Class
| Class I | Flammable gases or vapors are present in the air in quantities sufficient to produce explosive or ignitable mixtures. |
| Class II | Combustible or conductive dusts are present. |
| Class III | Ignitable fibers or flyings are present, but not likely to be in suspension in sufficient quantities to produce ignitable mixtures. (Group classifications are not applied to this class.) |
Division
| Division 1 | The substance referred to by class is present during normal conditions. |
| Division 2 | The substance referred to by class is present only in abnormal conditions, such as a container failure or system breakdown. |
Group
| Group A | Acetylene |
| Group B | Hydrogen (or gases of equivalent hazard) |
| Group C | Ethylene (or gases of equivalent hazard) |
| Group D | Gasoline (or gases of equivalent hazard) |
| Group E | Metal Dust |
| Group F | Coal Dust |
| Group G | Grain Dust |
