OSHA issued a memorandum to Regional Administrators on April 22, 2015 to provide further guidance on evaluating the combustion hazards associated with dust accumulation. Currently, it is possible to infer from OSHA’s Directive, CPL 03-00-008, Combustible Dust National Emphasis Program (Reissued), that any dust accumulation of 1/32nd inch or more presents a deflagration or combustible hazard. However, combustible dust determinations are not that simple. Other factors need to be considered, including the area of dust accumulation (see section 6.1.3.2 of NFPA 654 (2013 edition) and the bulk density of combustible dusts.

The National Fire Protection Association’s consensus standard, NFPA 654, allows dust accumulation to exceed 1/32^nd inch for materials with a bulk density less than 75 lb/ft³. According to OSHA’s guidance “[b]ulk densities of combustible dusts depend on many factors including the type of material (e.g. wood, paper, plastic, metal, etc.), the dust particle size, and the dust particle shape.” The memorandum includes the mathematical formula NFPA uses to make this determination. The formula is based on LD (layer depth) and BD (bulk density).  Both layer depth and bulk density are explained more fully but the guidance does instruct Compliance Safety and Health Officers (CSHOs) to consider the bulk density of a dust before determining whether a violation of 1910.22(a)(1), 1910.22(a)(2) or 1910.176(c) has occurred. Tissue dust is used as an example of a very low density dust which may not pose “a deflagration hazard even at an accumulation level of 1/4 inch, covering over five percent of the floor area of 1000 ft2, whichever is less.” The memorandum provides detailed instructions on how to test low density dust accumulations to determine their deflagration risk or combustibility.

According to Bloomberg BNA, OSHA does not anticipate that this guidance will materially alter the outcome of inspections. OSHA believes that most inspections involve dust accumulations so far in excess of the upper allowable limits that only in situations involving light dusts will bulk density be a deciding factor. However, this memorandum makes clear that the accumulation of 1/32^nd inch of dust alone does not justify a citation.

A copy of OSHA’s memorandum can be found here.