Dust can harm employees’ health, reduce visibility, damage equipment and annoy neighbours
Dust is an almost inevitable consequence of mineral working. But, unlike other environmental impacts, there are no comprehensive acceptability limits for dust. This is largely due to inadequacies in dust definitions, notably for larger-sized particles, and has implications for effective monitoring of emissions.
Within the site, dust is largely a health-and-safety issue for the operator. Excessive dust can be hazardous when inhaled or by obscuring visibility. Dust is also costly, clogging filters and leading to excessive energy use. Beyond the site boundary, it can exceed specified local authority air quality objectives and cause annoyance or nuisance to local residents and other commercial activities as well as impairing plant growth.
From a physical perspective, BS 6069 Part 2 defines dust as particulate matter up to 75µm diameter. In practice, larger sizes are also of practical interest but are not well specified. Many dust impacts, especially for the minerals industry, are related to physical, chemical and morphological properties of particles, rather than size. Density, colour and reflectivity are key parameters that influence dust mass and its visual impact.
Excessive exposure to smaller particles is associated with harm to health (through inhalation). In 1996, the 3rd report of the Quality of Urban Air Review Group (QUARG) noted that whilst particles up to 100µm enter the body through breathing, only those below about 5µm can reach deep into the lung. Health-related sampling conventions for fine dust particles have been developed and, for example, the ISO thoracic fraction is essentially those particles with a median aerodynamic diameter 10µm. The US EPA thoracic convention has been widely adopted and PM10 is a familiar expression for "particles essentially up to 10µm diameter". Increasing emphasis is being placed on yet smaller particles, notably PM2.5.
National and international limit values for acceptable concentrations of PM10 and other fractions of small particles have been defined and agreed. In the UK, these apply to both public and workplace exposures. The impacts of finer-sized dust particles have been regulated in relation to public health through national air quality objectives (AQO) and the workplace health and safety regime.
In the planning system, a clear assessment process for PM10 impacts from proposed minerals sites has been established (Minerals Policy Statement 2, Annex 1: Dust). Only where the impacts of PM10 are sufficiently great, after mitigation measures are taken into account and where PM10 concentrations are likely to exceed AQO levels, should an application to develop a minerals operation be refused. However, it seems likely that under government proposals to discard much planning guidance this explicit advice may soon be lost.
Compliance with limit values for the smaller fractions of dust is relatively straightforward to determine. Although there is a range of instrumentation and methodologies for monitoring PM10, the end result is more-or-less the same. Limit values for finer dust particles are given as mass concentrations over set periods.
The problems of coarse dust
However, the position is less clear for coarser particles. While there might be minimal epidemiological effects from coarse dust, its social, environmental and financial impacts can be considerable through soiling and potential effects on crops, ecosystems and high-technology equipment.
The coarser fraction of dust is harder to define but is often referred to as "nuisance dust". Unfortunately, this term can be misleading because "nuisance" has a specific meaning in environmental law while "nuisance dust" is usually taken to mean "generally visible particulate matter". Consequently, some practitioners prefer to use a term like "visible dust" because, in practice, such material can cause annoyance or loss of amenity (rather than necessarily constituting a statutory nuisance). The visual impacts of dust are experienced as both acute and chronic phenomena: from short-term dust clouds to long-term surface soiling.
Although it is unhelpful to consider visible dust exclusively in terms of particle size, it is generally accepted that this fraction is likely largely to comprise particles over 10µm. The 1995 Arup Environmental report, The Environmental Effects of Dust from Surface Minerals Workings, stated that the greatest proportion of dust from minerals workings was over 30µm and will largely deposit within 100m of sources. More recent guidance, published in 2010 by AEA Technology (Management, Mitigation and Monitoring of Nuisance Dust and PM10 Emissions Arising from the Extractive Industries: an overview), notes that assessment for potential impacts from PM10 emissions from minerals sites may also need consideration.
Therefore, since PM10 makes up a small proportion of dust at minerals sites and is relatively straightforward to assess, concerns over dust at minerals sites should focus on the visual impacts, loss of amenity and possible nuisance complaints that are more likely to arise from coarser fractions. Good site design and management are essential in keeping dust at acceptable levels. The challenge is to establish what is acceptable.
