I often find folks are surprised when they are told that dust accumulations on ledges, equipment, rafters, etc. is a MAJOR hazard. We all know about suspended dust, but as humans we tend to feel more safe when we can not see the hazard, a concern we all carry as safety professionals. But dust accumulations are indeed a MAJOR hazard. I like to say the first dust explosion is not the worse one…the first one rattles all the accumulated dust and stirs it up and this is like throwing gasoline on a fire. Almost all dust explosions have secondary and tertiary explosions causing extensive damage, more so than a flammable vapor/gas explosion. Annex D of NFPA 654 contains guidance on dust layer characterization and precautions. It indicates that immediate cleaning is warranted whenever a dust layer of 1/32″ thickness accumulates over a surface area of at least 5% of the floor area of the facility or any given room. The 5% factor should not be used if the floor area exceeds 20,000 ft2, in which case a 1,000 ft2 layer of dust is the upper limit. Accumulations on overhead beams, joists, ducts, the tops of equipment, and other surfaces should be included when determining the dust coverage area. Even vertical surfaces should be included if the dust is adhering to them. 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 material in Annex D is an idealized approach based on certain assumptions, including uniformity of the dust layer covering the surfaces, a bulk density of 75 lb/ ft3, a dust concentration of 0.35 oz/ ft 3, and a dust cloud height of 10 ft. Additionally, FM Data Sheet 7-76 contains a formula to determine the dust thickness that may create an explosion hazard in a room, when some of these variables differ.