Combustible dust: A hazard lying in wait

January 2003 Six workers are killed and dozens more are injured after fine plastic powder ignites at the West Pharmaceutical Services plant in Kinston, NC.	February 2008 An explosion kills 14 workers and injures 38 more at the Imperial Sugar refinery in Port Wentworth, GA.
January 2011 Three flash fires over a six-month period result in five worker deaths at the Hoeganaes Corp.’s powered iron manufacturing plant in Gallatin, TN.	May 2017 Corn dust ignites and kills five workers and injures 14 others at the Didion Milling Co. facility in Cambria, WI.

The common link in these incidents: combustible dust.

“Combustible dust explosions and fires can be deadly and incredibly destructive,” Chemical Safety Board Chair Steve Owens said when announcing the agency’s final investigation results for the Didion Milling explosion.

How can employers avoid similar catastrophes?

What’s combustible dust?

According to OSHA, combustible dust is any material that can burn rapidly when in a “finely divided form.”

The agency adds that “even materials that do not burn in larger pieces (such as aluminum or iron), given the proper conditions, can be explosible in dust form.”

Laura Moreno, senior engineer and standards lead for hazardous chemicals and materials at the National Fire Protection Association, says some workers may not be aware that they’re working near a combustible dust “because it might not have caused them any trouble in the past and is a seemingly innocuous material.”

Moreno pointed to flour, sugar and corn starch as examples.

How combustible dust is created?

Combustible dusts can form in a wide range of industries and industrial processes. Among them: agriculture, food production, textiles, chemical manufacturing, recycling facilities, woodworking operations and metal works.

“Dusts are created when materials are transported, handled, processed, polished, ground and shaped,” the Canadian Center for Occupational Health and Safety says. “Dusts are also created by abrasive blasting, cutting, crushing, mixing, sifting or screening dry materials. The buildup of dried residue from the processing of wet materials can also generate dusts.

“Essentially, any workplace that generates dust is potentially at risk.”

That dust can accumulate and build up on surfaces. Even a small amount of buildup on rafters, roofs, ceilings, ducts and windowsills can lead to a serious incident.

What causes combustible dust incidents?

Fire needs fuel, oxygen and an ignition source (a spark from static electricity, for example). In many combustible dust explosions or fires, two additional elements are present, according to CCOHS:

• Dust particles in the “right concentration” suspended in the air
• The confinement of that dust cloud in an enclosed or limited space, allowing pressure to build

However, CCOHS notes, some flash fires may occur without the confinement of a dust cloud.

Secondary explosion dangers

In a written response to Safety+Health, OSHA states that “secondary explosions” have been another major cause of death and damage. In those occurrences, the initial explosion sends more dust into the air, where it too can ignite.

“An initial (or primary) explosion in processing equipment or in an area where fugitive dust has accumulated may dislodge more accumulated dust into the air or damage a containment system (such as a duct, vessel or collector),” OSHA says. “Secondary explosions can be far more destructive than a primary explosion due to the increased quantity and concentration of dispersed combustible dust.”

OSHA standards

OSHA doesn’t have a specific standard on combustible dust, despite calls for one in multiple Chemical Safety Board investigation reports. However, provisions in OSHA’s general industry standards “address certain aspects of combustible dust hazards.” Among those provisions is 1910.22(a)(1), under the standard on walking-working surfaces: “All places of employment, passageways, storerooms, service rooms, and walking-working surfaces are kept in a clean, orderly and sanitary condition.”

The agency issued a proposed rule on combustible dust in October 2009, but withdrew that rulemaking in March 2017. The agency launched a National Emphasis Program on combustible dust in October 2007 and revised it in January 2023.

Assessing the hazards

To control combustible dusts, one of the first steps is conducting a hazard assessment, OSHA says. That assessment should include any materials handled by employees, organizational operations, byproducts of manufacturing, hidden and open spaces, and all possible ignition sources. To determine whether a powder or dust is explosive or combustible, use Safety Data Sheets or internet searches, Stonehouse Process Safety CEO Vahid Ebadat said.

“However, if this information cannot be found for some materials, Go/No-Go Explosibility Screening should be considered,” he said. “This is a relatively inexpensive laboratory test that would require about 100 grams of a representative sample of the powder and/or dust.”

If a dust or powder is explosive, Ebadat said the next step is to perform a dust hazard analysis with help from a subject matter expert.

“DHA is a systematic evaluation of potential dust fire, deflagration (or flash fire), and explosion hazards in a process or facility and prioritized recommendation of practical measures for their elimination or control,” he said.

Controlling combustible dust

The next focus for employers is containing combustible dust, Moreno said. That includes capturing dust at the point of release from equipment, using a dust collection system and following housekeeping methods “to clean up any fugitive dust so it doesn’t pose a hazard.”

OSHA adds that employers should:

• Use surfaces that minimize dust accumulation and facilitate cleaning.
• Provide access to all hidden areas to permit inspection.
• Inspect for dust residues in open and hidden areas at regular intervals.
• Use cleaning methods that don’t generate dust clouds if ignition sources are present.
• Use only vacuum cleaners approved for dust collection.
• Locate relief valves away from dust deposits.

OSHA also has advice on ignition control and injury/damage control methods.

Among the agency’s tips:

• Use appropriate electrical equipment and wiring methods.
• Control static electricity, including bonding of equipment to ground.
• Control smoking, open flames, sparks and friction.
• Separate heated surfaces or heating systems from dusts.
• Direct vents away from work areas.

“It’s also important to use explosion protection methods such as deflagration venting or suppression to mitigate combustible dust hazards within equipment,” Moreno said.