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The Hazards of Workplace Dust

Published: 01st Sep 2012 in OSA Magazine

Bucket elevator conveyors are used for vertically conveying bulk materials. Dust is generated and dispersed as buckets are loaded with product, move through the elevator, and then unload the product at the top.

Dusts may be defined as particulate aerosols produced by mechanical processes such as breaking, grinding, and pulverising. Smaller size and higher specific surface area may enhance the ability of dusts to become airborne. Particle size ranges from less than 1 um up to 1 mm and are generally regular in shape with a length to width ratio less than 3:1. Problems arise when the dusts generated by vertical conveyance of the product are combustible.

How hazards escalate

Manufacturing facilities generating combustible dust are not following safe work procedures as outlined by Conveyor Equipment Manufacturers Association (CEMA) during inspection, cleaning or maintenance operations on bucket elevator conveyors. Recently, two fatalities occurred as a result of a combustible dust flash fire during maintenance procedures on a bucket elevator.

Bucket elevators are the most common location of primary explosions in the grain industry but are not as prevalent in manufacturing sectors. The need for conveyor safety plans addressing combustible dust fire and explosion hazards aren’t as well recognised in other sectors.

Explosion pentagon

Five elements, which must occur simultaneously, are necessary for a dust explosion to occur: fuel, heat (ignition source), oxygen, suspension, and confinement. These form the five sides of the explosion pentagon. All elements of the explosion pentagon, other than an ignition source, are inherent to bucket elevators used in both the grain and manufacturing sectors. To ensure safe operation and prevent the occurrence of combustible dust fires and explosions, OSHA regulations must consider incorporating dust zoning (CENELEC – ATEX, IEC and Australian standards).

Case study

Manufacturing facilities generating combustible dust are not following safe work procedures as outlined by Conveyor Equipment Manufacturers Association (CEMA) during inspection, cleaning, or maintenance operations. Control of hazardous energy (Lockout / Tagout) procedures designed to safeguard employees during startup are often ignored.

On January 31, 2011, a fatal incident occurred due to failure to follow Lockout / Tagout (LOTO) procedures during maintenance operations on a bucket elevator at a Tennessee manufacturing facility that produces atomised iron powder for the automotive industry. A combustible dust flash fire claimed two lives.
dust flash fire
Employees were also not using respiratory protection equipment. Aerosol particle size greatly influences where deposition occurs in the respiratory tract, and the site of deposition often determines the degree of hazard represented by the exposure. Particle size of the iron dust was likely too coarse to present a respirable hazard although the presence of dust can be an indicator of fire and explosion risk. Perhaps because of this lack of protection, risk was not perceived.

“Dust control equipment was working in the past, so they didn’t have any visible dust in the annealing furnace building air. Their proprietary atomisation process is controlled and appears to have a narrow band of particle sizes, then, being a metallic iron material, it doesn’t get too chewed up in the conveyors,” (Anderson).

Bucket elevator #12 had been shut down until maintenance personnel could inspect it. The elevator was reported to be malfunctioning due to a misaligned belt, which could become an ignition source.

“If the belt was off-track, it could rub against the leg casing, creating friction, possibly catching fire then dropping into the boot pit aflame,” (Astad).
A dust collector associated with the elevator was also reported to have been out of service for two days prior to the incident.

Elevator #12 was located downstream of an annealing furnace and conveyed fine iron powder to storage bins. The elevator had experienced off-track conditions three times in the six months prior to the incident. The inspection panel at the base of the elevator was typically removed without implementing LOTO procedures. Witness statements indicate that the access panel near the head was also open.

Two maintenance mechanics on the night shift were standing alone on an elevated platform adjacent to an open access panel at the bucket elevator head, checking the belt alignment. LOTO procedures were not followed prior to starting maintenance operations (US Chemical Safety Board).

Neither believed the belt was off-track. Since the elevator had been shut down due to a malfunction, product remained in the buckets. They requested via two-way radio that the operator in the control room restart the elevator. The elevator could neither be seen nor heard from the control room.

Bucket elevator access/inspections covers were left off the bucket elevator during the restart. Employees were aware of dangers of placing hands inside the elevator, but the risk of a dust explosion from a potentially explosive atmosphere existing within the elevator wasn’t effectively communicated.

