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It could happen at any woodworking plant. Any time. Anywhere. If your company processes wood flour, or produces fine particles of wood dust during machining operations, you have the potential for a combustible dust explosion.

What Is Combustible Dust?
Although there is no universally accepted definition, the Occupational Safety and Health Administration refers to combustible dusts as fine particles that present an explosion hazard when suspended in air in certain conditions.

Combustible fine particles of wood dust, also known as “deflagrable” wood dust, are classified as having a diameter of 420 microns (0.425mm, 425 um micrometers, 0.0165 inch) or smaller, and having a moisture content of less than 25 percent. In other words, the particles are sized small enough to pass through a U.S. No. 40 standard sieve.

However, not every material is a potential source for combustion. According to OSHA’s Hazard Communication Guidance for Combustible Dust, the physical properties used to determine combustible dust include:
• Minimum Ignition Energy (MIE): “which predicts the ease and likelihood of ignition of a dispersed dust cloud;”
• Minimum Explosible Concentration (MEC): “which measures the minimum amount of dust dispersed in air required to spread an explosion;”
• Kst dust deflagration index: which “measures the relative explosion severity compared to other dusts. The larger the value for Kst, the more severe the explosion.” Wood flour, for example has a strong explosion class rating of st2. Silica, on the other hand, has a zero rating.

If you have any doubt about the potential explosiveness of the material, have your dust tested by a certified facility.

 Secondary explosions or deflagrations can occur when pressure waves from the first incident
disperse and ignite combustible dust that has accumulated on surfaces. These secondary
explosions are usually more devastating than the primary one.
For any fire to occur, there must be fuel, ignition and
oxygen (Classic Fire Triangle). However, for a
combustible dust explosion, you must also add
dispersion and confinement — the “Dust Explosion
Pentagon.” Removal of any one element prevents an
explosion, though not necessarily a fire.

Proposed: National Standard
Between 1980 and 2008, OSHA recorded 422 incidents of combustible dust-related incidents. Of those, 53 occurred in wood products facilities.

Based on its findings, OSHA has begun the process of developing a standard that will address the fire and explosion hazards of combustible dust. The agency has issued an Advanced Notice of Proposed Rulemaking (ANPR 74 FR 54334) requesting comments, data and other information, on the hazards of combustible dust in the workplace.

Approximately two months ago, OSHA held the first of its Combustible Dust Stakeholder Meetings in Washington, DC, to solicit comments on the proposal to create a national standard. According to Jordan Barab, Deputy Director of Occupational Safety and Health, the goal of a standard regulating combustible dust hazards is to “protect workers.” He also noted at the Stakeholder Meeting that the “rulemaking process is slow and painful.”

Feedback to OSHA’s ANPR on Combustible Dust was due last month. OSHA asked for comments on 69 questions which, with questions within questions, actually totaled about 200. One question discussed Hazard Communication and the “Right to Know,” which requires MSDS (material safety data sheets). The ComDust standard is looking to require MSDS sheets on combustible materials. That raises the following questions: How can a piece of wood have an MSDS — a safety sheet for chemicals? Who creates the MSDS? If wood is only combustible if it is 420 microns or smaller, has less than 25 percent moisture and is contained within a dust explosion pentagon (see chart above), does it require an MSDS?

Page 31 of the ANPR states, “In some cases, the hazards of certain dust are widely known (for example, wood dust). In these cases, testing to determine whether the dust is explosive may not be necessary.”

Under the ANPR, methods of combating combustible dust explosions include:
• Hazard assessment: recognition, assessment, communication (HazCom Standard, MSDS), industry standards (National Fire Protection Assn.) and state and local codes (NFPA).
• Engineering controls: primary (building) to prevent the accumulation of dust on beams and surfaces, and secondary (equipment) such as the use of dust collection and sprinkler systems. Administrative controls include written rules and procedures.
• Housekeeping: identifying and eliminating fugitive dust. Use proper dust collection systems and filters, and make sure to minimize dust escaping from loose ductwork or bags.
• Explosion protection: including spark arrestor systems.
• Worker training.

OSHA’s NEP for combustible dust applies to 64 industries.
Of the more than 1,000 inspections conducted, 25% were at
wood products companies and 6% were at furniture companies.

On Now: National Emphasis Program
With plans for a national standard underway, OSHA in the meantime has released its Status Report on the Combustible Dust National Emphasis Program. The revised program targets 64 industries, including furniture and other wood products. In the two years since the inception of the initial NEP, more than 1,000 inspections have taken place, including 235 at wood products facilities and another 51 at furniture manufacturing plants. (See chart, page 32.)

Overall in its inspections, OSHA found more than 4,900 violations. Of those related to combustible dust, most fell under the hazard communication standard, with 548 violations, followed by 388 violations of the housekeeping standard (i.e., accumulation of dust on the floor — just a 1/32-inch-thick layer of fugitive dust can be combustible). (See examples of general duty clause violations on page 32.)

Until an OSHA standard is in place, the agency has referenced NFPA standards in combustible dust citations issued under the NEP. The NFPA standards most applicable to the woodworking industry are:
• 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids.
• 664: Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities.

However, there is a lot of discussion by manufacturers regarding the fact that the development of an OSHA Combustible Dust Standard could cause confusion should the OSHA standard continue to reference NFPA standards. For example, according to NFPA 654, the AHJ (Authority Having Jurisdiction) is “responsible for enforcing the requirements of a code or standard, or for approving equipment, materials, an installation, or a procedure.” But under OSHA, an Inspector is responsible. So who would be responsible?

Cost would also become an issue. NFPA charges for copies of its standards. So if an OSHA regulation were to refer to an NFPA standard, then a copy of the NFPA standard must be obtained and reviewed. Also, each NFPA standard typically refers to other standards. For example, NFPA 654 references approximately 35 other NFPA standards. At approximately $30 per standard, times 35, the cost could become a burden, especially for a small business. But more than that is the time that would be needed to digest all the data in the various standards. In order for a standard to be effective, it needs to be simplified.

Legislative Efforts
Congress also has gotten involved in the combustible dust issue. Sponsored by Rep. George Miller (D-CA), and cosponsored by Rep. John Barrow (D-GA) and Rep. Lynn Woolsey (D-CA), the “Worker Protection Against Combustible Dust Explosions and Fires Act of 2009” (H.R. 849) directs the Secretary of Labor to create a standard regulating combustible particulate solids and their dusts.

H.R. 849 was referred to the House Subcommittee on Workforce Protections back in March, where it remains under review.

Jamison Scott is a corporate officer of Air Handling Systems and also the chairman of the Wood Machinery Manufacturers of America’s Combustible Dust Task Force. Scott was in attendance at the Combustible Dust Stakeholder Meeting held in December. For more information, he can be reached at [email protected], or visit A copy of his report from the Stakeholder Meeting is in the News Archives on

Karen Koenig contributed to this article.

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