The ADR dangerous goods classes

Every dangerous good belongs to a class. The class tells you which hazard comes first: explosion, fire, poison, corrosion or radiation. It decides the danger labels, the packaging and many of the duties during carriage. ADR has 13 classes, numbered 1 to 9, with Classes 4, 5 and 6 split into subclasses. Here’s the overview you need for the exam.

The 13 classes at a glance

The list is in ADR 2.1.1.1. The examples come from Chapter 3.2, Table A.

Class	Name in ADR	Example
1	Explosive substances and articles	fireworks (UN 0336)
2	Gases	propane (UN 1978)
3	Flammable liquids	petrol (UN 1203), diesel (UN 1202)
4.1	Flammable solids, self-reactive substances, polymerizing substances and solid desensitized explosives	sulphur (UN 1350)
4.2	Substances liable to spontaneous combustion	white phosphorus (UN 1381)
4.3	Substances which, in contact with water, emit flammable gases	calcium carbide (UN 1402)
5.1	Oxidizing substances	sodium chlorate (UN 1495)
5.2	Organic peroxides	organic peroxide type D, liquid (UN 3105)
6.1	Toxic substances	potassium cyanide, solid (UN 1680)
6.2	Infectious substances	clinical waste (UN 3291)
7	Radioactive material	excepted package (UN 2910)
8	Corrosive substances	sulphuric acid (UN 1830)
9	Miscellaneous dangerous substances and articles	lithium ion batteries (UN 3480)

Many classes use packing groups to show the degree of danger: I for high, II for medium and III for low danger (ADR 2.1.1.3). Classes 1, 2, 5.2, 6.2 and 7, and the self-reactive substances of Class 4.1, have no packing groups.

Classes 1 and 2

Class 1: explosives

Class 1 is split into six divisions (ADR 2.2.1.1.5). 1.1 means a mass explosion hazard, an explosion that affects almost the entire load virtually instantaneously. 1.2 is a projection hazard, 1.3 a fire hazard with a minor blast or projection hazard. With 1.4 the effects are largely confined to the package. 1.5 covers very insensitive substances with a mass explosion hazard, 1.6 extremely insensitive articles.

A compatibility group letter is added (ADR 2.2.1.1.6). It shows which explosives may be loaded together. Fireworks usually have the letter G, for example 1.4G for UN 0336. The letter S means the effects stay inside the package unless fire has damaged it. Division and letter together make the classification code, such as 1.1D.

Class 2: gases

A gas is a substance with a vapour pressure above 300 kPa (3 bar) at 50 °C, or one that is completely gaseous at 20 °C and 101.3 kPa (ADR 2.2.2.1.1). By physical state, ADR distinguishes compressed, liquefied, refrigerated liquefied and dissolved gases and aerosols, among others (2.2.2.1.2). By hazard, there are groups with letters (2.2.2.1.3):

• A asphyxiant, O oxidizing, F flammable, T toxic,
• combinations such as TF, TC, TO, TFC and TOC (C stands for corrosive).

Where a gas fits more than one group, groups with T take precedence over all others, and groups with F over A and O. State and group make up the classification code: propane is 2F (liquefied, flammable), compressed oxygen 1O (compressed, oxidizing), chlorine 2TOC.

Class 3: flammable liquids

Class 3 covers liquids with a flash-point of not more than 60 °C (ADR 2.2.3.1.1). The flash-point is the lowest temperature of a liquid at which its vapours form a flammable mixture with air (definition in 1.2.1). A note to the same paragraph says that diesel, gas oil and heating oil (light) with a flash-point above 60 °C but not above 100 °C still count as Class 3, UN 1202.

The packing group depends on the flash-point and the initial boiling point (ADR 2.2.3.1.3):

• I: initial boiling point 35 °C or below,
• II: flash-point below 23 °C, initial boiling point above 35 °C,
• III: flash-point from 23 °C to 60 °C, initial boiling point above 35 °C.

