Combustible Dusts | DSEAR

How to Assess Combustible Dusts as Part of Your DSEAR Risk Assessment

Combustible dusts present a significant but often underestimated explosion and fire risk in many workplaces, including manufacturing, processing, storage, food production, woodworking, metalworking, pharmaceuticals, and recycling operations.

Under the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR), employers must identify, assess, and control the risks arising from combustible dusts wherever they may form explosive atmospheres or fire hazards.

This article explains how combustible dusts are assessed within a DSEAR risk assessment, how hazardous areas are classified, and why dust layers are just as important as airborne dust clouds.

Understanding Combustible Dust Hazards

Combustible dust is any finely divided solid material that can burn rapidly or explode when dispersed in air and exposed to an ignition source. Common examples include wood dust, flour, sugar, plastics, rubber, coal, pharmaceuticals, and many metals.

A key misconception is that only visible dust clouds are dangerous. In reality, combustible dust hazards arise from both airborne dust clouds and settled dust layers, and these risks can develop gradually over time.

The following principles are fundamental to understanding combustible dust risks and must be considered as part of any DSEAR assessment:

Explosive dust atmospheres occur only when airborne dust concentrations fall within a specific explosive range. While a dust cloud with an extremely high concentration may not be explosive, there remains a significant risk that, if the concentration reduces, it could pass into the explosive range. Not every dust release will automatically result in an explosive atmosphere; this depends on the nature of the release and the surrounding conditions.

Dust clouds are rarely evenly distributed, and variations in concentration can exist within the same cloud. These potential differences must be considered for all operating and release scenarios. Where dust is not effectively removed by local exhaust ventilation or other extraction systems, it will settle over time, forming layers or accumulations. The rate at which dust settles depends on factors such as particle size and material properties.

It is important to recognise that even a low-level or dilute continuous release can, over time, lead to the formation of a hazardous dust layer. Combustible dust hazards therefore arise in two main ways: the creation of an explosive dust cloud from any source of release, including the disturbance of settled dust layers; and the formation of dust layers that may not readily become airborne but can ignite through self-heating, contact with hot surfaces, or exposure to thermal radiation, leading to fire or equipment overheating. Once ignited, a dust layer may also act as an ignition source for an explosive dust atmosphere.

Because both explosive dust clouds and hazardous dust layers may be present, all potential ignition sources should be eliminated wherever practicable. Where ignition sources cannot be completely avoided, suitable measures must be implemented to minimise the presence of dust and/or ignition sources, such that the likelihood of both occurring at the same time is reduced to a level where the risk is considered negligible.

Area Classification for Explosive Dust Atmospheres

Area classification is the structured process used to identify where and how often explosive dust atmospheres may occur. It forms the foundation for selecting suitable equipment, managing ignition sources, and implementing control measures under DSEAR.

Area classification for dust atmospheres is not a tick-box exercise. It requires informed judgement, engineering knowledge, and an understanding of how the process operates in practice.

Key Factors Considered During Area Classification

The assessment takes into account a range of interrelated factors, including:

Dust combustibility and ignition characteristics may be established through laboratory testing, published data, or specialist advice. However, published data must always be treated with caution and validated for the specific material and application, as values can vary significantly.

The Dust Area Classification Procedure

Under DSEAR, areas where combustible dusts may be present are classified into Zones 20, 21 or 22, based on the likelihood and duration of an explosive dust atmosphere.

Zone 20 is an area in which an explosive dust cloud is present continuously, for long periods, or frequently, such as inside silos, filters, or dust handling equipment.

Zone 21 applies where an explosive dust cloud is likely to occur occasionally during normal operation, for example, near filling points, bag emptying stations, or transfer locations.

Zone 22 is an area where an explosive dust cloud is not expected during normal operation; if it does occur, it will persist only for a short time, often due to abnormal conditions such as equipment failure or poor housekeeping.

Although these zone definitions relate specifically to airborne dust clouds, areas where dust layers may accumulate and could be disturbed to form a cloud must also be considered as part of the area classification.

The area classification process for combustible dust atmospheres follows a structured approach:

Step 1 – Identify the Dust and Its Characteristics

The first stage is to confirm whether the material is combustible and determine its relevant ignition and explosion properties. This includes consideration of particle size, moisture content, minimum ignition temperatures for both dust clouds and dust layers, and electrical resistivity.

Based on these characteristics, the dust is assigned to the appropriate dust group:

Understanding the dust group is essential for equipment selection and ignition source control.

Step 2 – Identify Sources of Dust Release and Accumulation

The next step is to identify where explosive dust mixtures may be present or where dust may be released during normal operation, maintenance, or foreseeable abnormal conditions.

This typically involves reviewing:

The potential for dust layers to form and later become airborne must always be considered.

Step 3 – Assess Likelihood and Define Zones

The final step is to evaluate how likely it is that dust will be released and form an explosive atmosphere in different parts of the installation. This includes considering both frequency and duration of releases.

Only after this evaluation can dust zones be defined and their boundaries established. These decisions should be recorded on an area classification drawing and supported by written justification.

