According to NFPA 654 “A combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape.”

According to NFPA 654, 3.3.7 “Propagation of combustion zone at a volictiy that is less than the speed of sound in the un-reacted medium.”

Combustible particulate solid small enough to be deflagrable.
Deflagrable particle suspended in air.
Deflagrable particulate of high enough concentration.
Adequate igniter.

Any combustible solid material with a distinct particles or pieces. They can include fibers, chips, fines, dusts, and flakes.

The smaller the particle size the more dangerous the particulate and greater likelihood of explosion.

Typically in the Highest compartments of buildings, such as rafters, top of ductwork, top of lights, etc.

When sufficient energy per unit (temperature) of time and volume is applied to a suspended deflagrable particle.

Limiting, or eliminating accumulated fugitive dust in the workplace.

Rough calculations show that the available surface area of bar joists is approximately 5 % of the floor area and the equivalent surface area for steel beams can be as high as 10%.

The minimum concentration that will propagate a flame front through a suspension of the particulate in air or other medium of concern.”