If you have classified dangerous goods for road or air, the classes of IMDG will look immediately familiar. Same nine hazard classes, same UN numbers, same packing groups. That familiarity is the trap, because the classes are where ocean classification starts, not where it finishes.
The IMDG Code (maintained by the International Maritime Organization) adds three layers that no road or air classification produces: segregation, stowage and marine pollutant status. This guide covers what carries over, what does not, and what will get a container refused at the terminal gate.
The classes of IMDG carry over unchanged
The IMDG Code uses the same taxonomy as everything else, because all of it descends from the UN Model Regulations:
- Class 1 explosives, divisions 1.1 to 1.6
- Class 2 gases — 2.1 flammable, 2.2 non-flammable non-toxic, 2.3 toxic
- Class 3 flammable liquids
- Class 4 flammable solids, spontaneously combustible, dangerous when wet
- Class 5 oxidisers and organic peroxides
- Class 6 toxic and infectious substances
- Class 7 radioactive material
- Class 8 corrosives
- Class 9 miscellaneous dangerous goods, including lithium batteries
If you want the classes themselves in detail, our guide to the nine classes of hazardous materials covers each with examples. What follows is the ocean-specific material.
Segregation: the concept air shippers never meet
A parcel on an aircraft is one package among many. A container on a vessel sits for weeks beside whatever else was loaded into it, in a hold that may reach 50 °C, on a ship that rolls.
So the IMDG Code governs what may travel near what. Substances are assigned to segregation groups (acids, alkalis, chlorates, heavy metal salts, and others), and a segregation table sets the minimum separation between any two.
| Term | Requirement in a container | Requirement between containers on deck |
|---|---|---|
| Away from | Effectively segregated so incompatible goods cannot interact dangerously | May be stowed in the same vertical line if separated horizontally by 3 m |
| Separated from | Not permitted in the same container | One container space horizontally or vertically |
| Separated by a complete compartment or hold from | Not permitted in the same container | Increased separation, typically two container spaces |
| Separated longitudinally by an intervening complete compartment or hold from | Not permitted in the same container | The most demanding separation in the code |
The practical consequence for a consolidator: a container of individually compliant goods can be non-compliant as a unit. Nitric acid and sodium hydroxide are each ordinary Class 8 consignments. Put them in the same box and you have a segregation breach.
Marine pollutants
This is the flag that catches shippers moving from air or road to ocean for the first time.
A marine pollutant is a substance the code identifies as harmful to the marine environment. It is assessed independently of hazard class, so a material can be a marine pollutant whether or not it is otherwise remarkable. Many detergents, cleaning concentrates, pesticides, marine paints and certain metal compounds qualify.
The consequences are the marine pollutant mark on the package, a declaration on the dangerous goods documentation, and in some cases stowage restrictions. Nothing in a 49 CFR road classification will tell you about it — the flag simply does not exist in that system.
Stowage categories
Every IMDG entry carries a stowage category, from A through E for general cargo, which decides where on the vessel the consignment may travel. On deck, under deck, or on deck only with restrictions. Categories tighten for passenger vessels.
You will not normally choose the stowage yourself (the carrier does), but the category is part of the classification, and it affects both cost and available sailings. A material restricted to on-deck stowage on a passenger vessel has meaningfully fewer routes available.
Keeping up with the classes of IMDG
The IMDG Code runs on a two-year amendment cycle, and it works differently from the annual air edition in a way that catches people out.
Each amendment gets a voluntary application period followed by a mandatory one. For roughly twelve months, two versions are simultaneously valid — you may apply the new amendment or continue with the previous one, but not mix them. Then the new version becomes mandatory and the old one stops being an option.
Two practical consequences. First, "which amendment are you working to?" is a real question with more than one correct answer at certain points in the cycle, and your documentation should say. Second, the overlap period is a trap for anyone cherry-picking: applying a relaxation from the new amendment while relying on the old text elsewhere is not permitted.
Because the classes of IMDG themselves rarely move, it is tempting to treat an amendment as irrelevant. The changes that matter are usually in the entry-level detail (segregation, stowage, packing provisions and the marine pollutant listings), which is precisely the material that a road or air classification never covered in the first place.
The current amendment status is published by the IMO's dangerous goods pages.
The paperwork ocean adds
Beyond the shipping papers you would produce for road:
- Dangerous goods declaration — UN number, proper shipping name, class, packing group, flash point where relevant, marine pollutant status, EmS emergency schedule.
- Container packing certificate — signed by whoever packed the unit, certifying it was clean, correctly loaded, braced, placarded and segregation-compliant.
- EmS codes — emergency schedules for fire and spillage, referenced from the code.
The packing certificate is the one small shippers most often omit, and terminals do check for it. The declaration format and the required data elements are set out in the code itself; the IMO's dangerous goods guidance is the authoritative starting point, and national maritime administrations publish their own implementing detail on top.
Reusing an air classification at sea
Here is the shape of the problem. A homeware brand has been air-freighting a cleaning concentrate for years, with a clean IATA classification: UN1760, corrosive liquid n.o.s., Class 8, packing group III. Correct and unremarkable.
Freight rates push them to ocean, and the same classification data goes onto the booking. The consignment is refused at the terminal for two reasons neither the air paperwork nor the road paperwork would have surfaced. The formulation is a marine pollutant, so the mark and the declaration entry are missing. And it has been consolidated with an alkaline product in the same container, which the segregation table does not allow.
Nothing was wrong with the original classification. It was simply answering a different question.
The UN number transfers between modes. The operational requirements do not. Ocean asks two things air never asks: what is this near, and what does it do to seawater.
Classify7 ruleset team, on why determinations are pinned to a mode as well as a ruleset edition
- Carrying an air or road classification straight onto an ocean booking without checking marine pollutant status.
- Consolidating compatible-looking Class 8 goods that sit in opposing segregation groups.
- Omitting the container packing certificate.
- Assuming an amendment cycle you can ignore — the IMDG Code updates on a two-year cycle with a voluntary and then a mandatory period.
- Leaving the flash point off the dangerous goods declaration for Class 3 consignments, where it is required.
Classifying against the classes of IMDG specifically
The workable approach is to treat mode as an input to classification rather than something applied afterwards. Send the product and the intended mode together, and get back the class, packing group and the mode-specific flags in one determination — so that a consignment classified against the classes of IMDG carries its marine pollutant status and segregation group from the start.
For the road and air side of the same question, see our guides to the 49 CFR Hazardous Materials Table and the IATA DGR 2026 edition. If you are consolidating other people's cargo rather than your own, manual versus automated hazardous cargo classification covers the volume problem directly.
