Classification tells the carrier what your cargo is. Segregation tells them where it can be put. It is the reason a dangerous goods booking cannot be confirmed as quickly as an ordinary one: the vessel planner has to find a stowage position that satisfies the segregation requirements against everything else already loaded.
The rules are in the IMDG Code, currently Amendment 42-24, mandatory since 1 January 2026.
The four segregation terms
The segregation table gives one of four requirements for each pair of classes. Each has a precise meaning, and for closed cargo transport units carried on deck each translates into a minimum distance:
- Away from. Effectively segregated so that incompatible goods cannot interact dangerously in an accident, but they may be in the same compartment or hold, or on deck, provided a minimum horizontal separation of 3 metres.
- Separated from. In different compartments or holds when stowed under deck. On deck, a minimum horizontal separation of 6 metres.
- Separated by a complete compartment or hold from. Either vertical or horizontal separation. On deck, 12 metres horizontally.
- Separated longitudinally by an intervening complete compartment or hold from. The strictest. Vertical separation alone does not satisfy it. On deck, 24 metres horizontally with an intervening complete compartment.
A blank in the table means no general segregation requirement between those classes, and a number refers to a specific provision that overrides the general rule.
Segregation groups
Class alone is not enough, because substances within a class can be incompatible with each other. The code therefore defines segregation groups based on chemistry, including acids, alkalis, ammonium compounds, chlorates, chlorites, cyanides, heavy metals and their salts, hypochlorites, lead compounds, mercury compounds, nitrites, perchlorates, permanganates, peroxides, powdered metals and azides.
A substance can belong to more than one group, and a substance not formally in a group may still carry a special provision requiring segregation from named materials. This is why the technical name matters on a generic entry, and why an n.o.s. description without the technical name is rejected: without it the planner cannot apply the segregation groups.
On deck and under deck
The dangerous goods list assigns each substance a stowage category, which determines whether it may be carried on deck only, under deck only, or either. The logic:
- On deck stowage gives ventilation and rapid access for firefighting or, in the worst case, jettison. It exposes cargo to weather and sea.
- Under deck stowage protects from weather but confines any release, and firefighting is much harder.
Some substances are permitted on deck only, some require stowage away from living quarters or heat sources, and some require protection from direct sunlight. Temperature controlled organic peroxides and self-reactive substances have their own regime again.
Segregation between containers
Closed cargo transport units give some inherent separation, so the requirements between closed containers are less onerous than between open units or between packages in a hold. But the distances above still apply, and they apply in three dimensions: vertically as well as fore, aft and athwartships.
Do not mix incompatible dangerous goods inside a single container unless you have checked the segregation table for the pair. A consolidator putting two customers' dangerous goods into one LCL box without checking is a real and recurring cause of incidents.
What this means commercially
- Book earlier. Dangerous goods bookings need acceptance from the carrier's DG desk, which is a manual review.
- Expect refusals. A vessel that already has an incompatible parcel may simply have no legal position left for yours.
- Expect a surcharge, and expect it to vary by class. Class 1 and Class 7 are in a different commercial category from Class 9.
- Declare accurately the first time. A reclassification after loading can mean discharge at the next port at your cost.
- For LCL, ask what else is in the box. Many consolidators will not accept dangerous goods at all, which is usually the honest answer.
See also the IMDG and IATA rules and the dangerous goods declaration.
General guidance. Segregation requirements come from the segregation table, the stowage category and the special provisions for each specific UN number in the IMDG Code in force.
