The nine classes of hazardous materials are the backbone of every transport regulation on earth. Once you know which class a product sits in, you know roughly what the label looks like, what packaging standard applies, and which carriers will start asking questions.
What trips people up is that the numbers look like a ranking and are not one. Class 9 (the miscellaneous class, the one that sounds like a filing cabinet for leftovers) contains lithium batteries, which are more tightly controlled in the air network than plenty of things in Class 3. This guide walks all nine, with divisions, ordinary examples and the trap attached to each.
How the nine classes of hazardous materials are organised
The classes sort materials by the kind of harm they do in transport. Not by toxicity, not by how frightening the label is. A material burns, or corrodes, or releases pressure, or poisons — and that behaviour decides the class.
Three ideas make the rest legible:
- Divisions refine a class. Class 2 splits into 2.1 flammable gas, 2.2 non-flammable non-toxic gas, 2.3 toxic gas — and the division, not the class, drives carrier acceptance.
- Packing groups grade severity within a class: I high, II medium, III low. Classes 1, 2 and 7 do not use them.
- Subsidiary risks capture secondary hazards. Something both flammable and corrosive gets two labels and one shipping name.
Class 1 — Explosives
Six divisions, from 1.1 mass explosion hazard through to 1.6 extremely insensitive articles. Fireworks, ammunition, airbag inflators, safety fuses.
Almost entirely out of reach for ordinary parcel networks, and requiring specific approvals and an EX number for most articles. If your product is Class 1, you are already working with a specialist.
Class 2 — Gases
Three divisions: 2.1 flammable gas, 2.2 non-flammable non-toxic gas, 2.3 toxic gas.
This is where aerosols live, which makes it one of the most common classes in consumer shipping. Deodorant, dry shampoo, spray paint, cooking sprays, insect repellent. UN1950 covers aerosols generally and the division depends on the propellant and contents.
The recurring error: assigning a packing group. Class 2 has none. A shipping paper showing "2.1, PG II" is wrong on its face.
Class 3 — Flammable liquids
The single largest class by volume in e-commerce. The test is flash point: below 60 °C and you are in Class 3.
Perfume, hand sanitiser, nail varnish, solvent adhesives, essential oil blends, reed diffuser bases, some inks. Packing group follows flash point and initial boiling point, which is why the same product family can span PG II and PG III depending on formulation.
Class 4 — Flammable solids and reactive substances
- 4.1 flammable solids — matches, some fire-lighters, certain powders
- 4.2 substances liable to spontaneous combustion — oily rags, some metal powders
- 4.3 substances which emit flammable gases on contact with water — calcium carbide, some metal hydrides
Division 4.3 catches people out with hobby and industrial chemicals that appear inert in the box and are anything but if the box gets wet.
Class 5 — Oxidisers and organic peroxides
5.1 oxidising substances, 5.2 organic peroxides. Pool chlorine, some hair bleaches, certain fertilisers, hardeners for resin kits.
These do not burn well themselves; they make everything near them burn better. That is why segregation rules matter more here than in most classes, and why a mixed pallet can be non-compliant even when every item on it is individually fine.
Class 6 — Toxic and infectious substances
6.1 toxic substances, 6.2 infectious substances. Pesticides, some laboratory reagents, diagnostic specimens, medical samples.
Division 6.2 has its own documentation and packaging world (UN3373 for Category B biological substances is the one most people meet) and is rarely a casual shipment.
Class 7 — Radioactive material
Smoke detector sources, certain industrial gauges, some medical isotopes. No packing groups; a transport index and category system instead. Almost always a specialist consignment.
Class 8 — Corrosives
Materials that destroy skin or corrode metal. Drain cleaner, descaler, bleach at strength, battery acid, some sodium hydroxide solutions.
The defining feature of Class 8 is that it is concentration-dependent. Household bleach at 5 % and industrial hypochlorite at 15 % are not the same classification problem, and a dilute solution may fall out of scope entirely. Packing group turns on contact time to destroy skin tissue or corrosion rate on steel or aluminium.
