Dangerous goods are divided into nine classes according to their main hazard: explosives, gases, flammable liquids, flammable solids, oxidisers and organic peroxides, toxic and infectious substances, radioactive material, corrosives and miscellaneous dangerous goods. Several classes are split into divisions. The same nine classes are used in the IMDG Code for sea, ADR for road and 49 CFR in the US, as they all come from the UN Model Regulations.
How classification works
Classification means testing and assessing a substance or article against internationally agreed criteria, then assigning it to the class that matches its main hazard. Any further hazards are shown as subsidiary hazards. In the IMDG Code, classification is covered in Part 2, and the result points you to an entry in the Dangerous Goods List with its UN number, proper shipping name and packing group.
Most classes also use packing groups to show the degree of danger: PG I (high danger), PG II (medium danger) and PG III (low danger). Classes 1, 2 and 7, and some other entries, do not use packing groups.
The examples below are typical, but classification always depends on the actual product.
Class 1: Explosives
Substances and articles that can explode, or that are made to produce an explosive or pyrotechnic effect. Class 1 has six divisions based on how the explosive behaves:
- Division 1.1: mass explosion hazard, where almost the whole load explodes at once (for example, some blasting explosives).
- Division 1.2: projection hazard but no mass explosion hazard (for example, some ammunition).
- Division 1.3: fire hazard with a minor blast or projection hazard, but no mass explosion hazard (for example, some propellants and display fireworks).
- Division 1.4: minor explosion hazard, with effects largely confined to the package (for example, small arms cartridges and consumer fireworks).
- Division 1.5: very insensitive substances that have a mass explosion hazard but are unlikely to be set off in normal transport (for example, some blasting agents).
- Division 1.6: extremely insensitive articles with no mass explosion hazard.
Explosives are also given a compatibility group letter (A to H, J, K, L, N or S), which controls which explosives may be stowed together. Division 1.4S is the least restrictive.
Class 2: Gases
Compressed, liquefied, refrigerated liquefied and dissolved gases, as well as aerosols and some articles containing gas.
- Division 2.1 Flammable gases: gases that ignite in air, such as propane, butane, hydrogen and many aerosols.
- Division 2.2 Non-flammable, non-toxic gases: for example nitrogen, carbon dioxide, argon and compressed air. These can still cause asphyxiation or, if oxidising, increase a fire.
- Division 2.3 Toxic gases: gases that are toxic or corrosive to people if inhaled, such as chlorine and ammonia, anhydrous.
Class 3: Flammable liquids
Liquids with a flash point of not more than 60 °C (closed cup), which means they give off flammable vapour at relatively low temperatures, plus liquid desensitised explosives. Examples include petrol (UN 1203), acetone (UN 1090), ethanol, many paints, inks, adhesives and perfumery products. Packing group depends on flash point and initial boiling point. Flammable liquids with lower flash points are generally placed in PG I or II.
Class 4: Flammable solids and similar substances
- Division 4.1 Flammable solids, self-reactive substances, solid desensitised explosives and polymerising substances: solids that ignite easily or can cause fire through friction, such as sulphur, matches and some metal powders. Self-reactive substances can decompose violently without oxygen.
- Division 4.2 Substances liable to spontaneous combustion: materials that can heat up and catch fire on their own in air, such as white phosphorus, some oily rags and cotton waste, and some types of activated carbon.
- Division 4.3 Substances which, in contact with water, emit flammable gases: “dangerous when wet” materials, such as sodium, calcium carbide and some metal hydrides.
Class 5: Oxidising substances and organic peroxides
- Division 5.1 Oxidising substances: substances that are not necessarily combustible themselves, but can cause or help other materials to burn, usually by giving off oxygen. Examples include ammonium nitrate, calcium hypochlorite and hydrogen peroxide solutions.
- Division 5.2 Organic peroxides: organic compounds with the –O–O– structure, used as hardeners and polymerisation initiators. They can be thermally unstable, and some need temperature control during transport.
