U. S. Department of Labor

Occupational Safety and Health Administration

Directorate of Standards and Guidance

Office of Safety Systems

SHIB 07-31-2005

Safety and Health Information Bulletin

Combustible Dust in Industry: Preventing and Mitigating

 the Effects of Fire and Explosions

Purpose

This Safety and Health Information Bulletin

(SHIB) highlights:

• Hazards associated with combustible

dusts;

• Work practices and guidelines that

reduce the potential for a combustible

dust explosion, or that reduce the danger

to employees if such an explosion

occurs; and,

• Training to protect employees from these

hazards.

This Safety and Health Information Bulletin is not a

standard or regulation, and it creates no new legal

obligations. The Bulletin is advisory in nature, informational in content, and is intended to assist employers in

providing a safe and healthful workplace. Pursuant to

the Occupational Safety and Health Act, employers

must comply with hazard-specific safety and health

standards promulgated by OSHA or by a state with an

OSHA-approved state plan. In addition, pursuant to

Section 5(a)(1), the General Duty Clause of the Act,

employers must provide their employees with a workplace free from recognized hazards likely to cause

death or serious physical harm. Employers can be cited

for violating the General Duty Clause if there is a

recognized hazard and they do not take reasonable

steps to prevent or abate the hazard. However, failure

to implement any recommendations in this Safety and

Health Information Bulletin is not, in itself, a violation of

the General Duty Clause. Citations can only be based

on standards, regulations, and the General Duty Clause.

Background

Organic Dust Fire and Explosion:

Massachusetts (3 killed, 9 injured)

In February 1999, a deadly fire and explosion

occurred in a foundry in Massachusetts. The

Occupational Safety Health Administration (OSHA)

and state and local officials conducted a joint

investigation of this incident. The joint investigation

report1

 indicated that a fire initiated in a shell

molding machine from an unknown source and then

extended into the ventilation system ducts by

feeding on heavy deposits of phenol formaldehyde

resin dust. A small primary deflagration occurred within

the ductwork, dislodging dust that had settled on the

exterior of the ducts. The ensuing dust cloud provided

fuel for a secondary explosion which was powerful

enough to lift the roof and cause wall failures.

Causal factors listed in the joint investigation report

included inadequacies in the following areas:

• Housekeeping to control dust accumulations;

• Ventilation system design;

• Maintenance of ovens; and,

• Equipment safety devices.
Organic Dust Fire and Explosion: North Carolina

(6 killed, 38 injured)

In January 2003, devastating fires and explosions

destroyed a North Carolina pharmaceutical plant that

manufactured rubber drug-delivery components. Six

employees were killed and 38 people, including two

firefighters, were injured. The U.S. Chemical Safety

and Hazard Investigation Board (CSB), an

independent Federal agency charged with investigating

chemical incidents, issued a final report2 concluding

that an accumulation of a combustible polyethylene

dust above the suspended ceilings fueled the

explosion. The CSB was unable to determine what

ignited the initial fire or how the dust was dispersed to

create the explosive cloud in the hidden ceiling space.

The explosion severely damaged the plant and caused

minor damage to nearby businesses, a home, and a

school. The causes of the incident cited by CSB

included inadequacies in:

• Hazard assessment;

• Hazard communication; and

• Engineering management.

The CSB recommended the application of provisions

in National Fire Protection Association standard

NFPA 654, Standard for the Prevention of Fire

and Dust Explosions from the Manufacturing,

Processing, and Handling of Combustible

Particulate Solids, as well as the formal adoption of

this standard by the State of North Carolina.

Organic Dust Fire and Explosion: Kentucky (7

killed, 37 injured)

In February 2003, a Kentucky acoustics insulation

manufacturing plant was the site of another fatal dust

2

explosion. The CSB also investigated this incident.

Their report3 cited the likely ignition scenario as a

small fire extending from an unattended oven which

ignited a dust cloud created by nearby line cleaning.

This was followed by a deadly cascade of dust

explosions throughout the plant. The CSB identified

several causes of ineffective dust control and

explosion prevention/mitigation involving inadequacies

in:

• Hazard assessment;

• Hazard communication;

• Maintenance procedures;

• Building design; and,

• Investigation of previous fires.