OSHA regulations instructing employers to secure bucket elevator access covers prior to restarting equipment after maintenance procedures were found. These regulations do not address the risk of combustible dust flash fires or explosions.

When the bucket elevator was restarted combustible iron dust was lofted into the air, forming a dust cloud. The dust cloud ignited and a fireball engulfed the workers, causing their burn injuries. Both maintenance mechanics succumbed to their burn injuries. The first mechanic died from his injuries two days later. The second mechanic suffered for nearly four months.

What went wrong?

Control measures failed. According to Reasoner’s Swiss Cheese model, existing or unexpected hazards are prevented from causing human losses by a series of barriers. Each barrier has unintended weaknesses, or holes, which open and close at random. The holes lined up just right in Tennessee on January 31, 2011, allowing a fatal accident to happen.

LOTO regulations failed to address explosive atmospheres with access/inspection covers left off and product remaining in the buckets when restarting bucket elevators. LOTO procedures were not followed prior to beginning maintenance operations. Employees were not adequately trained on LOTO procedures.

Employees were not trained to recognise the hazards associated with potentially explosive atmospheres when accumulated dust is dispersed in air or to identify potential ignition sources prior to restarting bucket elevators. Some had never seen MSDS (material safety data sheets) for the combustible iron dust.
Written housekeeping procedures did not exist. Dust accumulations existed in and near Bucket elevator #12, providing fuel. Flash fires and near misses occurred periodically, but were rarely reported.

PPE, the last line of defence, was not enough to prevent the victims’ burn injuries when other control measures failed. Combustible metal dust fires burn very hot. Flame resistant clothing worn by the victims was consumed by the fireball that engulfed them. The few remnants that remained fell from their charred bodies onto the floor of the platform they were standing on.

OSHA regulations versus best industry practises

Bucket elevators used in the grain sector are regulated under OSHA Grain Facility Standard 29 CFR 1919.272. Bucket elevators in the manufacturing sector are not regulated by a formal OSHA combustible dust standard.

Best industry practises do not provide specific safety guidance for identifying the hazards of a combustible dust fire or explosion during bucket elevator LOTO procedures.

Instead of specific combustible dust regulations during LOTO procedures, OSHA relies on National Fire Protection Association (NFPA) Combustible Dust Standards (Best Industry Practises) and generally issues citations via the General Duty Clause.

NFPA Combustible Dust Standards:
• NFPA 61 Agricultural and Food Processing Facilities
• NFPA 484 Combustible Metals
• NFPA 654 Chemicals
• NFPA 664 Wood Processing Facilities
• NFPA 665 Sulfur Fires and Explosions

Grain sector engineering controls should be included, such as bearing temperature sensors, belt alignment sensors, and belt speed sensors help minimise the risk of combustible dust fires and explosions. These engineering controls, if required by OSHA regulations, would also minimise the risk of these incidents in the manufacturing sector.

Safety labels alerting employees to the hazards of bucket elevators have been developed by the Conveyer Equipment Manufacturers Association. Although warning labels are placed on bucket elevators during installation, there is no explosive atmosphere warning label.


OSHA Hazardous (Classified) Locations are defined for electrical ignition sources only. “The National Electrical Code (NEC) defines hazardous locations as those areas ‘where fire or explosion hazards may exist due to flammable gases or vapors, flammable liquids, combustible dust, or ignitable fibers or flyings’” (OSHA).

Class II locations are those areas made hazardous by the presence of combustible dust. The National Electrical Code specifies that hazardous material may exist in several different kinds of conditions described as either normal or abnormal.
• Class II, Division I – The hazard would be expected to be present in everyday production operations or during frequent repair and maintenance activity (normal)
• Class II, Division II - The hazardous material is expected to be confined within closed containers or closed systems and will be present only through accidental rupture, breakage or unusual faulty operation (abnormal)

European Union members follow the ATEX Directive, which is comprised of two parts. One directive defines what equipment is allowed in an environment with an explosive atmosphere. The other directive applies to the protection of workers exposed to explosive atmospheres. Both electrical and non-electrical ignition sources are included. Facilities in the European Union must follow the directives and protect employees from explosion risk.

