The Basics of Dangerous Goods Classification

Correct classification is the starting point of every compliant dangerous goods shipment. Before anyone thinks about labels, packaging or transport documents, the substance or article itself must be correctly identified and assigned to the right UN number and class. Under ADR, this is done using the dangerous goods list in Table A of Chapter 3.2 together with the detailed criteria in Part 2.

In practice, the process begins with basic information about the product: its composition, physical state, flash point, toxicological data, corrosivity, reactivity with water or oxidisers, and any specific test results. If the exact name appears in the alphabetical index (Table B), you follow the reference to Table A and apply all requirements linked to that UN number. If it cannot be found by name, the substance must be classified according to the procedures and criteria in Part 2, which determine whether it is dangerous and which UN number it should receive. 

ADR groups dangerous goods into classes based on the primary type of hazard they present. These classes are:
• Class 1 Explosive substances and articles
• Class 2 Gases
• Class 3 Flammable liquids
• Class 4.1 Flammable solids, self-reactive substances and solid desensitized explosives
• Class 4.2 Substances liable to spontaneous combustion
• Class 4.3 Substances which, in contact with water, emit flammable gases
• Class 5.1 Oxidizing substances
• Class 5.2 Organic peroxides
• Class 6.1 Toxic substances
• Class 6.2 Infectious substances
• Class 7 Radioactive material
• Class 8 Corrosive substances
• Class 9 Miscellaneous dangerous substances and articles

For many entries, ADR also assigns a packing group (I, II or III) to reflect the degree of danger within a class: I for high danger, II for medium, III for low. The packing group influences the choice of packaging, test standards and some parts of the transport documentation. Subsidiary hazards must also be considered: a substance may be allocated a primary class and one or more subsidiary risk labels if it presents multiple significant hazards. 

Once the correct UN number, proper shipping name, class, packing group and any subsidiary hazards are determined, all other requirements flow from this classification: packaging instructions, tank codes, mixed loading rules, tunnel restrictions, marking and labelling, and documentation. This is why ADR stresses that the consignor must only offer consignments that conform to ADR and that classification is a core obligation.

For companies, robust internal procedures are essential: collecting up-to-date safety data, involving competent dangerous goods safety advisers, and regularly reviewing classifications when formulations or regulations change. A correct classification not only reduces the risk of incidents on the road, but also ensures that inspections, customs controls and multimodal transfers (road–rail–sea–air) run smoothly, with the same UN number and class recognised across modes.