That’s why petrol is packing group II and diesel packing group III.

Classes 4.1 to 9

• 4.1: readily combustible solids and solids that may cause fire through friction, plus self-reactive and polymerizing substances and solid desensitized explosives (2.2.41.1.1).
• 4.2: substances that ignite in contact with air. Pyrophoric substances ignite within five minutes even in small quantities, self-heating substances heat up without an energy supply (2.2.42.1.1).
• 4.3: substances that give off flammable gases in contact with water (2.2.43.1.1).
• 5.1: substances that may not burn themselves but can cause or feed a fire, generally by yielding oxygen (2.2.51.1.1).
• 5.2: organic peroxides with the -O-O- structure. They can decompose and give off heat, and some must be carried under temperature control (2.2.52.1.1, 2.2.52.1.3 and 2.2.52.1.4).
• 6.1: substances that can harm health or kill in small quantities if inhaled, absorbed through the skin or swallowed (2.2.61.1.1).
• 6.2: substances known or reasonably expected to contain pathogens (2.2.62.1.1). Category A, such as UN 2814, is the most dangerous. BIOLOGICAL SUBSTANCE, CATEGORY B (UN 3373) has no transport category in column (15).
• 7: radioactive material (2.2.7.1.1). Packages are assigned to category I-WHITE, II-YELLOW or III-YELLOW according to the transport index and the surface dose rate (5.1.5.3.4).
• 8: substances that cause irreversible damage to skin or, if they leak, damage other goods or the means of transport (2.2.8.1.1). Acids and alkalis are typical examples.
• 9: substances and articles with a hazard not covered by any other class (2.2.9.1.1). These include asbestos (M1), lithium and sodium ion batteries (M4), environmentally hazardous substances such as UN 3082 and 3077 (M6 to M8) and elevated temperature substances such as UN 3257 (M9 and M10).

You can also see the class in the hazard number on the orange plate. The first digit shows the main hazard: 3 for petrol (33), 8 for sulphuric acid (80), 9 for UN 3257 (99).

When a substance has more than one hazard

Plenty of substances are toxic and flammable, or toxic and corrosive. The class follows the main hazard. ADR 2.1.3.5.3 sets the hazards that always come first: Class 7 (apart from most excepted packages), then Class 1, Class 2, liquid desensitized explosives of Class 3, self-reactive substances and solid desensitized explosives of Class 4.1, pyrophoric substances of Class 4.2, Class 5.2, Class 6.1 substances meeting the packing group I inhalation toxicity criteria, and Class 6.2. For every other combination, the table of precedence of hazards in 2.1.3.10 applies (ADR 2.1.3.5.4).

The subsidiary hazards don’t disappear. They show up as extra danger labels in column (5) of Table A. Methanol (UN 1230) is Class 3 with labels 3 and 6.1. Chlorine (UN 1017) is Class 2 with labels 2.3, 5.1 and 8. Our guide to danger labels shows what each one looks like.

Learn to spot the classes

In the exam it’s usually enough to match the class, its name and its label. Start with the ones you see every day: 2, 3, 8 and 9.

In the free test you get an explanation and the ADR reference after every answer. The app has every question on the classes, so you can focus on your weak spots.

Sources

ADR 2025: sections 2.1.1 and 2.1.3 (2.1.3.5.3, 2.1.3.5.4, 2.1.3.10), paragraphs 2.2.1.1.5 and 2.2.1.1.6, 2.2.2.1.1 to 2.2.2.1.3, 2.2.3.1.1 and 2.2.3.1.3, the class definitions for Classes 4.1 to 9 in Chapter 2.2, the definition of ‘flash-point’ in 1.2.1, paragraph 5.1.5.3.4 and Chapter 3.2, Table A. In Great Britain, ADR applies through the Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2009. This guide explains the basics. When you classify an actual substance, the wording of ADR is what counts.