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Hazard

Combustible dust

Hazard Knowledge

The Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) include the definition of a dangerous substance as:

‘any dust, whether in the form of solid particles or fibrous materials or otherwise, which can form an explosive mixture with air or an explosive atmosphere’

A dust explosion can occur when suspended, solid, combustible particles are ignited, potentially releasing enormous amounts of energy. Increasing the surface area of a combustible solid enhances the ease of ignition, resulting in the dust burning more rapidly than the corresponding bulk solid.

Suspended dust particles behave in a similar way to gases and a flammable dust and air mixture can form within certain limits. The most violent explosions usually result from dust and air mixtures that are fuel-rich. This means that the oxygen available in the air cannot burn all the dust and partly burnt, glowing material often remains after the explosion. This can reignite if more air becomes available. The shape and size of the dust particles, and other factors, strongly affects the force of the explosion and the explosive limits. Only weak explosions are likely where the mean particle size of the dust exceeds 200 microns or the moisture content exceeds 16%.

Secondary dust explosions may occur when the blast wave from a primary explosion entrains dust layers already present, creating a large dust and air combustible mixture that is ignited by the first explosion.

Incident commanders should be aware that dust can collect in structures and on surfaces such as rafters, roofs, suspended ceilings, ducts, crevices, dust collectors and other equipment. Fire and rescue service activities such as using jets and tactical ventilation can create or disturb fine dusts or powders that may be present in a range of situations.

Refer to the Health and Safety Executive (HSE) publication Safe handling of combustible dusts: Precautions against explosions for further information.