The criteria for a dust hazard are typically based on an evaluation of dust accumulations in areas of the facility. The combustible dust accumulation level that would support a fire and/or explosion, if there was an ignition source present, depends on numerous factors. For some dusts, accumulation limits are specified. For other dusts, accumulation limits can be calculated. The following standards set dust threshold accumulation limits:
NFPA 61-2017—Agricultural and food processing facilities:
Establishes an accumulation limit of 1/8 inch over 5% of the foot print area.
NFPA 85-2019—Combustible coal dust:
Accumulation limits are not specifically set but include housekeeping provisions to keep dust deposits “less than the minimum required to create an explosion hazard.” These provisions direct the employer to NFPA 654 for the use of calculation methods described in NFPA 654. [Note: The binding OSHA requirement on combustible coal dust in the operation and maintenance of electric power generation, control, transformation, transmission, and distribution lines and equipment is 29 CFR 1910.269(v)(11)(xii).]
NFPA 484-2019—Combustible metals:
Typically, accumulations should minimally not be allowed to accumulate to a level that obscures the color of the surface beneath it, and could be stricter based on the material and the dust hazard analysis. Before conducting site visits, refer to the considerations specific to the particular metal or metal alloy.
NFPA 654-2017—Particulate solid dusts:
Provides calculation methods for determining the hazards of many particulate solid dust accumulations based on the factors listed earlier in this section.
NFPA 655-2017—Sulfur:
Provides calculation methods for determining the hazards of sulfur accumulations.
NFPA 664-2017—Wood:
Provides information and calculation methods for characterizing wood dust accumulations. Establishes an accumulation limit of 1/8 inch over the foot print area for smaller areas and as an average of 1/8 inch of accumulation on upward facing surfaces for larger building compartments.
OSHA’s Grain Handling Facilities standard 29 CFR 1910.272 has requirements for limiting dust accumulations in grain handling facilities.
29 CFR1910.272(j)(2)(ii): “The employer shall immediately remove any fugitive grain dust accumulations whenever they exceed 1/8 inch (0.32 cm) at priority housekeeping areas, pursuant to the housekeeping program, or shall demonstrate and assure, through the development and implementation of the housekeeping program, that equivalent protection is provided.”
29 CFR 1910.272(j)(1): “The employer shall develop and implement a written housekeeping program that establishes the frequency and method(s) determined best to reduce accumulations of fugitive grain dust on ledges, floors, equipment, and other exposed surfaces.”
Note: The summaries above are not all inclusive. Exceptions and additional criteria often apply. Therefore, CSHOs should review the applicable NFPA standard(s) to ensure that the criteria for the dust hazards are understood, rather than simply relying on the above summaries. If an OSHA standard specifically refers to or incorporates an older version of an industry standard, the older version applies.
Source: https://www.osha.gov/sites/default/files/otm_secIV_chap6.pdf