United Nations (UN) Numbers are used on dangerous goods labels and in MSDS. They consist of a four-digit number assigned by the UN Committee of Experts on the Transport of Dangerous Goods. UN Numbers are assigned to one substance or a group of substances. They are therefore not necessarily unique to one chemical and can cover a group of chemicals with similar hazardous properties. Nevertheless, UN Numbers are a useful starting point for a person wishing to identify the hazards associated with a particular substance.
Dangerous Goods are grouped in nine classes, according to their common dangers. They are classified in this way as each class has very different properties and risks involved in not only handling them but also transporting them. Each class has a distinctive diamond-shaped symbol with the class number at the bottom of the diamond.

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