Class 6: Toxic and infectious substances
- Division 6.1 Toxic substances: substances that can cause death or serious harm if swallowed, inhaled or absorbed through the skin. Classification is based on toxicity data such as oral LD50. Examples include many pesticides, cyanides, arsenic compounds and some solvents.
- Division 6.2 Infectious substances: substances known or reasonably expected to contain pathogens, such as bacteria, viruses, parasites, fungi or prions, that can cause disease in humans or animals. They are split into Category A (UN 2814 and UN 2900) and Category B (UN 3373, “biological substance, category B”), with clinical waste under separate entries.
Class 7: Radioactive material
Any material containing radionuclides where both the activity concentration and total activity exceed set values. Examples include medical isotopes, industrial radiography sources and uranium hexafluoride. Class 7 has no divisions, but uses its own package types (excepted, industrial, Type A, Type B and Type C) and transport index. Its requirements are based on the IAEA Regulations for the Safe Transport of Radioactive Material.
Class 8: Corrosive substances
Substances that cause irreversible damage to skin, or that can seriously damage other goods or the ship if they leak, for example by corroding steel or aluminium. Examples include sulphuric acid (UN 1830), hydrochloric acid, sodium hydroxide and wet-filled lead-acid batteries (UN 2794).
Class 9: Miscellaneous dangerous substances and articles
Substances and articles that present a danger in transport but do not meet the criteria of any other class. Examples include:
- lithium-ion and lithium-metal batteries (for example UN 3480 and UN 3090), and sodium-ion batteries from Amendment 42-24
- environmentally hazardous substances, solid or liquid (UN 3077 and UN 3082), often shipped as marine pollutants
- dry ice (carbon dioxide, solid, UN 1845)
- asbestos, vehicle airbag inflators and seatbelt pretensioners
- vehicles and engines, and elevated temperature substances.
Marine pollutants
Marine pollutants are not a separate class. In the IMDG Code, a substance can be a marine pollutant as well as belonging to any class. If it has no other hazard, it is usually shipped under Class 9, UN 3077 or UN 3082. Marine pollutants carry the marine pollutant mark in addition to any hazard labels, following MARPOL Annex III.
How Hazcheck can help
Hazcheck Search gives instant look-up of class, subsidiary hazards, packing group and special provisions for any UN number across the IMDG Code, ADR and 49 CFR. Hazcheck Validate checks declared classes and segregation on every booking. To build your team’s knowledge, see our Dangerous Goods Transport General Awareness Course, IMDG Code e-learning and ADR Standard Course, or contact us.
Frequently asked questions
How many classes of dangerous goods are there?
Nine. Classes 1, 2, 4, 5 and 6 are split into divisions, giving 20 classes and divisions in total.
Who decides the class of a product?
The consignor (shipper) is responsible for classifying dangerous goods correctly. This is usually based on test data and the safety data sheet, but always check against the classification criteria rather than relying only on section 14 of the SDS.
Can a substance belong to more than one class?
A substance has one primary class but can have one or more subsidiary hazards. For example, a flammable liquid that is also toxic is Class 3 with a 6.1 subsidiary hazard, and must show both labels.
What is a packing group?
A packing group shows the degree of danger within a class: PG I is high, PG II is medium and PG III is low. It affects packaging standards, quantity limits and whether limited quantity concessions apply.
Sources
- IMO – Dangerous goods
- IMO Publishing – IMDG Code, 2024 Edition
- HSE – Carriage of dangerous goods: introduction
- Vehicle Certification Agency – What are dangerous goods?
- eCFR – 49 CFR § 173.2 Hazardous material classes
- eCFR – 49 CFR § 173.50 Class 1 divisions
- eCFR – 49 CFR § 173.115 Class 2 divisions
- eCFR – 49 CFR § 173.134 Division 6.2 infectious substances
- eCFR – 49 CFR § 173.136 Class 8 definitions