Metal Dust Fire and Explosion: Indiana (1 killed,

1 injured)

Finely dispersed airborne metallic dust can also be

explosive when confined in a vessel or building. In

October 2003, an Indiana plant where auto wheels

were machined experienced an incident which was

also investigated by the CSB. A report has not yet

been issued, however, a CSB news release4

 told a

story similar to the previously discussed organic dust

incidents: aluminum dust was involved in a primary

explosion near a chip melting furnace, followed by a

secondary blast in dust collection equipment.

Related Experience in the Grain Handling

Industry

In the late 1970s a series of devastating grain dust

explosions in grain elevators left 59 people dead and

49 injured. In response to these catastrophic events,

OSHA issued a “Grain Elevator Industry Hazard
Alert” to provide employers, employees, and other

officials with information on the safety and health

hazards associated with the storage and distribution of

grain.

In 1987, OSHA promulgated the Grain Handling

Facilities standard (29 CFR 1910.272), which

remains in effect. This standard, other OSHA

standards such as Emergency Action Plans (29 CFR

1910.38), and updated industry consensus standards

all played an important role in reducing the occurrence

of explosions in this industry, as well as mitigating their

effects. The lessons learned in the grain industry can

be applied to other industries producing, generating,

or using combustible dust.

Elements of a Dust Explosion

Elements Needed for a Fire (the familiar “Fire

Triangle”):

1. Combustible dust (fuel);

2. Ignition source (heat); and,

3. Oxygen in air (oxidizer).

Additional Elements Needed for a Combustible

Dust Explosion:

4. Dispersion of dust particles in sufficient quantity

 and concentration; and,

5. Confinement of the dust cloud.

The addition of the latter two elements to the fire

triangle creates what is known as the “explosion

pentagon” (see Figure 1). If a dust cloud (diffused

fuel) is ignited within a confined or semi-confined

vessel, area, or building, it burns very rapidly and may

explode. The safety of employees is threatened by the

ensuing fires, additional explosions, flying debris, and

collapsing building components.

An initial (primary) explosion (see Figure 2) in

processing equipment or in an area were fugitive dust

has accumulated may shake loose more accumulated

dust, or damage a containment system (such as a duct,

vessel, or collector). As a result, if ignited, the

additional dust dispersed into the air may cause one or

more secondary explosions (see Figure 2). These can

be far more destructive than a primary explosion due

to the increased quantity and concentration of

dispersed combustible dust.

Blast

Wave

Primary Explosion Secondary Explosion

Dust cloud formed Heat from primary explosion ignites 

dust cloud

Figure 2

Figure 1

3
4

If one of the elements of the explosion pentagon is

missing, a catastrophic explosion can not occur.

Two of the elements in the explosion pentagon are

difficult to eliminate: oxygen (within air), and

confinement of the dust cloud (within processes or

buildings). However, the other three elements of

the pentagon can be controlled to a significant

extent, and will be discussed further in this

document.

Facility Dust Hazard Assessment

A combustible dust explosion hazard may exist in a

variety of industries, including: food (e.g., candy,

starch, flour, feed), plastics, wood, rubber, furniture,

textiles, pesticides, pharmaceuticals, dyes, coal,

metals (e.g., aluminum, chromium, iron, magnesium,

and zinc), and fossil fuel power generation. The vast

majority of natural and synthetic organic materials,

as well as some metals, can form combustible dust.

NFPA’s Industrial Fire Hazards Handbook5

states that “any industrial process that reduces a

combustible material and some normally

noncombustible materials to a finely divided state

presents a potential for a serious fire or explosion.”

Facility Analysis Components

Facilities should carefully identify the following in

order to assess their potential for dust explosions:

• Materials that can be combustible when

finely divided;

• Processes which use, consume, or produce

combustible dusts;

• Open areas where combustible dusts may

build up;

• Hidden areas where combustible dusts

may accumulate;

• Means by which dust may be dispersed in

the air; and

• Potential ignition sources.

The applicable Federal, state, and local laws and

regulations must be identified and followed. The

two predominant model fire codes which have been

adopted by many jurisdictions in this country are the

International Code Council’s International Fire Code®

and NFPA’s Uniform Fire Code®. Both of these model

codes reference many of the NFPA consensus standards

related to dust explosion prevention and mitigation which

are discussed below. In the absence of a legal mandate

to comply with these consensus standards, they should

be considered a very useful source of guidance on this

topic.

