‘Dust’ is generally regarded as particulate matter up to 75 µm (micron) diameter and can be considered in two categories. Fine dust, essentially particles up to 10 µm, is commonly referred to as PM10. Coarser dust (essentially particles greater than 10 µm) is generally regarded as ‘nuisance dust’ and can be associated with annoyance.
PM10 is measured to agreed standards and forms part of National Air Quality Strategy (NAQS) objectives (AQO) and comprises what is defined as health risk dust. There are no official standards (such as AQO) for dust annoyance or 'nuisance dust'.
The expression ‘nuisance dust’ relates to the human perception of, or reaction to, some aspect of dust pollution, such as the long-term, chronic, soiling of surfaces or the visibility of acute, short-lived, dust clouds. In the absence of standards, ‘custom and practice’ criteria for assessing nuisance dust have been developed.
Dust sources and generation
Although it is a widespread environmental phenomenon, dust is also generated through many human activities. This includes activities such as materials handling operations, vehicle and machinery movements at the sites of quarries and mines, heavy industry, waste and recycling, construction and demolition, power generation, agriculture (especially arable farming) docks, harbours and roads and railways.
Dust is generally produced by mechanical action on materials and is carried by moving air when there is sufficient energy in the airstream. More energy is required for dust to become airborne than for it to remain suspended. Dust is removed through gravitational settling (sedimentation), washout (for example during rainfall or by wetting) and by impaction on surfaces (e.g. on vegetative screening). Dust can be re-suspended where conditions allow, such as from bare ground.
Dust propagation through air is influenced by many factors including particle size, wind energy and disturbance activities. Large dust particles generally travel shorter distances than small particles. It is often considered that particles greater than 30 µm will largely deposit within 100 metres of sources, those between 10 – 30 µm to travel up to 250 – 500 metres and particles less than 10 µm to travel as far as 1 km or more from sources. These distances may be exceeded.
When propagated, dust travels on a pathway towards its final resting place which may arise from settlement, washout or impaction. This final location is the receptor; clearly certain locations are more important and sensitive than others. Table 7 of the National Planning Policy Framework (NPPF) Technical Guidance, identifies broad categories of high, medium and low dust sensitive facilities. The most sensitive receptors are hospitals and clinics, high-tech industries, painting and furnishing and food processing facilities.