UN (United Nations) number

A four-digit number that the United Nations Committee of Experts on the Transport of Dangerous Goods assigns to a potentially hazardous material, such as an explosive, flammable liquid, or toxic substance. UN numbers help emergency response teams identity hazardous substances, particularly when transported across international borders.

The list of UN numbers and their corresponding materials is documented in the UN Recommendations on the Transport of Dangerous Goods, also known as the Orange Book. UN numbers are internationally recognized.

UN numbers are assigned only to hazardous substances; non-hazardous substances do not have UN numbers.