Dust Combustibility

The primary factor in an assessment of these hazards is

whether the dust is in fact combustible. Any “material

that will burn in air” in a solid form can be explosive

when in a finely divided form.6

 Combustible dust is

defined by NFPA 654 as: “Any finely divided solid

material that is 420 microns or smaller in diameter

(material passing a U.S. No. 40 Standard Sieve) and

presents a fire or explosion hazard when dispersed and

ignited in air.” The same definition is used for

combustible metal dust in NFPA 484, Standard for

Combustible Metals, Metal Powders, and Metal

Dusts. One possible source for information on

combustibility is the Material Safety Data Sheet (MSDS)

for the material. In some cases, additional information

such as test results will be available from chemical

manufacturers.

Different dusts of the same chemical material will have

different ignitability and explosibility characteristics,

depending upon many variables such as particle size,

shape, and moisture content. Additionally, these variables

can change while the material is passing through process

equipment. For this reason, published tables of dust

explosibility data may be of limited practical value. In

some cases, dusts will be combustible even if the particle

size is larger than that specified in the NFPA definition,

especially if the material is fibrous.7

Industrial settings may contain high-energy ignition

sources such as welding torches. In these situations, test

methods for dust ignition and explosion characteristics

from ASTM International (originally the American

Society for Testing and Materials) would be of value. A

discussion of these test methods is in reference 8, and

the relevant OSHA and other standards are listed in the

“Sources of Additional Information” section of this

document.
initiate is where dust is concentrated. In equipment

such as dust collectors, a combustible mixture could

be present whenever the equipment is operating.

Other locations to consider are those where dust can

settle, both in occupied areas and in hidden concealed

spaces. A thorough analysis will consider all possible

scenarios in which dust can be disbursed, both in the

normal process and potential failure modes.

After hazards have been assessed and hazardous

locations are identified, one or more of the following

prevention, protection and/or mitigation methods may

be applied. The references and information sources at

the end of this document will assist in the decision

process for the methods suitable to specific work

sites. Additional guidance and requirements may be

available from local or state fire and building code

officials as well as OSHA Area or Regional Offices.

Dust Control

NFPA 654, Standard for the Prevention of Fire

and Dust Explosions from the Manufacturing,

Processing, and Handling of Combustible

Particulate Solids, contains comprehensive guidance

on the control of dusts to prevent explosions. The

following are some of its recommendations:

• Minimize the escape of dust from process

equipment or ventilation systems;

• Use dust collection systems and filters;

• Utilize 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;

• Clean dust residues at regular intervals;

• Use cleaning methods that do not generate

dust clouds, if ignition sources are present;

• Only use vacuum cleaners approved for dust

collection;

• Locate relief valves away from dust hazard

areas; and

• Develop and implement a hazardous dust

inspection, testing, housekeeping, and control

program (preferably in writing with established

frequency and methods).

5

Electrical Classification

The facility analysis must identify areas requiring

special electrical equipment classification due to the

presence (or potential presence) of combustible dust.

The OSHA Electrical standard (29 CFR Part 1910

Subpart S) contains general requirements for electrical

installations in hazardous areas. Detailed requirements

for equipment and wiring methods are in NFPA 70,

the National Electrical Code®. However, NFPA 70

does not define combustible dusts.

Further guidance on area classification is contained in

NFPA 499, Recommended Practice for the

Classification of Combustible Dusts and of

Hazardous (classified) Locations for Electrical

Installations in Chemical Process Areas. This

document uses the same definition of combustible dust

as NFPA 484 and NFPA 654. The overall dust

hazard designation for electrical requirements is Class

II. This is further broken down into Divisions which

represent the probability of dust being present at any

given time. Additionally, each dust is assigned a group

(E, F, or G), representing the dust types (metal,

carbonaceous, and other, respectively) with different

properties. For instance, group E dusts are electrically

conductive and electric current can pass through a

layer of such dust under favorable circumstances,

causing short circuits or arcs.

