Last edit: 14/08/2026
There are several myths about how dangerous combustible dust can be. Here are a few of them:
- Dust explosions happen only in coal mines and grain elevators.
- A large amount of dust is needed to cause an explosion.
- Gas explosions are always much more severe than dust explosions).
- Removing oxygen automatically makes the situation safe.
- There is no problem if dust is not visibly suspended in the air.
I agree: these are indeed myths. However, as a colleague of mine told me many years ago, “If all the dusts described as dangerous, in the risk assessments I read, were really that hazardous, NASA would have chosen to fly to the Moon with a dust-propelled rocket!”
The point behind the joke is that, although combustible dust hazards are very real, there is also a tendency to overestimate the risk since assessments are usually based on qualitative criteria. There is extensive literature on dust explosions, and much of it is supported by solid experimental research. NFPA has recently published an excellent standard dealing comprehensively with the subject:
NFPA 660:2025 – Standard for Combustible Dusts and Particulate Solids
NFPA 660 is a consolidated standard that brings together requirements previously contained in several NFPA standards dealing with combustible dust, including NFPA 652 – Standard on the Fundamentals of Combustible Dust, as well as a number of industry- and material-specific standards.
When assessing the risk of explosion caused, for example, by a gas release from a furnace gas skid, technical standards provide relatively well-defined quantitative methods. Release rates can be calculated using equations derived from fluid mechanics and thermodynamics.
With dust, the situation is often quite different. The assessment frequently relies much more heavily on qualitative criteria and engineering judgement.
One often-cited example is that a relatively thin layer of combustible dust—sometimes described as a layer thick enough to write your name in with a finger—may represent a significant hazard if it becomes dispersed into the air. Criteria of this kind are useful for identifying potentially hazardous situations. However, when applied without sufficient consideration of the actual process conditions, dust characteristics, quantities involved, likelihood of dispersion, and ignition sources, they can lead practitioners to overestimate the actual explosion risk.
That said, the basic mechanism of a dust explosion is quite easy to understand from everyday experience.
Any solid material capable of burning will generally burn faster as its degree of fragmentation increases. A piece of wood, once ignited, burns relatively slowly, releasing its energy over a comparatively long period of time. If the same wood is divided into small pieces, combustion becomes faster because the total surface area exposed to the air increases.
If fragmentation continues until very small particles are produced—typically fractions of a millimetre—and these particles are dispersed in air at a suitable concentration, the available surface area becomes extremely large. Combustion can then occur very rapidly, and the ignition energy required may become very small.
If this rapid combustion of a suspended dust cloud causes a sufficiently fast pressure rise, the result is a dust deflagration and, particularly when confinement is present, potentially a dust explosion.
Materials capable of giving rise to combustible dust explosions include:
- Natural organic materials: flour, wood, textiles, sugar, etc.
- Synthetic organic materials: plastics, organic pigments, pesticides, pharmaceutical compounds, etc.
- Coal and peat
- Metals: aluminium, magnesium, titanium, zinc, iron, etc.
The density of the powder for the blend to be explosive
Fig. 7.2 shows the explosive concentration range for a typical natural organic dust, such as cornflour, dispersed in air at room temperature and atmospheric pressure. This range is relatively narrow, extending over less than two orders of magnitude, approximately from 100 g/m³ to 2 kg/m³.
The explosive limits vary significantly from one dust to another. For example, zinc dust may have a minimum explosive concentration of approximately 500 g/m³.
Explosible dust clouds have a very high optical density, even at concentrations close to the lower explosive limit. This is illustrated in Fig. 7.2 by comparing the explosive concentration range with the much lower dust concentration limits typically associated with occupational exposure and workplace hygiene. These permissible airborne dust concentrations are generally three to four orders of magnitude lower than the minimum concentration required for a dust explosion.
In other words, the dust concentrations that may already be considered excessive or unacceptable in a workplace—and therefore subject to occupational hygiene controls—are far below the concentrations normally required to form an explosible dust cloud.
This leads to a useful general guideline when assessing the likelihood of an explosive atmosphere: a gas explosive atmosphere is often expected outside the containing pipe or vessel, following a release, whereas a dust explosive atmosphere is more commonly expected inside process equipment, where sufficiently high dust concentrations can readily occur. This should, of course, be regarded as a general principle rather than an absolute rule, since explosive dust clouds can also form outside equipment as a result of releases, deposits being disturbed, or secondary dispersion.
Consequently, the minimum explosible concentration (MEC)—which may be regarded as the mass concentration corresponding approximately to the lower explosive limit for dust—already represents a dust cloud of very high optical density. Such concentrations are therefore unlikely to occur continuously or routinely in normally occupied workplace areas.
Fig. 7.3 illustrates the high optical density associated with explosible dust clouds, based on a rule of thumb attributed to Intelmann and quoted by Zehr (1965):
“Consider a 25 W light bulb viewed from a distance of 2 m through the dust cloud. If the bulb is no longer visible, the dust concentration exceeds approximately 40 g/m³.”
This concentration is already of the same order of magnitude as, although still below, the MEC of many combustible dusts.
