IMO Classes.

Dangerous goods are split into nine classes by their main hazard. The numbering comes from the UN Model Regulations and is the same in the IMDG Code (sea), IATA DGR (air), ADR (road) and RID (rail).

The nine classes

Order of the numbers is not the order of danger. Each card shows the hazard labels used on packages for that class.

Class 1: Explosives

Division 1.1Substances and articles which have a mass explosion hazard

Division 1.2Substances and articles which have a projection hazard but not a mass explosion hazard

Division 1.3Substances and articles which have a fire hazard and either a minor blast hazard or a minor projection hazard or both, but not a mass explosion hazard

Division 1.4Substances and articles which present no significant hazard

Division 1.5Very insensitive substances which have a mass explosion hazard

Division 1.6Extremely insensitive articles which do not have a mass explosion hazard

Typical cargo: Fireworks, ammunition, detonators, airbag inflators (some are Class 9).

No packing groups

Class 2: Gases

Division 2.1Flammable gases

Division 2.2Non-flammable, non-toxic gases

Division 2.3Toxic gases

Typical cargo: LPG and aerosols (2.1), nitrogen and CO2 (2.2), chlorine and ammonia (2.3).

No packing groups

Class 3: Flammable liquids

Typical cargo: Petrol, diesel, paints, perfumes, alcohol-based products.

Packing groups I to III apply

Class 4: Flammable solids; substances liable to spontaneous combustion; substances which, on contact with water, emit flammable gases

Division 4.1Flammable solids, self-reactive substances, solid desensitized explosives and polymerizing substances

Division 4.2Substances liable to spontaneous combustion

Division 4.3Substances which in contact with water emit flammable gases

Typical cargo: Matches and sulphur (4.1), activated carbon and some oily rags (4.2), sodium and calcium carbide (4.3).

Packing groups I to III apply

Class 5: Oxidizing substances and organic peroxides

Division 5.1Oxidizing substances

Division 5.2Organic peroxides

Typical cargo: Ammonium nitrate fertilisers and hydrogen peroxide (5.1), hardeners for resins (5.2).

Packing groups I to III apply

Class 6: Toxic and infectious substances

Division 6.1Toxic substances

Division 6.2Infectious substances

Typical cargo: Pesticides and cyanides (6.1), clinical samples and medical waste (6.2).

Packing groups I to III apply

Class 7: Radioactive material

Typical cargo: Medical isotopes, industrial gauges, uranium ore concentrate.

No packing groups

Class 8: Corrosive substances

Typical cargo: Battery acid, caustic soda, many cleaning chemicals.

Packing groups I to III apply

Class 9: Miscellaneous dangerous substances and articles

Hazard label, Li batteries — Li batteries

Typical cargo: Lithium batteries, dry ice, magnetised material, environmentally hazardous substances.

Packing groups I to III apply

Packing groups

Set how strong the packaging must be. Not used for Classes 1, 2 and 7, Divisions 5.2 and 6.2, and self-reactive substances of Division 4.1.

Packing group	Degree of danger
I	Substances presenting high danger
II	Substances presenting medium danger
III	Substances presenting low danger

Sources: UN Model Regulations, Part 2, 2.0.1.1 and 2.0.1.3(archived copy); hazard labels: Wikimedia Commons, ADR markings (public domain); UN Sub-Committee note on polymerizing substances in Division 4.1; current edition: UNECE, Model Regulations Rev. 23; sea carriage: IMO, IMDG Code. Typical cargo lines are Navo's own examples.

A booking needs the UN number, proper shipping name, class, packing group and, for sea, the IMDG declaration. Lines accept or refuse per class and port.
Lithium batteries are Class 9 but have their own UN numbers (UN 3480, 3481, 3090, 3091) and extra rules by air.
See Container Types for tank containers used for liquid dangerous goods.