Other Hazard Analysis Considerations

The amount of dust accumulation necessary to cause

an explosive concentration can vary greatly. This is

because there are so many variables – the particle size

of the dust, the method of dispersion, ventilation

system modes, air currents, physical barriers, and the

volume of the area in which the dust cloud exists or

may exist. As a result, simple rules of thumb regarding

accumulation (such as writing in the dust or visibility in

a dust cloud) can be subjective and misleading. The

hazard analysis should be tailored to the specific

circumstances in each facility and the full range of

variables affecting the hazard.

Many locations need to be considered in an

assessment. One obvious place for a dust explosion to
The OSHA ventilation standard, 29 CFR 1910.94,

contains ventilation requirements for certain types of

operations (such as abrasives, blasting, grinding, or

buffing) which involve dusts, including combustible

dusts. Additionally, 29 CFR 1910.22(a)(1) requires

employers to keep work places and other areas clean,

which includes the removal of dust accumulations.

Ignition Control

NFPA 654, Standard for the Prevention of Fire

and Dust Explosions from the Manufacturing,

Processing, and Handling of Combustible

Particulate Solids, also contains comprehensive

guidance on the control of ignition sources to prevent

explosions. The following are some of its

recommendations:

• Use appropriate electrical equipment and

wiring methods;

• Control static electricity, including bonding of

equipment to ground;

• Control smoking, open flames, and sparks;

• Control mechanical sparks and friction;

• Use separator devices to remove foreign

materials capable of igniting combustibles from

process materials;

• Separate heated surfaces from dusts;

• Separate heating systems from dusts;

• Proper use and type of industrial trucks;

• Proper use of cartridge activated tools; and

• Adequately maintain all the above equipment.

The use of proper electrical equipment in hazardous

locations is crucial to eliminating a common ignition

source. The classification of areas requiring special

electrical equipment is discussed in the Facility Dust

Hazard Assessment section above. Once these areas

have been identified, special Class II wiring methods

and equipment (such as “dust ignition-proof” and

“dust-tight”) must be used as required by 29 CFR

1910.307 and as detailed in NFPA 70 Article 500. It

is important not to confuse Class II equipment with

Class I explosion-proof equipment, as Class II

addresses dust hazards, while Class I addresses gas,

vapor and liquid hazards.

The use of industrial trucks is regulated by OSHA’s

Powered Industrial Trucks standard (29 CFR

1910.178). Hazardous atmospheres including dust

concentrations are addressed in paragraph (c) of this

standard.

Where coal-handling operations may produce a

combustible atmosphere from flammable dust,

employers covered by the Electric Power Generation,

Transmission, and Distribution standard must eliminate

or safely control ignition sources. See 29 CFR

1910.269(v)(11)(xii).

Damage Control

NFPA 654, Standard for the Prevention of Fire

and Dust Explosions from the Manufacturing,

Processing, and Handling of Combustible

Particulate Solids, contains comprehensive guidance

to minimize the danger and damage from an explosion.

The following are some suggested protection methods:

• Separation of the hazard (isolate with

distance);

• Segregation of the hazard (isolate with a

barrier);

• Deflagration venting of a building, room, or

area;

• Pressure relief venting for equipment;

• Provision of spark/ember detection and

extinguishing systems;

• Explosion protection systems (also refer to

NFPA 69, Standard on Explosion

Prevention Systems);

• Sprinkler systems; and

• The use of other specialized suppression

systems.

Training

Employees

Workers are the first line of defense in preventing and

mitigating fires and explosions. If the people closest

to the source of the hazard are trained to recognize

and prevent hazards associated with combustible dust

6
in the plant, they can be instrumental in recognizing

unsafe conditions, taking preventative action, and/or

alerting management. While OSHA standards require

training for certain employees, all employees should be

trained in safe work practices applicable to their job

tasks, as well as on the overall plant programs for dust

control and ignition source control. They should be

trained before they start work, periodically to refresh

their knowledge, when reassigned, and when hazards

or processes change.

Employers with hazardous chemicals (including

combustible dusts) in their workplaces are required to

comply with 29 CFR 1910.1200, the Hazard

Communication standard. This includes having labels

on containers of hazardous chemicals, using material

safety data sheets, and providing employee training.

Management

A qualified team of managers should be responsible

for conducting a facility analysis (or for having one

done by qualified outside persons) prior to the

introduction of a hazard and for developing a

prevention and protection scheme tailored to their

operation. Supervisors and managers should be aware

of and support the plant dust and ignition control

programs. Their training should include identifying how

they can encourage the reporting of unsafe practices

and facilitate abatement actions.

