Flammable gases or vapors are present in the air in quantities sufficient to produce explosive or ignitable mixtures.
Combustible or conductive dusts are present.
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)
The substance referred to by class is present during normal conditions.
The substance referred to by class is present only in abnormal conditions, such as a container failure or system breakdown.