IMDG Class 4.2 is the hazard subclass for substances liable to spontaneous combustion. Two distinct hazard mechanisms sit inside the class: pyrophoric substances that ignite within 5 minutes of contact with air, and self-heating substances that heat up in contact with air without external energy and can reach autoignition over hours or days. Class 4.2 spans pure pyrophoric materials (white phosphorus, certain metal alkyls), activated catalysts (Raney nickel, some palladium-on-carbon), self-heating organic materials (some activated carbons, certain wet textile waste), and metal sulphide ores in some forms.
What defines Class 4.2
The class breaks into two test-defined groups:
| Group | Definition | Common examples |
|---|---|---|
| Pyrophoric | Ignites within 5 minutes of air contact under standardised test | White phosphorus (UN 1381), trimethylaluminium, finely-divided pyrophoric metals |
| Self-heating | Heats up to ignition over time in air without external energy under standardised test | Activated carbon (some grades), Raney nickel wetted with water, some sulphide concentrates |
The PG split is based on the severity of the spontaneous combustion behaviour. PG I covers pyrophoric substances. PG II covers self-heating substances meeting the more severe self-heating test criteria. PG III covers self-heating substances meeting the less severe criteria.
Common Chinese-export Class 4.2 substances
| Substance | UN number | PG | Notable |
|---|---|---|---|
| Activated carbon (self-heating type) | UN 1362 | III | Most “decolorising” or “deodorising” carbons; not all activated carbon is Class 4.2, depends on test results |
| White phosphorus | UN 1381 | I | Stored under water; high-toxicity precursor; export-controlled |
| Sulphide concentrates (some) | UN 1376 (iron sulphide) and similar | II or III | Some metal sulphide ores meet self-heating criteria |
| Raney nickel catalyst (wetted) | UN 1378 | II | Standard hydrogenation catalyst; ships water-wetted |
| Self-heating fish meal | UN 2216 | III | Some animal feed materials; not chemicals but routine 4.2 cargo |
For chemical buyers, activated carbon and Raney nickel are the most common Class 4.2 cargoes from China. White phosphorus is heavily restricted and rarely shipped commercially in 2026.
Packaging requirements
Class 4.2 packaging must prevent air contact during transport. Specific requirements vary by substance:
- Pyrophoric substances: hermetically sealed containers under inert gas (typically argon or nitrogen). Small parcels in glass ampoules sealed inside metal cans. Larger volumes in stainless steel cylinders with valves rated for inert-atmosphere service.
- Self-heating wetted catalysts (Raney nickel): drums or IBCs filled with the catalyst submerged in water. The water-wetting must be maintained throughout transit. Drums marked with “Keep Wet” warnings.
- Activated carbon (UN 1362): standard packaging in supersacks or drums, but the packaging must allow heat dissipation. Closed sealed containers without ventilation can accumulate heat over the voyage.
The water-wetting maintenance for Raney nickel is the most common Class 4.2 packaging failure. A drum that develops a leak in transit dries out the catalyst and creates a self-heating risk. Carriers periodically reject Class 4.2 catalyst cargo at the load port if the drums show signs of leakage.
Segregation at sea
Class 4.2 must be stowed:
- “Away from” Class 1 (explosives), Class 2.1 (flammable gases), Class 3 (flammable liquids)
- “Separated from” Class 5.1 (oxidisers), Class 5.2 (organic peroxides), Class 8 acids, the oxidiser combination is the highest risk
- “Segregated from” Class 7 (radioactives)
For self-heating cargo specifically, stowage away from heat sources (engine room bulkheads, hot bunker tanks, deck cargo exposed to sun) is operational practice even when not strictly required.
Documentation and licensing
Standard DG documentation. Class 4.2 has additional licensing layers in some jurisdictions:
- White phosphorus is on Chinese dual-use export control lists and on most importing countries’ precursor monitoring lists. Export licence required from China; import licence often required at destination.
- Some metal sulphide concentrates trigger mineral export licensing under separate Chinese resource-management rules.
- Catalysts containing precious metals (palladium, platinum) trigger separate customs valuation scrutiny, the catalyst itself may be Class 4.2, but the precious metal content drives the customs entry value.
Operator note: the activated-carbon test gap
Not all activated carbon is Class 4.2. Whether a specific carbon meets the self-heating test depends on the carbon’s pore structure, surface chemistry, and moisture content. Coconut-shell activated carbons typically pass (not Class 4.2). Coal-based activated carbons often fail (Class 4.2 UN 1362). The factory’s product-specific test result determines the classification.
A buyer ordering “activated carbon” generically without specifying the substrate may receive a Class 4.2 cargo without realising it. The freight cost and packaging requirements differ. Always specify the substrate type and request the carbon’s self-heating test certificate before booking. Carriers sometimes refuse activated carbon cargo without the test certificate confirming it is non-self-heating.
Related terms
IMDG umbrella code. UN number. Packing group. Class 4.1 covers flammable solids and self-reactives. Class 4.3 covers substances dangerous when wet. Segregation table for stow compatibility.