References

1. Occupational Safety and Health Administration

(OSHA), the Massachusetts Office of the State Fire

Marshall, and the Springfield Arson and Bomb Squad.

“Joint Foundry Explosion Investigation Team Report.”

OSHA, Springfield, MA, (No date).

2. Chemical Safety and Hazard Investigation Board

(CSB). “Investigation Report: West Pharmaceutical

Services, Inc. Dust Explosion.” CSB, Washington,

DC, September 2004.

3. Chemical Safety and Hazard Investigation Board

(CSB). “Investigation Report: CTA Acoustics, Inc.

Combustible Dust Fire and Explosions.” CSB,

Washington, DC, September 2004.

4. Chemical Safety and Hazard Investigation Board

(CSB). “CSB Investigators Find Likely Source of

Dust Explosion at Indiana Automotive Plant.” CSB

News Release, Washington, DC, November 5, 2003.

5. National Fire Protection Association (NFPA).

“Industrial Fire Hazards Handbook,” 3rd Edition.

NFPA, Inc., Quincy, MA, 1990.

6. Cross, J., and Farrer, D., “Dust Explosions,” New

York: Plenum Press, 1982.

7. Cashdollar, K.L., “Overview of Dust Explosibility

Characteristics,” Journal of Loss Prevention in the

Process Industries, v. 13, pp. 183-199, 2000.

8. Britton, L.G., Cashdollar, K.L, Fenlon, W., Frurip,

D., Going, J., Harrison, B.K., Niemeier, J., and Ural,

E.A., “The Role of ASTM E27 Methods in Hazard

Assessment Part II: Flammability and Ignitability,”

Process Safety Progress, v. 24, pp. 12-28, 2005.

General Information

FM Global, “Prevention and Mitigation of

Combustible Dust Explosions and Fire,” Data Sheet

No. 7-76, January 2005.

Eckhoff, Rolf K. “Dust Explosions in the Process

Industries,” 3rd Edition, Gulf Professional Publishing,

2003.

Bartknecht, W. “Dust Explosions: Course, Prevention,

and Protection,” Springer- Verlag, 1989.

Hatwig, M., and Steen, H. (eds.), “Handbook of

Explosion Prevention and Protection,” Wiley-VCH,

2004.

Frank, Walter. “Dust Explosion Prevention and the

Critical Importance of Housekeeping,” Process

Safety Progress, vol. 23, no. 3, September 2004, pp.

175-184.

7
Amyotte, P., Kahn, F., and Dastidar, A. “Reduce

Dust Explosions the Inherently Safer Way,” Chemical

Engineering Progress, vol. 99, no. 10, October 2003,

pp. 36-43.

Ebidat, Vahid. “Is Your Dust Collection System an

Explosion Hazard?” Chemical Engineering Progress,

vol. 99, no. 10, October 2003, pp. 44-49.

Center for Chemical Process Safety (CCPS).

“Guidelines for Safe Handling of Powders and Bulk

Solids.” CCPS, American Institute for Chemical

Process Safety, New York, New York, January

2005.

Sources of Additional Information

Note: This SHIB was developed using the latest

information and requirements from the references

below. Editions are not listed here, since users of this

document should refer to the most current editions.

Code of Federal Regulations (CFR) [Standards]

http://www.gpoaccess.gov

U.S. Government Printing Office

732 N. Capitol Street, NW

Washington, DC 20401

Telephone: (866) 512-1800 (toll-free)

OSHA Standards, Interpretations, and

Publications

http://www.osha.gov

U.S. Department of Labor/OSHA

OSHA Publications Office

200 Constitution Ave., NW, N-3101

Washington, DC 20210

Telephone: (202) 693-1888

or by Fax: (202) 693-2498

Related OSHA standards found in 29 CFR:

§1910.22 - General Requirements: Housekeeping·

§1910.38 - Emergency Action Plans

§1910.294 - Ventilation

§1910.107 - Spray Finishing Using Flammable and

Combustible Materials

§1910.146 - Permit-Required Confined Spaces

(references combustible dust)

§1910.178 - Powered Industrial Trucks

§1910.269 - Electric Power Generation,

Transmission and Distribution (coal handling)

