Following the model put forth by the United Nations, the Department of Transport (DOT) divides hazardous materials into nine different classes, and mailing hazardous materials requires that you obtain a placard and make sure the contents of the package are clearly labelled on the package itself.

These classes are:

1.Explosives

2.Gases

3.Flammable Liquids

4.Flammable Solids

5.Oxidizing Agents and Organic Peroxides

6.Toxic and Infectious Substances

7.Radioactive Substances

8.Corrosive Substances

9.Miscellaneous

Adhesives are, luckily, much easier to ship than many other hazardous 

materials or dangerous goods, but are still considered dangerous because 

of their toxicity and their flammability, placing them into classes 3 and 6.