FAQ

A combustible dust is any material (finely chopped into small particles) which can spread in the air, ignite and initiate an explosion in the presence of an ignition source.

Combustible dust can be presented:

• Organic products (such as sugar, flour, grain, sawdust, etc.)
• with carbon-containing materials (charcoal, ash)
• textile fibres (cotton)
• most metals
• some non-metallic inorganic materials

Some of these materials are not normally flammable, but they can ignite or explode if the particles are the right size and the right concentration. Thus any activity that creates dust must be investigated to determine the risk of this dust becoming combustible. Dust accumulates on surfaces such as rafters, roofs, suspended ceilings, ducts, crevices, dust collectors and other equipment. When dust rises under certain circumstances, there is the potential for a serious explosion. An accumulation of even small amounts of dust can cause serious damage.

The technical definitions of combustible dust vary. For example, the Hazardous Goods Regulations (for the Workplace Hazardous Materials Information System (WHMIS) 2015) define combustible dust as “a mixture or substance in the form of fine particulate matter which, when ignited, can ignite or explode when dispersed in air”.

Another example is the Alberta Health and Safety Code, which defines combustible dust as “dust that can create an explosive atmosphere when suspended in the air in flammable concentrations”.

The Occupational Safety and Health Administration (OSHA) in the United States defines combustible dust as “a solid material consisting of individual particles or pieces, regardless of size, shape or chemical composition, which presents a fire or deflagration hazard when suspended in air or some other oxidising environment in a concentration range”.

The National Fire Protection Agency (NFPA) defines combustible dust as “a finely dispersed solid substance in the form of combustible particles that presents a risk of sudden ignition or Subscribe to DeepL Pro to edit this document. Visit www.DeepL.com/profor more information. explosion when suspended in air or a process-specific oxidising environment in a range of concentrations” in Standard 652-2019 “Combustible dust basics”.

Many materials can become a flammable substance under certain conditions. Workspaces can be equipped with such materials for use, or create dust during the work process. Examples of such materials are:

• agricultural products such as egg whites, milk powder, corn starch, sugar, flour, cereals, potatoes, rice, etc.
• metals such as aluminium, bronze, magnesium, zinc, etc.
• chemical dust such as coal, sulphur, etc.
• pharmaceutical products
• pesticide products
• rubber
• wood
• textiles
• plastics

There are many other types of materials that can become combustible dust. The US Food and Drug Administration has created a poster (link) that lists additional examples.

Dust explosions have occurred in many different types of workplaces and industries, including:

• Agriculture (e.g. fertiliser plants, composting plants, elevators, silos, etc.)
• Food production (e.g. sugar factories, bakeries)
• Chemical production (e.g. rubber, plastics, pharmaceuticals)
• Manufacture of fabric products
• Woodworkers
• Metalworking and production (e.g. zinc, magnesium, aluminium, iron)
• Recyclers (e.g. paper, plastics, metals)
• Coal mining, processing or coal-fired power plants

Dust is generated when materials are transported, processed, polished, ground and shaped. It is also generated by blasting, cutting, crushing, mixing, sieving or screening dry materials. The accumulation of dry residue from processing wet materials can also lead to dust formation. In fact, any workplace that generates dust is potentially at risk.

The main danger is that combustible dust can ignite, causing a sudden fire and causing an explosion.

A particular product or material may also cause health effects such as lung disease or cancer, which are not addressed in this document.

Three elements are needed for a fire. These elements are folded into a “fire triangle”.

1. Fuel for combustion
2. Oxygen
3. Source of ignition (heat, spark, etc.)
4. Dispersion of dust particles at the right concentration
5. Dust cloud isolation

Dispersion means that the dust particles are dispersed in the air. (It is worth noting that a rapid fire can happen without the fifth element of dust cloud isolation).
Isolation means that the dust is in a confined or enclosed space. This allows pressure to build up, increasing the likelihood of an explosion.

Deflagration is a term often used to describe explosions of combustible dust. Deflagration is a common fire such as in a gas cooker, burning wood or paper and even the combustion of petrol vapours inside a car cylinder. In deflagration, the burning substance releases heat, hot gases and particles or sparks that ignite and spread the fire.

When combustible dust ignites, there are usually 2 explosions, known as primary and secondary explosions. A primary dust explosion is the first explosion. It occurs when there are deposits of dust in an enclosed space (such as a container, room, equipment component containers) which ignite and explode.

The primary explosion picks up other accumulated dust. When this dust enters the air, it too is ignited. This secondary dust explosion is often more destructive than the primary explosion.

The main requirements for a dust explosion to occur are that the dust is in suspension and ignited. In practice, a number of conditions must be met for a dust explosion to occur, including:

• The dust must be combustible and release enough heat when burning to maintain the fire.
• The dust must be capable of forming a suspended mixture in the air.
• The size of the dust particles must allow the dust to spread fire.
• The concentration of the dust suspension must be within the explosive range.
• The source of ignition must be in contact with the dust mixture.
• The atmosphere must contain enough oxygen to sustain combustion.
• There is a form of restriction or guardrail that allows pressure to build up.

There are various aspects to be taken into account – dust particle size, method of dispersion, ventilation system characteristics, air flows, ignition sources, dust cloud localisation, physical barriers, etc.

