Combustible dust explosions have long been a recognized hazard in many industries, including agriculture, mining, power generation, and food processing. The United States Chemical Safety and Hazard Investigation Board conducted a study of dust explosion incidents between 1980 and 2005, which identified 281 incidents that killed 119 workers and injured 718. In 2006, CSB issued a formal recommendation to OSHA to promulgate a comprehensive combustible dust standard, based in part on NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids
In 2007, OSHA issued Instruction CPL 03-00-006, “Combustible Dust National Emphasis Program,” and reissued the NEP in 2008 in response to a combustible dust explosion at a sugar refinery. Two years later, OSHA published an Advance Notice of Proposed Rulemaking for Combustible Dust, in which the agency sought input on issues related to the hazards of combustible dust in the workplace in preparation for developing a combustible dust standard. In its own study of the NEP for combustible dusts, OSHA discovered that its inspectors cited the Occupational Safety and Health Act’s General Duty Clause almost seven times as often for combustible dust-related citations than for all other citations, which underscored the need for a combustible dust standard. Unfortunately, under the current administration, which campaigned to reduce “burdensome” regulations on industry, OSHA removed the combustible dust standard from its regulatory agenda, citing “resource constraints and other priorities.” THE DUST EXPLOSION PENTAGON Historically, one of the difficulties in controlling combustible or explosive dust hazards is defining exactly what constitutes a combustible or explosible dust atmosphere. Five elements are needed for a combustible dust explosion to occur: fuel (combustible dust), an ignition source (a flame, spark, or hot surface), an oxidizer (air, oxygen), dust dispersion in air, and confinement. These five elements are known as the “dust explosion pentagon”; without any one of them, an explosion will not occur. Confinement may be in the form of four walls and a roof, as in the case of a process building, or a process vessel or silo, which traditionally would be considered confined spaces. The OSHA confined spaces standard for general industry (29 CFR 1910.146) defines a hazardous atmosphere with respect to airborne combustible dust as “at a concentration that meets or exceeds its LFL” (lower flammable limit), and further states that “this concentration may be approximated as a condition in which the dust obscures vision at a distance of 5 feet (1.52 meters) or less.” How murky must one’s vision be to be “obscured”? Merriam-Webster defines “obscure,” in the context of vision, as “dark, dim; shrouded in or hidden by darkness; not clearly seen or easily distinguished.” If I were in a confined space with a potential combustible dust hazard, these definitions would not provide me much comfort in determining whether the concentration of dust was approaching the explosive range. Then along comes the publication of the 2019 edition of NFPA 652, Standard on the Fundamentals of Combustible Dust. The National Fire Protection Association has published other commodity-specific standards for combustible dusts, including NFPA 484, which addresses combustible metals; NFPA 61, which addresses agricultural and food processing facilities; NFPA 664 for wood processing and woodworking facilities; and NFPA 655 for the prevention of sulfur fires and explosions. While these commodity-specific standards are still in effect, NFPA 652 was designed to provide the general requirements for management of combustible dust fire and explosion hazards, and to provide consistency among the various industries, processes, and types of combustible dusts.
