The nine classes — in everyday words

ADR — the international agreement that governs dangerous goods by road in the UK and across Europe — divides every hazardous substance into one of nine numbered classes.This lesson covers the first three, which together deal with the most dramatically energetic hazards you'll encounter.Class 1 is explosives.An explosive is a substance or article that can very rapidly release energy as a blast, shock, or heat — think fireworks at a display, the ammunition in a police or military convoy, or the small blasting caps used in construction.The danger is obvious: a single ignition source at the wrong moment can trigger a chain reaction.Because of this, Class 1 goods have some of the strictest carriage rules in ADR.Class 2 is gases.Gases are dangerous partly because they are pressurised — a punctured cylinder can become a projectile — and partly because many gases are also flammable, toxic, or both.The LPG canister on a barbeque is Class 2, as is the propane powering a forklift truck, and the oxygen cylinder used in a hospital.The pressurised state is key: damage to the container changes the risk immediately.Class 3 is flammable liquids.These are liquids that give off enough vapour at relatively low temperatures to ignite from a spark or a flame.Petrol, with UN number 1203, is the everyday example almost every driver in the UK will recognise.White spirit, paint thinners, and methylated spirits all belong here too.The key risk is the invisible vapour — it travels along the ground and can reach an ignition source metres away from the spill itself.All three classes have their own diamond-shaped hazard label, and a vehicle carrying above the ADR threshold must display orange-plate markings and placards so emergency services can identify the load at a glance.

Classes 4 and 5 cover hazards that are a step less obvious than a can of petrol or a gas cylinder, which is exactly why it pays to understand them clearly.Class 4 is split into three sub-classes, each describing a different route by which a solid substance becomes a fire hazard.Class 4.1 — flammable solids — are materials that can catch fire from a brief contact with a flame or spark and keep burning.Matches are the textbook example; so are some metal powders.Class 4.2 — spontaneously combustible — are materials that can ignite all by themselves if exposed to air or moisture, without any external flame.Oily rags soaked in linseed oil are the classic workshop hazard: piled together, they generate enough heat to self-ignite.This is also where pyrophoric substances sit.Class 4.3 — water-reactive — are materials that emit flammable gases when they get wet.Sodium metal reacts violently with water; if you tried to extinguish a sodium fire with a hose, you would make it dramatically worse.The key takeaway is never to use water on a Class 4.3 incident.Class 5 moves from 'things that burn' to 'things that make burning worse'.An oxidiser does not itself catch fire, but it supplies the oxygen that a fire needs to become far more intense.Class 5.1 covers oxidising substances — swimming-pool chlorine tablets (calcium hypochlorite) are a common UK road-transport example.Class 5.2 covers organic peroxides, which appear in some adhesive hardeners and industrial processes.The practical danger: a spill of a Class 5.1 oxidiser into a Class 3 flammable liquid is not just twice as dangerous — the oxidiser can cause an ordinary fire to burn far hotter and faster than normal firefighting equipment is designed for.

Classes 6 and 7 both involve hazards you cannot always see, smell, or feel, which makes clear labelling and correct handling procedures especially important.Class 6 is divided into two very different sub-classes.Class 6.1 — toxic substances — are materials that can poison humans or animals through swallowing, skin contact, or breathing in fumes.Pesticides transported to farms are a common UK road-transport example.Industrial chemicals such as cyanide compounds also fall here.Class 6.2 — infectious substances — are materials that contain, or are reasonably expected to contain, pathogens (bacteria, viruses, or other micro-organisms) capable of causing disease in humans or animals.Clinical lab samples, blood specimens, and diagnostic cultures are typical examples.The packaging rules for 6.2 are particularly strict because a leak in transit could expose postal workers, couriers, or emergency services to harmful biological material.Class 7 — radioactive material — covers substances that spontaneously emit ionising radiation.Medical isotopes used in hospital scans and industrial gauges are the most common UK road-transport examples.What makes Class 7 unusual is that the radiation cannot be detected without specialist equipment, and the health effects of over-exposure can be delayed.Because of this, ADR imposes dedicated provisions for Class 7 — additional paperwork and strict quantity controls — that go beyond the rules for most other classes.

Classes 8 and 9 round out the nine-class system and together cover two very different types of hazard.Class 8 is corrosives.A corrosive substance chemically attacks and destroys living tissue (especially skin and eyes) and can also degrade metals, plastics, and other materials on contact.In UK road transport you'll come across Class 8 goods regularly: concentrated bleach heading to a cleaning contractor, sulphuric acid (UN1830) in industrial batteries or manufacturing deliveries, strong drain-cleaning fluids, and car-battery acid.The hazard is contact — a leaking Class 8 package can burn a driver's hands or eat through the floor of a van.This is why Class 8 packaging rules require robust, chemically resistant containers and why the diamond label (showing a liquid dripping onto a hand and a surface) is one of the most instantly recognisable in the system.Class 9 is the miscellaneous class — it exists specifically for dangerous substances and articles that have a genuine road-transport hazard but do not fit neatly into Classes 1 through 8.The single most important Class 9 example in modern UK logistics is lithium batteries (UN3480 for lithium-ion cells or batteries; there are related UN numbers for batteries packed with or contained in equipment).Lithium batteries are not flammable like petrol, not corrosive like acid, not explosive like Class 1 — but if they are damaged, short-circuited, or incorrectly charged, they can short, overheat and catch fire in a way that is very difficult to extinguish.Because of the explosion in e-commerce and electric-vehicle logistics, lithium batteries now appear in large volumes on UK roads every day, making Class 9 far more practically significant than its 'miscellaneous' label might suggest.Class 9 also includes environmentally hazardous substances.