IMDG Code Classes (Ocean Freight Dangerous Goods)

Research institutions that ship chemicals, reagents, or biological materials internationally by sea — rather than by air courier — fall under a different rulebook than the one most lab shipping guidance assumes. The IMDG Code (International Maritime Dangerous Goods Code) is the International Maritime Organization’s mandatory code governing how dangerous goods are classified, packed, marked, and stowed for carriage by vessel. It shares its underlying nine-class hazard system with air and ground transport, but it adds requirements that have no equivalent in IATA’s air-freight rules — which is where labs that only have an air-shipping reference sheet get caught out.

Why the Classes Are the Same Everywhere, but IMDG Adds a Layer

The IMDG Code, the IATA Dangerous Goods Regulations, and the US DOT’s 49 CFR Parts 171-180 all trace back to the same source: the United Nations Recommendations on the Transport of Dangerous Goods (informally the ‘Orange Book’). That’s why a solvent’s or a biological specimen’s UN number, proper shipping name, hazard class, and packing group stay the same whether it travels by plane, truck, or ship — classification is mode-independent. What the IMDG Code adds is a set of sea-specific handling and stowage requirements layered on top of that shared classification, because a vessel voyage involves risks — extended transit time, confined cargo holds, proximity to crew accommodation, no mid-voyage diversion to a hazmat response team — that don’t apply the same way to a two-hour flight or a same-day truck run.

The Nine UN Hazard Classes

Every dangerous good carried under the IMDG Code falls into one of the same nine classes used across all transport modes:

• Class 1 — Explosives
• Class 2 — Gases: compressed, liquefied, or dissolved gases, including calibration-gas cylinders and some cryogenic configurations
• Class 3 — Flammable liquids: many common lab solvents above the applicable flash-point threshold
• Class 4 — Flammable solids, substances liable to spontaneous combustion, and substances dangerous when wet
• Class 5 — Oxidizing substances and organic peroxides
• Class 6 — Toxic and infectious substances, including Category A and Category B (UN 3373) biological substances
• Class 7 — Radioactive material
• Class 8 — Corrosives
• Class 9 — Miscellaneous dangerous goods, including dry ice (UN 1845) and lithium batteries

For the full decision logic behind assigning one of these to a specific lab material, see CASRAI’s lab chemical hazard classes primer and, for dry ice specifically, the dry ice shipping regulations guide.

What’s Specific to the IMDG Code

Four features of the IMDG Code have no direct equivalent in air-only shipping references:

• Emergency Schedules (EmS). Every entry in the IMDG Dangerous Goods List carries a two-part EmS code — one for fire response (F-series) and one for spillage response (S-series) — pointing to standardized procedures in IMO’s EmS Guide, so a ship’s crew can respond to an incident involving that specific cargo without needing the shipper on the line.
• Marine Pollutant designation. Independent of a substance’s primary hazard class, the IMDG Code requires an additional ‘Marine Pollutant’ marking (and packaging/documentation treatment) for any material that meets defined aquatic-toxicity and bioaccumulation criteria. A reagent whose primary hazard is, say, Class 3 flammability can also require Marine Pollutant marking if it meets those separate criteria — something an air-only classification would never flag.
• Segregation groups. The Code defines eighteen segregation groups (acids, alkalis, oxidizing substances, and so on) that determine which dangerous goods cannot be stowed together or even in adjacent cargo spaces, going well beyond a safety data sheet’s general incompatibility notes.
• Stowage categories. Each entry is assigned a lettered stowage category describing where aboard the vessel it may be carried — on deck only, below deck permitted, minimum distance from accommodation or machinery spaces — a concept that simply doesn’t exist for air cargo.

The Code is revised on a two-year amendment cycle by IMO’s Maritime Safety Committee, tracking the same underlying UN Model Regulations updates that also drive periodic changes to the IATA DGR and 49 CFR.

When a Lab Shipment Falls Under the IMDG Code

The IMDG Code applies once a package meeting one of the nine hazard-class definitions is offered for carriage aboard a vessel — this includes full ocean freight shipments and any leg of a multimodal shipment that travels by sea. It is the correct reference, not the IATA DGR, whenever an institution: ships reagents, samples, or equipment overseas by ocean container rather than air; uses a freight forwarder that routes part of an international shipment by sea for cost or capacity reasons; or exports bulk chemical stock to an international collaborator or contract site. Staff preparing IMDG-governed shipments need IMDG-specific training — IATA DGR certification alone does not cover the Code’s segregation, stowage, and EmS requirements; see CASRAI’s hazmat shipping training and certification guide for how training requirements are structured across modes.

Worked Examples

Example 1. A university core facility ships a case of flammable HPLC solvent (Class 3, Packing Group II) by ocean container to a partner lab overseas because air freight surcharges on flammable liquids make sea transport significantly cheaper for a large-volume shipment. The solvent is classified identically to how it would be for an air shipment, but the freight forwarder must also determine its IMDG stowage category and confirm it isn’t in a segregation group that conflicts with other cargo already booked on that sailing.

Example 2. A diagnostics lab exports a UN 3373 Category B biological substance by sea as part of a bulk sample-transfer agreement. Because Category B biological substances also frequently trigger dry ice (Class 9) requirements for temperature control, the shipment carries both classifications, an EmS code for each, and its own stowage category — none of which is determined differently than under IATA rules for the underlying classification itself, only in the sea-specific handling layered on top.

Counter-Example

A lab shipping the same flammable solvent domestically by ground courier, entirely within the continental United States, is governed by 49 CFR Parts 171-180, not the IMDG Code — the material’s UN number, class, and packing group are the same, but none of the sea-specific requirements (EmS codes, stowage category, segregation groups) apply, because the shipment never travels by vessel. Similarly, an air-only international shipment routed entirely by cargo aircraft falls under the IATA DGR, not the IMDG Code, even though the two frameworks classify the exact same material identically.