Essentially, a combustible dust is any fine material that has the ability to catch fire and explode when mixed with air. Combustible dusts can be from:
most solid organic materials (such as sugar, flour, grain, wood, etc. )
many metals, and
some nonmetallic inorganic materials.
Some of these materials are not "normally" combustible, but they can burn or explode if the particles are the right size and in the right concentration.
Therefore any activity that creates dust should be investigated to see if there is a risk of that dust being combustible. Dust can collect on surfaces such as rafters, roofs, suspended ceilings, ducts, crevices, dust collectors, and other equipment. When the dust is disturbed and under certain circumstances, there is the potential for a serious explosion to occur. The build-up of even a very small amount of dust can cause serious damage.