The Dust Explosion Pentagon
One of the objectives of the standard is to prevent injury from explosions through the design, construction, and maintenance of facilities, processes, and equipment and the implementation of management systems. The standard directly addresses confinement, one of the necessary elements for a combustible dust explosion according to the dust explosion pentagon. NFPA 652 defines an enclosure as a “confined or partially confined volume.” When a combustible dust ignites in a confined area, pressure may build until the structure of the space fails, resulting in the sudden release of pressure, which may result in the resuspension of dust in the air and secondary explosions. THE DUST HAZARD ANALYSIS DEFINED Central to NFPA 652 is the requirement for owners or operators of facilities with potentially combustible dust to conduct a Dust Hazard Analysis. A DHA is intended to determine combustibility hazards; identify and assess fire, flash fire, and explosion hazards; manage those hazards; and communicate the hazards to affected personnel. In the written discussion of the origin and development of NFPA 652, facility owners’ and operators’ lack of awareness of combustible dust hazards was the driving force in establishing the DHA as a fundamental step in creating a plan for safeguarding facilities that handle, store, or process combustible dusts. The standard sets a deadline of Sept. 7, 2020, for completion of the DHA. NFPA 652 provides some clarity on the issue of defining a combustible dust atmosphere, in the sense that it requires the owner or operator of a facility with potentially combustible dust to, among other things, determine the combustibility and explosibility hazards of the materials it handles. Chapter 5 of the standard, “Hazard Identification,” requires the owner or operator to determine whether the materials are combustible or explosible and, if so, to “characterize their properties” in support of the development of a DHA. NFPA 652 permits two options for determining the combustibility or explosibility of dust: the use of historical or published data, or the analysis of representative samples of the dust in question.  There is already much published data with respect to the combustibility of various types of dusts. OSHA published a combustible dust poster (PDF) that identifies 67 agricultural products or dusts, 12 carbonaceous dusts, 14 chemical dusts, 5 metal dusts, and 18 plastic dusts that, if processed in powdered form, pose a hazard of combustible dust explosion. More specific information about more than 7,000 combustible dusts is available from the Institute for Occupational Safety and Health of the German Social Accident Insurance, which developed a database of the combustion and explosion characteristics of dusts, also known as the GESTIS-DUST-EX database. This database provides information about a particular dust’s particle size, explosibility class (based on the rate of pressure rise), and minimum ignition energy. The database does not, however, provide the Minimum Explosible Concentration (MEC) of the dusts, a piece of data that could be invaluable in determining whether an explosion hazard actually exists in a given situation. According to NFPA 652, if the owner or operator chooses the testing option, explosibility of the dust may be determined either by the “go/no-go” screening test methods in ASTM E1226, Standard Test Method for Explosibility of Dust Clouds, or ASTM E1515, Standard Test Method for Minimum Explosible Concentration of Combustible Dusts. The ASTM E1226 test simply determines whether a sample of dust has the potential to be explosive, and the ASTM E1515 test determines the minimum concentration for explosivity of a dust cloud. So, an owner or operator of a facility in which potentially combustible dusts are handled or stored is now directed to define the minimum concentration of the dust suspended in air that could lead to a deflagration. 
One of the difficulties in controlling combustible or explosive dust hazards is defining exactly what constitutes a combustible or explosible dust atmosphere.
Chemical Safety and Hazard Investigation Board: Combustible Dust Safety. Chemical Safety and Hazard Investigation Board: “Dust Incidents, 2006–2017” (PDF, October 2018). Institute for Occupational Safety and Health of the German Social Accident Insurance: GESTIS-DUST-EX. National Fire Protection Association: NFPA 652, Standard on the Fundamentals of Combustible Dust (2019). OSHA: Combustible Dust (PDF).  OSHA: Combustible Dust National Emphasis Program (2007). OSHA: Occupational Safety and Health Standards, General Environmental Controls, Permit-required Confined Spaces.
