The classes defines the general nature of hazardous material in the surrounding atmosphere.
| Class | Hazardous Material in Surrounding Atmosphere |
| Class I | Hazardous because flammable gases or vapors are present in the air in quantities sufficient to produce explosive or ignitable mixtures. |
| Class II | Hazardous because combustible or conductive dusts are present. |
| Class III | Hazardous because ignitable fibers or flying's are present, but not likely to be in suspension in sufficient quantities to produce ignitable mixtures. (Group classifications are not applied to this class.) |
The division defines the probability of hazardous material being present in an ignitable concentration in the surrounding atmosphere.
| Division | Presence of Hazardous Material |
| 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. |
The group defines the hazardous material in the surrounding atmosphere.
| Group | Hazardous Material in Surrounding Atmosphere |
| Group A | Acetylene |
| Group B | Hydrogen, fuel and combustible process gases containing more than 30% hydrogen by volume or gases of equivalent hazard such as butadiene, ethylene, oxide, propylene oxide and acrolein. |
| Group C | Ethyl and ethylene or gases of equivalent hazard. |
| Group D | Gasoline, acetone, ammonia, benzene, butane, cyclopropane, ethanol, hexane, methanol, methane, natural gas, naphtha, propane or gases of equivalent hazard. |
| Group E | Combustible metal dusts, including aluminum, magnesium and their commercial alloys or other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards in connection with electrical equipment. |
| Group F | Carbonaceous dusts, coal black, charcoal, coal or coke dusts that have more than 8% total entrapped volatiles or dusts that have been sesitized by other material so they present an explosion hazard. |
| Group G |
All reasonable efforts have been been made to accurately compile the information contained here. However, no warranty is made with regard to any such information, and Piping Specialties, Inc., cannot assume any responsibility for any possible errors or any direct or indirect result of the use of such information.
