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UN numbers or UN IDs are four-digit numbers that identify dangerous goods hazardous substances and articles (such as explosives, flammable liquids, toxic substances, etc.) in the framework of international transport. Some hazardous substances have their own UN numbers (e.g. acrylamide has UN2074), while sometimes groups of chemicals or products with similar properties receive a common UN number (e.g. flammable liquid, not otherwise specified, have UN1993). A chemical in its solid state may receive a different UN number than the liquid phase if their hazardous properties differ significantly; substances with different levels of purity may also receive different UN numbers.

UN numbers range from UN0001 to about UN3500 and are assigned by the United Nations Committee of Experts on the Transport of Dangerous Goods. They are published as part of their Recommendations on the Transport of Dangerous Goods, also known as the Orange Book. These recommendations are adopted by the regulatory organization responsible for the different modes of transport.

For more details, see List of UN Numbers.

NA numbers (North America), also known as DOT numbers are issued by the United States Department of Transportation and are identical to UN numbers, except that some substances without a UN number may have an NA number. These additional NA numbers use the range NA8000 - NA9999.

Associated with each UN number is a hazard identifier, which encodes the general hazard class and subdivision (and, in the case of explosives, their compatibility group). For instance, the hazard identifier of acrylamide is 6.1 and the one of cigarette lighters is 2.1. If a substances poses several dangers, then subsidiary risk identifiers may be specified. It is not possible to deduce the hazard class(es) of a substance from its UN number: they have to be looked up in a table.

It has been suggested that this article or section be merged with Dangerous goods. (Discuss)

The hazard classes and their divisions are:

• Hazard Class 1: Explosives
  • Division 1.1: Substances and articles which have a mass explosion hazard
  • Division 1.2: Substances and articles which have a projection hazard but not a mass explosion hazard
  • Division 1.3: Substances 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 1.4: Substances and articles which present no significant hazard
  • Division 1.5: Very insensitive substances which have a mass explosion hazard
  • Division 1.6: Extremely insensitive articles which do not have a mass explosion hazard
• Hazard Class 2: Gases
  • Division 2.1: Flammable gases
  • Division 2.2: Non-flammable, non-toxic gases
  • Division 2.3: Toxic gases
• Hazard Class 3: Flammable Liquids
• Hazard Class 4: Flammable Solids (Flammable Solid, Spontaneously Combustible, Dangerous When Wet)
  • Division 4.1: Flammable solids, self-reactive substances and solid desensitized explosives
  • Division 4.2: Substances liable to spontaneous combustion
  • Division 4.3: Substances which in contact with water emit flammable gases
• Hazard Class 5: Oxidizers and Organic Peroxides
  • Division 5.1: Oxidizing substances
  • Division 5.2: Organic peroxides
• Hazard Class 6: Toxic/Poisonous and Infectious Substances Labels (PG III, Inhalation Hazard, Poison, Toxic)
  • Division 6.1: Toxic substances
  • Division 6.2: Infectious substances
• Hazard Class 7: Radioactive Materials (I, II, III, and Fissile)
• Hazard Class 8: Corrosives
• Hazard Class 9: Miscellaneous Dangerous Goods

• UN Recommendations on the Transport of Dangerous Goods. Part 2 defines the hazard classes and their divisions and Part 3 contains a complete list of all UN numbers and their hazard identifiers.
• The Emergency Response Guidebook from the U.S. Department of Transportation contains a list of all assigned NA numbers along with recommended emergency procedures.
• Lablemaster Label Finder software and labeling system.