
- safety training: hazards, safe handling and emergency response
The MCA expects the training to be refreshed regularly and records to be kept. Many carriers and forwarders expect refresher training at least every two years.
Packaging & labels
Buy from suppliers who provide UN test certificates and closing instructions:
- UN-approved fibreboard boxes (4G) and combination packs
For PG II and III liquids and solids, with inner receptacles - UN-approved steel and plastic drums and jerricans
1A1/1A2, 1H1/1H2 and 3H1, for larger liquid volumes - DG hazard labels and 250 mm container placards
Class labels, marine pollutant marks, LQ marks and orientation arrows - Absorbent and cushioning material
Vermiculite and absorbent pads, where the packing instruction requires them - Lithium battery marks and Class 9 labels
For batteries shipped by sea under special provision 188 or as Class 9
Links marked (Ad) may be affiliate links, which means we may earn a small commission if you buy, at no extra cost to you. Always check that packaging is UN-approved for your substance, packing group and mode of transport.
Sources
- International Maritime Organization (IMO), IMDG Code: 2024 edition (Amendment 42-24), mandatory from 1 January 2026 (accessed October 2026).
- IMO, Dangerous goods: overview of IMDG and the CTU Code.
- Hazcheck, IMDG Amendment 43-26 summary: voluntary from 1 January 2027, mandatory from 1 January 2028.
- Maritime and Coastguard Agency (MCA), MGN 699 (M): carriage of cargoes guidance (UK).
- GOV.UK, Moving dangerous goods: UK overview, including links for each mode of transport (DfT).
Guidance only. This is a simplified summary, not the official text and not legal advice. Always use the current IMDG Code, your carrier's DG rules and a trained DG professional or dangerous goods safety adviser.