Class 9 — Miscellaneous dangerous goods
Everything genuinely hazardous in transport that does not fit the other eight. That sounds like a residual category and it is the most commercially important class in modern parcel shipping, because it holds:
- Lithium batteries — UN3480, UN3481, UN3090, UN3091
- Dry ice — UN1845
- Environmentally hazardous substances — UN3077, UN3082
- Magnetised material, some elevated-temperature substances
Note how differently the carriers treat items inside this single class. Dry ice moves through the network routinely with a net weight marking. Standalone lithium batteries are cargo aircraft only. Same class, entirely different operational reality — which is exactly why class alone never gives you a shipping decision.
| Class | Name | Divisions | Packing groups | Everyday example |
|---|---|---|---|---|
| 1 | Explosives | 1.1–1.6 | No | Fireworks |
| 2 | Gases | 2.1, 2.2, 2.3 | No | Aerosol deodorant |
| 3 | Flammable liquids | — | I, II, III | Perfume |
| 4 | Flammable solids, reactives | 4.1, 4.2, 4.3 | I, II, III | Matches |
| 5 | Oxidisers, organic peroxides | 5.1, 5.2 | I, II, III | Pool chlorine |
| 6 | Toxic, infectious | 6.1, 6.2 | I, II, III (6.1) | Pesticide |
| 7 | Radioactive | — | No | Industrial gauge |
| 8 | Corrosives | — | I, II, III | Drain cleaner |
| 9 | Miscellaneous | — | III, or none | Lithium battery, dry ice |
Subsidiary risks, and why one class is rarely the whole story
Plenty of materials do more than one dangerous thing. A concentrated cleaning acid burns skin and gives off flammable vapour. A pesticide concentrate is toxic and harmful to the marine environment.
The regulations handle this with a primary class and one or more subsidiary risks. The primary class decides the proper shipping name and the packing group. The subsidiary risks add labels, and they can change segregation and carrier acceptance without changing anything else about the entry.
Which hazard becomes primary is not a judgement call — there is a precedence table in the regulations that resolves it. Where a material meets the criteria for several classes, that table decides which one governs, and it is one of the more counter-intuitive parts of the system. A material can meet Class 3 and Class 8 criteria and be assigned Class 8 as primary, with Class 3 subsidiary.
The practical failure is a package carrying one label where it should carry two. It is a visible defect, it gets caught at induction, and it is entirely avoidable. Whenever a classification comes back with a single class, it is worth asking explicitly whether anything else applies — the UN Model Regulations maintained by UNECE set out the precedence rules the national and modal codes all inherit.
The assumption that costs money
A retailer we would describe as fairly sophisticated built an internal shipping rule that read, roughly: escalate anything Class 1 to 5 for manual review, let 6 to 9 through automatically. The logic was that higher numbers meant lower risk.
Class 9 went through on the fast path. Class 9 is where the lithium batteries are. Every spare power bank in the catalogue flowed straight into the air network as a general parcel, with no state-of-charge control and no packing instruction on the paperwork, for as long as it took someone to notice.
Class number tells you what kind of harm a material does. It does not tell you how restricted it is, and it never tells you whether a given carrier will take it. Those are three separate questions and only the first one is answered by the class.
Classify7 ruleset team
- Treating class number as a severity ranking.
- Putting a packing group on a Class 2, Class 7 or lithium battery entry, none of which take one.
- Recording the class but not the division, then discovering 2.1 and 2.2 have very different carrier positions.
- Missing a subsidiary risk, so the package carries one label where it needs two.
- Assuming segregation is someone else's problem — Class 5 next to Class 3 on a pallet is your problem.
From the nine classes of hazardous materials to a shipment
The class is a waypoint, not a destination. Knowing where a product sits among the nine classes of hazardous materials tells you the shape of the problem; to actually ship you need the UN number, proper shipping name, packing group where applicable, and the carrier's position on that exact entry. Our UN number lookup reference covers the next step, and packing groups I, II and III explains how severity is graded within a class. The full class criteria are set out in 49 CFR part 173.
To see the encoded entries for any single class, with the carrier matrix beside them, browse the hazard class pages. Or send your product straight to the free classifier and let it resolve the class, division and subsidiary risk together.