Hazardous areas are classified by:
• The type of hazard (gas, vapour, dust or fibre)
• The likelihood of the hazard being present in flammable concentrations
• The (auto) ignition temperature of the hazardous material
• Dusts and fibres are also defined in terms of their ignition properties

Employers must also use zones to classify hazardous areas. There are three zones for dusts.
• Zone 20 - Dust cloud likely to be present continuously or for long periods
• Zone 21 - Dust cloud likely to be present occasionally in normal operation
• Zone 22 – Dust cloud unlikely to occur in normal operation

Zoning is a concept we are all familiarwith. For example, when driving on thehighway we note speed zones,construction zones or school zones; weassess the risk accordingly and modifyour behaviour in an effort to be safer.

In a laboratory, zones are defined, the hazards are communicated, and proper control measures (safety glasses, compatible gloves, goggles, fume hoods, secondary containment) are selected to ensure one’s safety and the safety of those working nearby.

Mechanical heating is considered and is fundamental to the explosion protection concept in ATEX. In the United States, only electrical ignition sources are considered when defining Class II locations. Zoning is optional. The United States simply must change the way hazardous areas are classified and include zoning in explosion protection concepts.

Improving safe work practises

Existing OSHA LOTO regulations for the control of hazardous energy (ignition sources) should include combustible dust (explosive atmosphere) hazards. Recognising explosive atmospheres in LOTO minimises accidents.

Risks of explosive atmospheres within bucket elevators should be communicated to employees via outreach, education and training. Leaving an access cover off provides the opportunity for a dust cloud or fireball to exit the opening during bucket elevator restart. An off-track condition exacerbates the problem. Intense frictional heat generated by the belt rubbing on the inside of the elevator casing can ignite a dust cloud.
workplace dust
Hazardous (classified) locations should be defined and implemented for all ignition sources. Identifying all ignition sources comprehensively communicates the risk.

Combustible dust fire and explosion hazards must be recognised in addition to unexpected energisation or startup of process equipment that could cause harm to employees in close proximity.

LOTO procedures that address combustible dust hazards (explosive atmospheres) must be included in OSHA Regulations and followed
prior to maintenance operations on bucket elevators.

Engineering controls, such as a black box to record mechanical data – similar to flight data recorder – for use in accident investigations should be implemented for bucket elevators located inside a building (Kauffman, Dr C William).

Trapped Key Interlock Switches could be installed to ensure bucket elevators cannot start if access doors are open. This would minimise the occurrence of combustible dust fires and explosions as well as injuries incurred from moving parts.

Alternative process equipment, such as screw conveyers, have fewer inherent risks for a dust fire/explosion and can be installed in place of bucket elevators in some manufacturing processes where vertical bulk material transport is necessary.
Often, vertical screw conveyors require less capital investment and installation cost, require less space, and can be installed at almost any angle.


Even if dust is not visible and respiratory protection is not required, dust particles can unexpectedly become suspended in air, presenting fire and explosion hazards.

Best industry practises do not provide specific safety guidance during bucket elevator LOTO procedures. Specific LOTO regulations (OSHA) instructing employers to secure bucket elevator access covers prior to restarting equipment after maintenance procedures exist; however, these regulations do not address the risk of combustible dust flash fires or explosions.

Control of hazardous energy – LOTO – procedures designed to safeguard employees during startup of Bucket Elevator #12 were ignored. Access covers were left open, creating fire and explosion hazards. Personnel remained in close proximity of process equipment while it was restarted.

Combustible dust fire and explosion hazards must be recognised in addition to unexpected energisation or startup of process equipment that could cause harm to employees in close proximity.

Applying lessons learned from the bucket elevator #12 combustible dust flash fire/explosion to communicating the hazard and improving safe work practises can save lives. 

Published: 01st Sep 2012 in OSA Magazine


Teresa A Long

Teresa A Long, Combustible Dust Policy Institute, Santa Fe, Texas, is dedicated to raising awareness of occupational health and safety hazards. Teresa served as the global focal point for reactor/train qualifications and FDA compliance audit programmes within the Polyolefins R&D characterisation group at Dow Chemical. Prior to that, Teresa performed analytical testing and method development while working at American Electric Power’s Dolan Engineering Laboratory. Teresa earned an AAS in Chemical Technology from Texas State Technical College in 2002 and a certificate in Environmental Health and Safety Technology (Applied Health Physics) in 2003. She is pursuing an Environmental Science degree at University of Houston-Clear Lake, majoring in Industrial Hygiene.


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