As noted above, PM10 concentrations can be measured by a variety of techniques but with reasonably comparable results. That is not the case for visible dust. Data obtained from one method are rarely comparable with another and, more importantly, there are no universal acceptance thresholds.
PM10 sampling methods are usually active, involving some form of powered device drawing ambient air through a measurement cell or a filter. Visible dust sampling methods are usually passive, allowing dust to accumulate on or in a sampler over periods of days or weeks.
Many of the methods currently used for visible dust monitoring are derived from approaches developed under Her Majesty’s Inspectorate of Pollution’s (HMIP) regime. In essence, these focus on dust mass or dust soiling. The dust mass approach involves collecting dust in some form of open container over a period of time, and weighing it; the dust soiling approach involves allowing a surface to be discoloured by dust and measuring the obscuration and/or loss of reflectance caused by discolouration.
Dust flux and dust deposition
Passive dust samplers collect dust in flux (directional sampling) or in settlement (depositional sampling). Dust flux is the rate of dust travel from source to receptor; dust deposition its settlement at the receptor. Consequently, dust in flux is moving (usually driven by air currents) and deposited dust has stopped moving. The distinction between dust flux and dust deposition is frequently misunderstood.
From the "source-pathway-receptor" model of pollutant dispersion, directional (dust flux) sampling methods are more appropriate for monitoring on the pathway between source and receptor, and deposition methods for receptor (or proxy-receptor) monitoring. This has implications for dust sampling strategies and sampler location.
It follows that directional samplers are generally best placed at or near the site boundary, to assess dust flux between the suspected source, or sources, and potential receptors. Likewise, deposition samplers are best located at, or near, (by definition) off-site receptors. However, this raises questions about security and the risk of tampering or vandalism: if a deposition sampler is located in a "hostile" garden, will dust be artificially introduced? If it is placed in a "friendly" garden, will it be removed?
Methods have been developed to assess dust flux or dust deposition, either by mass or by soiling. For both of these approaches, custom-and-practice acceptance criteria have been developed and are widely applied. One commonly-applied dust mass criterion, derived from HMIP data and calibrated for the Frisbee-type gauge in 1998, is that 200mg/m2/day is a threshold for "complaints likely". Less common, however, is the necessary qualification for that criterion: "Residential areas and outskirts of towns". In other words, the threshold applies to dust fall at the receptor.
Consequently, on many occasions deposition samplers have been located within the site boundary and yet limit values applicable to an off-site receptor have been applied. Since the majority of dust particles from minerals sites are relatively coarse and unlikely to travel great distances, this means that unnecessary constraints can be placed on minerals operators, through inappropriate use of dust sampling equipment.
For soiling measurements, a limit of acceptability for sticky pad samplers of 0.5 per cent effective area coverage (EAC)/day is said to be a threshold for "possible complaints" and 5.0 per cent EAC/day for "serious complaints". For glass slides, 20–25 soiling units per week (averaged over four weeks) is the boundary of acceptability.
However, all these criteria were not necessarily calibrated thoroughly and have not been reviewed recently. As sampling techniques have evolved, criteria developed for one method have been adapted for another. Criteria and limit values have sometimes been based on an insufficient knowledge of sampler characteristics.
In particular, unless calibrated for the locality (to take differences in dust density and visibility into account), mass-based criteria are unlikely properly to describe the visual impact of dust. Work is needed on the definition of criteria and limit values, and sampling practices for coarse dust fractions, followed by discussion of best practices for securing effective monitoring that is proportionate and fit for purpose.
It is arguable that with social changes and environmental controls since the 1960s, the public perception of nuisance dust has changed and needs to be recalibrated by reviewing existing empirical thresholds for the range of directional and depositional dust monitoring devices now in use. This is a social sciences issue as much as it is a technical matter.
Hugh Datson is senior environmental scientist at DustScan and Brian Marker is an independent consultant. The authors thank Professor Geoffrey Walton for his help in the production of this article.
