2.1 UN Classification System, UN Numbers, and Packing Groups

The UN Classification System and Dangerous Goods Hierarchy

Core Concept: ADR Part 2 defines the unified global framework for classifying dangerous goods. Classified dangerous goods use the applicable UN entry, proper shipping name, hazard class and, where assigned, packing group. Not every hazardous product is regulated by ADR, and not every ADR entry has a packing group.

1. The Structure of ADR Part 2 and the 9 Hazard Classes

Transport of dangerous goods by road across Europe and participating UNECE states relies on a classification system established by the UN Committee of Experts on the Transport of Dangerous Goods. ADR organizes dangerous goods into nine numerical classes based on their predominant physical, chemical, or biological hazards:

• Class 1: Explosive substances and articles
• Class 2: Gases (compressed, liquefied, dissolved, refrigerated cryogenic)
• Class 3: Flammable liquids
• Class 4: Flammable solids, self-reactive substances, polymerizing substances, pyrophoric substances, and water-reactive substances (divided into 4.1, 4.2, and 4.3)
• Class 5: Oxidizing substances (5.1) and organic peroxides (5.2)
• Class 6: Toxic substances (6.1) and infectious substances (6.2)
• Class 7: Radioactive material
• Class 8: Corrosive substances
• Class 9: Miscellaneous dangerous substances and articles (including lithium batteries and environmentally hazardous substances)

Classification is not merely an administrative exercise: it dictates every downstream transport obligation, including packaging integrity, vehicle placarding, driver training requirements, tunnel transit restrictions, and emergency countermeasures.

2. UN Numbers and Proper Shipping Names (PSN)

Every dangerous good is identified by a unique four-digit serial number known as the UN number (e.g., UN 1203 for petrol/motor spirit, UN 1978 for propane). UN numbers eliminate linguistic ambiguities across international borders.

Along with the UN number, ADR assigns an official Proper Shipping Name (PSN) (ADR Chapter 3.1.2). Proper Shipping Names fall into four distinct categories:

1. Single entries for well-defined substances or articles: Directly identify a specific substance (e.g., UN 1090 ACETONE).
2. Generic entries for well-defined groups of substances: Cover groups sharing a common chemical family (e.g., UN 1263 PAINT or UN 1133 ADHESIVES).
3. Specific N.O.S. (Not Otherwise Specified) entries: Cover chemicals having a specific chemical or technical nature not specifically listed (e.g., UN 1987 ALCOHOLS, N.O.S.).
4. General N.O.S. entries: Cover substances meeting hazard criteria of a class or division without specific chemical definition (e.g., UN 1993 FLAMMABLE LIQUID, N.O.S.).

Special Provision 274 and Technical Names

When an N.O.S. or generic entry is subject to Special Provision 274 (indicated in Column 6 of Table A in Chapter 3.2), the Proper Shipping Name must be supplemented with the technical or chemical name of the hazardous constituents enclosed in parentheses. For example:

• UN 1993 FLAMMABLE LIQUID, N.O.S. (contains ethanol and ethyl acetate)
• UN 2924 FLAMMABLE LIQUID, CORROSIVE, N.O.S. (contains methanol and formic acid)

Commercial trade names alone (such as brand names or marketing designations) are prohibited from substituting for official technical names.

3. Packing Groups (PG): Levels of Danger

With certain critical exceptions, substances in Classes 3, 4.1, 4.2, 4.3, 5.1, 6.1, 8, and 9 are assigned to one of three Packing Groups based on the degree of danger they present:

The X design-type rating meets the packing-group performance level for PG I, II or III; the packing instruction, chemical compatibility, closures, density and pressure limits must also be satisfied. The letter alone does not authorise an arbitrary product. Packaging certified for PG II (marked with a Y) can hold PG II and PG III goods, while packaging certified only for PG III (marked Z) can only hold low-danger PG III goods.

Hazard Classes Without Packing Groups

Crucially, not all ADR hazard classes utilize the Packing Group system. The following classes do not have packing groups:

• Class 1 (Explosives): Danger is characterized by 6 Divisions (1.1 to 1.6) and Compatibility Groups (A to S), not packing groups.
• Class 2 (Gases): Hazards are governed by pressure receptacle testing standards, physical state, and hazard division codes (A, O, F, T, etc.).
• Class 5.2 (Organic Peroxides): Regulated by formulation types (Type A through Type G) and temperature control requirements (SADT).
• Class 6.2 (Infectious Substances): Assigned to UN 2814 or UN 2900 with specialized packaging standards (P620), without packing group designation. Category B is assigned to UN 3373.
• Class 7 (Radioactive Material): Danger is classified by package design types (Excepted, Industrial, Type A, Type B, Type C) and activity levels, rather than packing groups.

4. Precedence of Hazards (ADR 2.1.3.10)

When a mixture, solution, or waste substance contains multiple dangerous characteristics spanning two or more classes, the consignor must determine the primary hazard class and any subsidiary hazards using the Precedence of Hazards principles set out in ADR 2.1.3.10.

Certain hazard classes and properties are excluded from the ordinary precedence table and require their specific classification rules rather than a universal ranked ladder. They take priority as the primary class, regardless of other present hazards:

1. Substances and articles of Class 1 (Explosives)
2. Substances and articles of Class 2 (Gases)
3. Liquid desensitized explosives of Class 3
4. Self-reactive substances and solid desensitized explosives of Class 4.1
5. Pyrophoric substances of Class 4.2
6. Substances of Class 5.2 (Organic peroxides)
7. Substances of Class 6.1, Packing Group I by inhalation toxicity (due to extreme, rapid lethality)
8. Substances of Class 6.2 (Infectious substances)
9. Substances of Class 7 (Radioactive material, except in excepted packages)

For chemical mixtures that possess overlapping hazards outside of these absolute priorities (such as a liquid that is both flammable under Class 3 and corrosive under Class 8), the consignor consults the Precedence of Hazards Table in ADR 2.1.3.10. In this matrix, the intersection of the two hazard classes and their respective packing groups determines which hazard becomes the primary class (dictating the transport class, Kemler code prefix, and main label) and which becomes the subsidiary hazard (requiring a subsidiary hazard label with its prescribed number at the bottom).

Source check: 17 October 2026 — ADR 2025, English text published by the Slovak Ministry of Transport, 2.1, 3.1.2 and 6.1.3.