To determine the level of risk from combustible dust, the risk assessment must be completed by a competent person (e.g. safety engineer, industrial/occupational hygienist, etc.). The risk assessment is a process which determines:

• whether the dust is flammable
• sources of combustible dust (e.g. materials and processes used in production methods)
• places where combustible dust forms (e.g. visible and concealed – equipment, beams, pipes, etc.)
• processes that allow dust to rise into the air and create a dust cloud
• The likelihood (possibility and extent) that the dust will present a sudden fire or explosion hazard
• presence of ignition sources (e.g. hot surfaces, flames, friction points, sparks, electrostatic discharges, etc.)
• the possibility of illness, injury or death in a dangerous situation as a result of a fire or explosion.
• The best practice is to keep the workplace as dust-free as possible.

Below are some questions that may help.

The process

• Do you produce or use materials (and their by-products) that can become dust?
• Do you have processes such as blasting, cutting, grinding, sieving, polishing, cleaning or other tasks that create dust?

Research/information

• Have you examined the dust present for flammability?
• Are there documented cases in the literature where materials in your workplace have been associated with a combustible dust explosion?

Sources of ignition

• Do you have sources of ignition (e.g. sparks, fire, flames, cookers, heat sources, cookers, welding flames)?
• Can dust enter or collect on the electrical enclosures of the equipment?
• Is there a smoking ban in your workplace?
• Are there measures in place to isolate sources of smoke and fire away from the workplace?

Cleaning

• Are you aware of any open spaces or above ground structures where dust can accumulate?
• Have you looked for hidden areas where dust can collect (e.g. behind suspended ceilings, inside ventilation or conveyor equipment, in ducts, on support beams, etc.)?
• Do you have cleaning activities to get rid of dust on a regular basis?
• Do you have a dust collection system?
• If so, does your dust extraction system meet combustible dust extraction standards and local requirements (e.g. fire regulations)?

Education and training

• Are employees aware of combustible dust and its dangers?
• Are staff adhering to cleaning rules and taking measures to reduce dust and remove sources of ignition?
• Have employees been trained in safe cleaning methods?

Once a hazard is identified, appropriate control measures must be implemented.

Remedy

• Use alternative products where possible that are non-combustible or less dangerous.
• Upgrade production processes to eliminate (or reduce) dust deposits (e.g. use of wet cutting, liquid ingredients, etc.).
• Design work surfaces to minimise dust accumulation (e.g. horizontal surfaces such as window sills, beams, light fittings, etc.).
• Eliminate “hidden” places where dust can accumulate unnoticed.
• Do not create dust when cleaning using brooms or compressed air hoses. Use only wet methods or hoovers approved for dust collection.

Replacement
Install smooth ceilings and other surfaces (instead of rough finishes) to minimise dust accumulation and make cleaning easier.

Engineering

• Use a dust collection or extraction system designed to remove or control combustible dust. Most types of fans tend to pick up dust, adding dust particles to the air and making the situation worse.
• Use a suitable dust extraction and collection system, with the inlet located as close as possible to the dust generation process. Follow the necessary standards and regulations when installing these systems. Place the dust collectors outdoors wherever possible.
• Set up the dust extraction process so that it can only work when the ventilation is working properly.
• Direct the blast ventilation away from areas where employees may be present.
• Use suitable electrical equipment and ventilation.
• Try to keep all mechanical and electrical equipment in good condition.
• Keep static electricity under control, which means connecting and earthing the equipment. Check all connected and earthed equipment regularly to ensure that the connections are in good condition.
• Control sources of ignition, such as conductive connection and earthing equipment.
• Check equipment that may wear out (e.g. bearings) as they can generate heat and become a source of ignition.
• Remove open flames, sparks, friction, heat sources and other ignition sources.
• Select and use intrinsically safe tools or equipment.
• Put covers over pipes and cables or, if possible, embed the pipes and cables in the walls to reduce the number of surfaces where dust can accumulate.

Administration

• Develop and implement a combustible dust inspection programme which describes how often inspections will be carried out and how dust will be controlled. Include inspection of machines, ducts and ventilation systems for dust.
• Develop a hot work permit system for activities such as welding and cutting.
• Develop an ignition control programme to eliminate or reduce ignition sources. Keep ignition sources away from dusty areas or use suitable controls.
• Develop safe storage practices for combustible dust.
• Ensure that fire and explosion protection equipment is installed and used in accordance with applicable standards or legislation.
• Develop a fire safety plan (link).
• Make regular checks for dust.
• Set a cleaning programme that properly and regularly removes dust.
• Use proper equipment and methods when dusting. Care must be taken to minimise dust clouds and only use hoovers approved for dust collection.
• Check the machines, air ducts and ventilation systems regularly for dust. Have it repaired or cleaned quickly.
• Inform all employees about combustible dust, the hazards and how they can help eliminate the risk of fire and explosions.

Employees shall:

• Receive training from their employer on how to identify combustible dust materials, how to handle combustible dust safely and how to follow emergency procedures.
• Familiarise yourself with the hazards of the material and the work area before starting work.
• Never bring ignition sources into an area where combustible dust is handled or processed.
• Wear all necessary personal protective equipment (PPE).
• Immediately report hazardous conditions that can lead to ignition of combustible dust, such as:
  • combustible dust leaks;
  • any problems with the process or equipment failure;
  • presence of sparks in the combustible dust area.