Hazard Class 4 — Flammable Solids, Spontaneously Combustible & Dangerous When Wet
A composite class covering three distinct ignition mechanisms.
Overview
Class 4 groups three distinct ignition behaviours under one class number. Division 4.1 covers solids that ignite easily from friction or a brief flame source. Division 4.2 covers substances that self-heat or ignite without an external source. Division 4.3 covers substances that react with water to release flammable gas.
All three divisions are tightly regulated for air transport, and several entries are forbidden on passenger aircraft.
Divisions within Class 4
| Division | Description |
|---|---|
| 4.1 | Flammable solids, self-reactive substances, polymerising substances and solid desensitised explosives |
| 4.2 | Substances liable to spontaneous combustion |
| 4.3 | Substances which in contact with water emit flammable gases |
Aviation context
Class 4 articles encountered in MRO are typically specialised: powdered metals used in repairs, certain self-reactive composites and adhesive systems, and lithium-related residues. Most MRO sites do not hold these in bulk, but they appear occasionally in repair-kit consumables and must be classified correctly.
Typical UN numbers in aviation MRO
| UN Number | Proper Shipping Name | Notes |
|---|---|---|
| UN1325 | Flammable solid, organic, n.o.s. (4.1) | Generic entry for organic flammable solids |
| UN1383 | Pyrophoric metal, n.o.s. (4.2) | Spontaneously ignites in air |
| UN1428 | Sodium (4.3) | Reacts violently with water |
| UN3089 | Metal powder, flammable, n.o.s. (4.1) | Some repair powders |
Packaging and marking essentials
- Packing instructions vary by division. Division 4.2 and 4.3 entries often require hermetically sealed inner packagings to prevent contact with air or water.
- Subsidiary risks are common — many 4.3 substances also carry a Class 8 corrosive subsidiary risk.
- Verify against the current IATA DGR Table 4.2 for passenger / cargo eligibility and the assigned packing instruction.
Frequently Asked Questions
Can pyrophoric materials travel by air?
Some 4.2 entries are forbidden on aircraft; others may be carried only on cargo aircraft under restrictive packing instructions. Always verify the specific UN number against the current IATA DGR.
How is a 4.3 substance packed?
Inner packagings must be hermetically sealed to prevent any contact with moisture. Specific PIs apply, and the outer package typically carries both the 4.3 hazard label and any subsidiary-risk label.
Need to classify a specific part?
DG Copilot identifies the UN number, hazard class and air transport status from a part number — in seconds.
Other hazard classes
This information is provided for guidance only. Always verify against the current IATA DGR edition. Final classification and DGD must be validated by a certified DG specialist.