As discussed, the current OSHA confined space standard defines a hazardous atmosphere with respect to combustible dust at a concentration that meets or exceeds its lower explosive limit. With NFPA 652, the owner or operator of a confined space containing combustible dust, such as a grain mill, for example, must now define the actual concentration of that dust that may cause an explosion. This information could then be used in confined space situations to more clearly define the level of hazard from airborne combustible dusts, which is a huge improvement from the “obscures vision at a distance of 5 feet” definition. In the case of potentially explosive gas or vapor scenarios in confined spaces, OSHA defines a hazardous atmosphere at a gas or vapor concentration in excess of 10 percent of its lower flammable limit, implying a safety factor of 10 from the actual explosive concentration. So, even if the concentration of dust that may give rise to an explosion in a confined space is known, the definition of combustible dust in OSHA’s Confined Spaces standard fails to adequately protect confined space entrants from explosion hazards, as there is no safety factor built into the definition. Once a dust is determined to be explosible or combustible, the owner or operator is required to perform a DHA. The DHA is the real meat of NFPA 652: it requires an owner or operator, now armed with information that the dusts they process are potentially explosive, to dive further into their operations to identify parts of the process that are vulnerable to dust fires or explosions. Chapter 7 of NFPA 652 describes the DHA process, which is required for all new processes and facility “compartments.” A compartment is defined in the standard as a subdivision of an enclosure, which itself is defined as a confined or partially confined volume. The DHA must assess the fire, deflagration, and explosion hazards in each facility compartment, and provide specific recommendations to manage those hazards. In general, the DHA must identify and evaluate the process or facility areas where fire, flash fire, and explosion hazards exist; where those hazards are present, the DHA must identify specific scenarios that may give rise to a fire and deflagration. The evaluation of fire and deflagration scenarios must include the identification of safe operating ranges, the safeguards that are in place to manage those fire and explosion scenarios, and recommendations for additional controls, including an implementation plan. Where combustible dust is present or potentially present in a process system, each part of the system must be evaluated for the potential to transport dust between parts of the system, the potential for fugitive dust to enter a building or building compartments, and the potential for deflagration propagation between parts of the process system. To be considered a fire hazard, an oxidizing atmosphere and a credible ignition source must be present. Where a sufficient quantity of combustible dust is present to propagate a deflagration, the addition of a credible suspension mechanism, such as vibration or air movement, constitutes a dust deflagration hazard. THE BEST HOPE In October 2018, a CSB study of combustible dust incidents found that an average of 9.5 incidents per year occurred during the period 2006 through 2017, a slight improvement on the annual average of 11 incidents the agency identified during 1980–2005. But the hazard clearly remains. Unless and until OSHA promulgates a combustible dust standard, the NFPA 652 requirement for a Dust Hazard Analysis is perhaps the best hope for prevention of future combustible dust incidents.   RUSH BOWERS, CIH, CSP, is senior industrial hygiene consultant at Terracon Consultants Inc. in Midvale, Utah. Send feedback to The Synergist.

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Standard Requires Completion of a Dust Hazard Analysis by September
A New Tool for Preventing Combustible Dust Incidents
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Disadvantages of being unacclimatized:
  • Readily show signs of heat stress when exposed to hot environments.
  • Difficulty replacing all of the water lost in sweat.
  • Failure to replace the water lost will slow or prevent acclimatization.
Benefits of acclimatization:
  • Increased sweating efficiency (earlier onset of sweating, greater sweat production, and reduced electrolyte loss in sweat).
  • Stabilization of the circulation.
  • Work is performed with lower core temperature and heart rate.
  • Increased skin blood flow at a given core temperature.
Acclimatization plan:
  • Gradually increase exposure time in hot environmental conditions over a period of 7 to 14 days.
  • For new workers, the schedule should be no more than 20% of the usual duration of work in the hot environment on day 1 and a no more than 20% increase on each additional day.
  • For workers who have had previous experience with the job, the acclimatization regimen should be no more than 50% of the usual duration of work in the hot environment on day 1, 60% on day 2, 80% on day 3, and 100% on day 4.
  • The time required for non–physically fit individuals to develop acclimatization is about 50% greater than for the physically fit.
Level of acclimatization:
  • Relative to the initial level of physical fitness and the total heat stress experienced by the individual.
Maintaining acclimatization:
  • Can be maintained for a few days of non-heat exposure.
  • Absence from work in the heat for a week or more results in a significant loss in the beneficial adaptations leading to an increase likelihood of acute dehydration, illness, or fatigue.
  • Can be regained in 2 to 3 days upon return to a hot job.
  • Appears to be better maintained by those who are physically fit.
  • Seasonal shifts in temperatures may result in difficulties.
  • Working in hot, humid environments provides adaptive benefits that also apply in hot, desert environments, and vice versa.
  • Air conditioning will not affect acclimatization.
Acclimatization in Workers