ADR Hazard Classes Explained: The 9 Classes of Dangerous Goods
ADR (Accord Dangereux Routier) is the international agreement governing the transport of dangerous goods by road. At the heart of ADR is a classification system that divides dangerous goods into 9 hazard classes, each with its own rules for packaging, labelling, placarding, and transport documentation. Understanding these classes is essential for any driver or operator involved in the transport of dangerous goods in the UK and Europe.
Why Are Hazard Classes Important?
The hazard class of a substance determines how it must be transported, what safety equipment the driver must carry, what emergency procedures apply in the event of an incident, and what training the driver must have. Transporting dangerous goods without the correct ADR classification, documentation, and driver training is a serious offence that can result in substantial fines and prosecution.
The 9 ADR Hazard Classes
Class 1: Explosives
Class 1 covers substances and articles that can explode, deflagrate, or react dangerously when subjected to heat, shock, or friction. Examples include fireworks, ammunition, detonators, and pyrotechnics. Class 1 is subdivided into 6 divisions (1.1 to 1.6) based on the type and severity of the explosive hazard. Transport of Class 1 goods requires specialist ADR training and, in many cases, police notification.
Class 2: Gases
Class 2 covers compressed gases, liquefied gases, dissolved gases, and refrigerated liquefied gases. It is subdivided into flammable gases (2.1, e.g., LPG, hydrogen), non-flammable non-toxic gases (2.2, e.g., nitrogen, carbon dioxide), and toxic gases (2.3, e.g., chlorine, ammonia). Examples include gas cylinders, aerosols, and cryogenic liquids.
Class 3: Flammable Liquids
Class 3 is one of the most commonly transported classes and covers liquids with a flash point below 60°C. Examples include petrol, diesel, aviation fuel, paints, solvents, and alcohols. The flash point determines the packing group (I, II, or III) and the level of risk.
Class 4: Flammable Solids
Class 4 is divided into three sub-classes: 4.1 (flammable solids, self-reactive substances, and desensitised explosives), 4.2 (substances liable to spontaneous combustion), and 4.3 (substances that emit flammable gases in contact with water). Examples include matches (4.1), white phosphorus (4.2), and sodium metal (4.3).
Class 5: Oxidising Substances and Organic Peroxides
Class 5.1 covers oxidising substances that can cause or intensify fire (e.g., ammonium nitrate, hydrogen peroxide). Class 5.2 covers organic peroxides, which are thermally unstable and can combust, explode, or react dangerously (e.g., benzoyl peroxide). Both sub-classes require careful temperature control during transport.
Class 6: Toxic and Infectious Substances
Class 6.1 covers toxic substances that can cause death or serious injury if swallowed, inhaled, or absorbed through the skin (e.g., pesticides, cyanides). Class 6.2 covers infectious substances that contain pathogens capable of causing disease in humans or animals (e.g., clinical waste, diagnostic specimens, biological cultures).
Class 7: Radioactive Material
Class 7 covers materials that emit ionising radiation above a certain threshold. Examples include medical radioisotopes, industrial radiography sources, and nuclear fuel. Transport of Class 7 materials is subject to strict regulatory controls and requires specialist training and equipment.
Class 8: Corrosive Substances
Class 8 covers substances that can cause severe damage to living tissue or other materials on contact. Examples include sulphuric acid, hydrochloric acid, sodium hydroxide, and battery acid. Corrosive substances can also cause secondary hazards such as fire or toxic gas release.
Class 9: Miscellaneous Dangerous Substances and Articles
Class 9 is a catch-all category for substances and articles that present a danger during transport but do not fit into Classes 1–8. Examples include lithium batteries (a growing area of concern due to the risk of thermal runaway), dry ice, magnetised materials, elevated temperature substances, and environmentally hazardous substances.
ADR Training Requirements
Drivers transporting dangerous goods in quantities above the ADR exemption thresholds must hold a valid ADR certificate. The certificate covers the core ADR requirements plus one or more specialist modules depending on the classes being transported (e.g., Tanks, Explosives, Radioactive). GS Driver Training offers ADR training courses covering all classes.
View our ADR training course or contact us for more information about ADR certification.
