There are nine (9) hazard classes. Their order does not indicate degree of danger.

1. Explosives - This class has six divisions; in addition, this class has 13 compatibility groups that identify the kind of explosive articles and substances deemed to be compatible, which is very important when handling this type of cargo.
2. Gases - This class comprises compressed gases, liquefied gases, gases in solution, and mixtures of one or more gases with one or more vapors of substances of other classes. This class is organized into three subdivisions based on the primary hazards of gases during transport.
3. Flammable Liquids - This class has no subdivisions and includes liquids or mixtures of liquids, liquids containing solids in solution or in suspension that give off a flammable vapor at a temperature of not more than 60.5° C (150° F) open-cup test, normally referred to as the flash point. For example, paints, varnishing, lacquers, etc.
4. Flammable Solids - This class is divided into three divisions and includes all flammable solids and substances liable to spontaneous combustion or substances that emit flammable gases in contact with water.
5. Oxidizers and Organic Peroxides - This class has two divisions: a. Division 5.1 - Oxidizing Substances, which themselves are not necessarily combustible, but may generally cause or contribute to the combustion of other material by yielding oxygen. b. Division 5.2 - Organic Peroxides, which are substances that are thermally unstable and may undergo exothermic, self-accelerating decomposition. They are sensitive to impact and friction.
6. Toxic Materials and Infectious Substances - This class has two divisions: a. Division 6.1 - Toxic Substances, which are liable to cause death if swallowed, inhaled or contacted by the skin. b. Division 6.2 - Infectious Substances, which include all those substances that are infectious to humans and/or animals, and which include microorganisms and organisms, biological products, diagnostic specimens, and medical waste.
7. Radioactive Materials - Radioactive materials are articles or substances, which spontaneously and continuously emit certain types of radiation that can be harmful to health but which cannot be detected by any of the human senses. In this group, the principal considerations are the article activity and the transport index (TI), which is a single number, assigned to a cargo and is used to provide control of radiation exposure.
8. Corrosives - Substances that in the event of leakage can cause severe damage by chemical action when in contact with living tissue or that can materially damage other freight or the means of transport.
9. Miscellaneous Dangerous Goods - Articles and substances that during air transport present a danger not covered by other classes.