Dust
Definitions
Dust is often used in a generic sense to describe particles which are capable of becoming airborne to disperse in the atmosphere prior to returning to surface. However, there are a number of definitions which are more specific.
Most particles are not an even shape, so the sizes will be general approximations. Although parts of the minerals industry refer to dust as all material under 2mm in size, BS 6069-2 166 defines dust as particulate matter in the size range of 1 - 75μm in diameter, with particles less than 1μm being classified as smoke or fumes. Dust particles will eventually settle out under their own weight, but may remain suspended for some time. Particles less than 10μm in diameter (known as PM10) are often inhalable, while particles less than 2.5μm are considered respirable (PM2.5). Therefore, anything less than 10μm can cause concern with health effects, while particles greater than 10μm are associated with public perception and nuisance. Further detailed information can be found in Section 2.1.2 of the ALSF project "Cleaner Quarries: Optimising Environmental Performance" L0076.
The Environment Agency 167 states that particles greater than 30 - 50μm tend to be deposited quickly and as such this particle size approximates to annoyance, or nuisance dust. Such settled particles may show up as a deposit on clean surfaces such as cars and window ledges.
Dust can be emitted from a stack as a plume, lifted by the tyres of a truck or simply picked up by the wind from the ground, a road or a stockpile. It can also be ejected into the air during blasting (Photograph 3) or released as the rock mass collapses (Photograph 4). Particles can be chemically active or inert, depending on their chemical composition (e.g. in environmental terms, limestone is active and sand is inert). The colour of dust varies widely from black, through brown, to white, e.g. cement or chalk dust.
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Photograph 3. Dust and fumes emitted from a blast near a test house |
Photograph 4. Dust emitted from collapsing rock pile, but likely to stay within this deep quarry |
Dusts are normally present in the atmosphere, at varying levels of concentration and can have a wide variety of man-made and natural origins. They can be products of combustion (e.g. fires, power stations and motor vehicles) and other industrial activities, or naturally occurring from volcanic action, desert storms or other geological activities (not a major source in the UK).
It is therefore important to recognise that there are a number of sources of dust which will not be connected to a mineral working. These will not always be readily distinguishable from site dust and so may give rise to unwarranted complaints.
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