ADR divides dangerous goods into 9 main classes: explosives, gases, flammable liquids, flammable solids, oxidizing substances and organic peroxides, toxic and infectious substances, radioactive material, corrosive substances, and miscellaneous dangerous substances. Because classes 4, 5 and 6 are split into subclasses, the ADR text uses 13 class numbers in total, each with its own label.
Why is classification the first step?
Which packaging, which vehicle, which document and up to which quantity exemption a substance can be carried with all depend on its class. Part 2 of ADR defines the criteria for each class; a substance is assigned to a UN number on the basis of these criteria and test data. Table A shows the class, packing group, labels and all special provisions for that UN number on a single line. Classification is the consignor's responsibility; the information is usually taken from section 14 of the safety data sheet.
The 9 classes and their divisions
| Class | Scope | Example substances | Label |
|---|---|---|---|
| 1 | Explosive substances and articles. 1.1 mass explosion, 1.2 projection hazard, 1.3 fire and minor blast, 1.4 minor hazard, 1.5 very insensitive substances with a mass explosion hazard, 1.6 extremely insensitive articles | Ammunition, dynamite, fireworks, airbag explosives | Orange background, exploding bomb; number for 1.4–1.6 |
| 2 | Gases. As labels: 2.1 flammable, 2.2 non-flammable and non-toxic, 2.3 toxic | LPG, acetylene; nitrogen, argon; chlorine, ammonia | Red flame; green cylinder; white skull |
| 3 | Flammable liquids | Petrol, diesel, alcohol, solvent-based paint | Red background, flame |
| 4.1 / 4.2 / 4.3 | Flammable solids and self-reactive substances / substances liable to spontaneous combustion / substances which emit flammable gases in contact with water | Sulphur, matches / white phosphorus / calcium carbide, sodium | Red-and-white stripes / white top, red bottom / blue |
| 5.1 / 5.2 | Oxidizing substances / organic peroxides | Ammonium nitrate, hydrogen peroxide / polymerisation initiators | Yellow, flame over circle / red top, yellow bottom |
| 6.1 / 6.2 | Toxic substances / infectious substances | Cyanides, some pesticides / patient specimens, medical waste | White, skull / biohazard |
| 7 | Radioactive material | Medical isotopes, industrial radiography sources, nuclear fuel | Trefoil symbol; 7A, 7B, 7C, 7E |
| 8 | Corrosive substances | Sulphuric acid, sodium hydroxide, battery acid | White top, black bottom |
| 9 | Miscellaneous dangerous substances and articles | Lithium batteries, environmentally hazardous substances, asbestos | Seven black vertical stripes; 9A for batteries |
For gases, ADR uses the 2.1/2.2/2.3 distinction for labels; in classification, however, letter codes such as A (asphyxiant), O (oxidizing), F (flammable), T (toxic) and their combinations (TF, TC, TOC) are used. For explosives, a compatibility group letter from A to S is added to the division number, for example 1.4S. Package labels measure at least 100 mm × 100 mm, and placards on vehicles and containers at least 250 mm × 250 mm.
What if a substance has more than one hazard?
Many substances present more than one hazard. Acrylonitrile is both flammable and toxic; its class is 3 and its subsidiary hazard is 6.1. In this case the package carries two labels. The precedence of hazards table in ADR 2.1.3.10 determines which hazard counts as the main class. Some classes, such as radioactive material, organic peroxides and explosives, always take precedence.
Is the packing group the same as the class?
No. The class describes the type of hazard, while the packing group describes its degree: PG I high, PG II medium, PG III low danger. Within the same class 3, petrol is PG II and diesel is PG III, and this difference changes the choice of packaging and the exemption limits. Details are in the entry on the packing group. Class-specific rules are covered in separate entries, for example class 7 radioactive material and class 9 miscellaneous dangerous goods.
Common mistakes
- Mistaking the GHS pictogram on the product label for an ADR label. The two serve different purposes and have different shapes.
- Forgetting the subsidiary hazard label; in that case emergency responders cannot see the substance's second hazard from the package.
- Treating class 9 as “unimportant” and loading lithium battery shipments like general cargo.
- Not checking mixed loading prohibitions when putting different classes in the same vehicle.
Whether different classes can be combined in the same vehicle is one of the first questions in planning dangerous goods transport.