§1910.272 - Grain Handling Facilities

§1910.307 - Hazardous (classified) Locations (for

electric equipment)

§1910.1200 - Hazard Communication

U.S. Chemical Safety and Hazard Investigation

Board

http://www.csb.gov

2175 K Street, NW, Suite 400

Washington, DC 20037-1809

Telephone: (202) 261-7600

Fax: (202) 261-7650

ASTM International

http://www.astm.org

100 Barr Harbor Drive

P. O. Box C700

West Conshohocken, PA 19428-2959

Telephone: (610) 832-9585

Fax: (610) 832-955

Related ASTM Standards:

• E789-95 Standard Test Method for Dust

Explosions in a 1.2-Litre Closed

Cylindrical Vessel

• E1226-00e1 Standard Test Method for

Pressure and Rate of Pressure Rise for

Combustible Dusts

• E1491-97 Standard Test Method for

Minimum Autoignition Temperature of

Dust Clouds

• E1515-03a Standard Test Method for

Minimum Explosible Concentration of

Combustible Dusts

• E2021-01 Standard Test Method for HotSurface Ignition Temperature of Dust

Layers

8
National Materials Advisory Board (NMAB)

Publications

National Academies Press

http://nap.edu

500 Fifth St., NW

Washington, DC 20001

Telephone: (202) 334-3313

Fax: (202) 334-2451

Related NMAB Publication:

• NMAB 353-4 (1982), Classification

of Dusts Relative to Electrical

Equipment in Class II Hazardous

Locations.

National Fire Protection Association (NFPA)

http://www.nfpa.org

1 Batterymarch Park

Quincy, MA 02169-7471

Telephone: (800) 344-3555

Related NFPA Standards:

• NFPA 61, Standard for the Prevention of

Fires and Dust Explosions in Agricultural

and Food Processing Facilities

• NFPA 68, Guide for Venting of

Deflagrations

• NFPA 69, Standard on Explosion

Prevention Systems

• NFPA 70, National Electrical Code®

• NFPA 91, Standard for Exhaust Systems

for Air Conveying of Vapors, Gases, Mists,

and Noncombustible Particulate Solids

• NFPA 120, Standard for Fire Prevention

and Control in Metal/Nonmetal Mining and

Metal Mineral Processing Facilities

• NFPA 432, Code for the Storage of

Organic Peroxide Formulations

• NFPA 480, Standard for the Storage,

Handling, and Processing of Magnesium

Solids and Powders

• NFPA 481, Standard for the Production,

Processing, Handling, and Storage of

Titanium

• NFPA 482, Standard for the Production,

Processing, Handling, and Storage of

Zirconium

• NFPA 484, Standard for Combustible

Metals, Metal Powders, and Metal Dusts

• NFPA 485, Standard for the Storage,

Handling, Processing, and Use of Lithium

Metal

• NFPA 495, Explosive Materials Code

• NFPA 499, Recommended Practice for the

Classification of Combustible Dusts and of

Hazardous (Classified) Locations for

Electrical Installations in Chemical Process

Areas

• NFPA 505, Fire Safety Standard for

Powered Industrial Trucks Including Type

Designations, Areas of Use, Conversions,

Maintenance, and Operation

• NFPA 560, Standard for the Storage,

Handling, and Use of Ethylene Oxide for

Sterilization and Fumigat

• NFPA 654, Standard for the Prevention of

Fire and Dust Explosions from the

Manufacturing, Processing, and Handling

of Combustible Particulate Solids

• NFPA 655, Standard for Prevention of

Sulfur Fires and Explosions

• NFPA 664, Standard for the Prevention of

Fires and Explosions in Wood Processing

and Woodworking Facilities

• NFPA 1124, Code for the Manufacture,

Transportation, Storage, and Retail Sales

of Fireworks and Pyrotechnic Articles

• NFPA 1125, Code for the Manufacture of

Model Rocket and High Power Rocket

Motors

International Code Council (ICC)

http://www.iccsafe.org

5203 Leesburg Pike, Suite 600

Falls Church, VA 22041

Telephone: 1-888-ICC-SAFE (422-7233)

Fax: (703) 379-1546

Related ICC Publication:

• International Fire Code

9