The documented zone information is then used as the basis for:

Documentation, Review, and Ongoing Management

All decisions made during the area classification process should be clearly recorded, including the reasoning behind zone selection and extent. This ensures transparency and supports future reviews.

Area classification must be reviewed:

Combustible dust risks are not static. Changes in materials, production rates, housekeeping standards, or maintenance practices can all significantly alter the risk profile.

Summary

Assessing combustible dusts under DSEAR requires more than identifying visible dust clouds. A robust assessment considers:

By following a structured area classification process and maintaining effective housekeeping and review arrangements, duty-holders can significantly reduce the risk of fires and explosions caused by combustible dusts.

If you would like help carrying out a combustible dust DSEAR assessment or reviewing an existing area classification, we offer specialist support.

What we offer

Combustible dusts present a significant but often underestimated explosion and fire risk in many workplaces, including manufacturing, processing, storage, food production, woodworking, metalworking, pharmaceuticals, and recycling operations.

Under the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR), employers must identify, assess, and control the risks arising from combustible dusts wherever they may form explosive atmospheres or fire hazards.

​

This article explains how combustible dusts are assessed within a DSEAR risk assessment, how hazardous areas are classified, and why dust layers are just as important as airborne dust clouds.

​

Understanding Combustible Dust Hazards

​

Combustible dust is any finely divided solid material that can burn rapidly or explode when dispersed in air and exposed to an ignition source. Common examples include wood dust, flour, sugar, plastics, rubber, coal, pharmaceuticals, and many metals.

​

A key misconception is that only visible dust clouds are dangerous. In reality, combustible dust hazards arise from both airborne dust clouds and settled dust layers, and these risks can develop gradually over time.

​

The following principles are fundamental to understanding combustible dust risks and must be considered as part of any DSEAR assessment:

​

​

​

​

​

Area Classification for Explosive Dust Atmospheres

​

Area classification is the structured process used to identify where and how often explosive dust atmospheres may occur. It forms the foundation for selecting suitable equipment, managing ignition sources, and implementing control measures under DSEAR.

​

Area classification for dust atmospheres is not a tick-box exercise. It requires informed judgement, engineering knowledge, and an understanding of how the process operates in practice.

​

Key Factors Considered During Area Classification

​

The assessment takes into account a range of interrelated factors, including:

​

Whether the dust present is combustible

The physical and ignition characteristics of the dust

How, where, and how often dust is released

Process design and operating conditions

Housekeeping and cleaning arrangements

Maintenance activities and foreseeable failures

The presence of other equipment that could act as an ignition source

​

Dust combustibility and ignition characteristics may be established through laboratory testing, published data, or specialist advice. However, published data must always be treated with caution and validated for the specific material and application, as values can vary significantly.

​

The Dust Area Classification Procedure

​

Under DSEAR, areas where combustible dusts may be present are classified into Zones 20, 21 or 22, based on the likelihood and duration of an explosive dust atmosphere.

Zone 20 is an area in which an explosive dust cloud is present continuously, for long periods, or frequently, such as inside silos, filters, or dust handling equipment.

Zone 21 applies where an explosive dust cloud is likely to occur occasionally during normal operation, for example, near filling points, bag emptying stations, or transfer locations.

Zone 22 is an area where an explosive dust cloud is not expected during normal operation; if it does occur, it will persist only for a short time, often due to abnormal conditions such as equipment failure or poor housekeeping.

Although these zone definitions relate specifically to airborne dust clouds, areas where dust layers may accumulate and could be disturbed to form a cloud must also be considered as part of the area classification.

​

The area classification process for combustible dust atmospheres follows a structured approach:

​

Step 1 – Identify the Dust and Its Characteristics

​

The first stage is to confirm whether the material is combustible and determine its relevant ignition and explosion properties. This includes consideration of particle size, moisture content, minimum ignition temperatures for both dust clouds and dust layers, and electrical resistivity.

​

Based on these characteristics, the dust is assigned to the appropriate dust group:

​

Group IIIA – combustible flyings

Group IIIB – non-conductive combustible dusts

Group IIIC – conductive combustible dusts

​

Understanding the dust group is essential for equipment selection and ignition source control.

​

Step 2 – Identify Sources of Dust Release and Accumulation

​

The next step is to identify where explosive dust mixtures may be present or where dust may be released during normal operation, maintenance, or foreseeable abnormal conditions.

​

This typically involves reviewing:

​

Process flow diagrams

Plant and equipment layouts

Transfer points, conveyors, mixers, silos, filters, and packaging areas

Locations where dust layers may accumulate, such as ledges, beams, cable trays, and equipment surfaces

​

The potential for dust layers to form and later become airborne must always be considered.

​

Step 3 – Assess Likelihood and Define Zones

​

The final step is to evaluate how likely it is that dust will be released and form an explosive atmosphere in different parts of the installation. This includes considering both frequency and duration of releases.

​

Only after this evaluation can dust zones be defined and their boundaries established. These decisions should be recorded on an area classification drawing and supported by written justification.

​

The documented zone information is then used as the basis for:

Ignition source assessment