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An effective approach to ambient dust monitoring in the UK
With development projects occurring in close proximity to sensitive receptors, such as residential housing, there is an increasing need to monitor the levels and impacts of dust.
What is dust?
Dust is generally understood to be an aerosol of solid particles, mechanically produced, with individual particle diameters of 0.1?m upwards. It can be a problem in almost any industry, from bakeries to building sites. Dependent upon the nature of the industry, the dust generated may be confined to the workplace e.g. in a bakery, or may be released in to the atmosphere and surrounding environment e.g. from a construction site.
Occupational exposure to workplace dust
The hazards from dust that are generated within the confines of the workplace can range from high lead content dusts to those that contain silica or cotton. There are many other substances which may be present in dust, dependent upon the particular process, which may be potentially hazardous to health. It is recognised that exposure to any dust in excessive amounts can create respiratory problems. The COSHH Regulations apply to ‘substances hazardous to health’ and these include dusts of any kind, when present in a substantial concentration in air.
The HSE state that employers should not carry out any work which may expose their employees to dusts unless:
- They have made a suitable and sufficient assessment of the risks to health from the dust(s) concerned and
- They have taken the necessary precautions before the work begins, to meet the other requirements of the COSHH Regulations
Many substances which can create dust have been given occupational exposure limits (OELs) under COSHH. Previously these OELs were set as a combination of Occupational Exposure Standards (OESs) and Maximum Exposure Limits (MELs). These have recently been amended in favour of Workplace Exposure Limits (WELs). A full list of these limits is produced in the HSE Guidance Note, EH40/2005.
Sampling for many different types of dust is generally undertaken in accordance with HSE booklet ‘General methods for sampling and gravimetric analysis of respirable and total inhalable dust (MDHS14/3)’. Two different types of sampling are commonly used:
- Personal sampling:- A sampler (sample head and filter) is attached to the operative to be monitored within his or her breathing zone, and the pump is connected to it by a length of flexible tubing and worn on a belt, harness, or in a pocket. Personal sampling instruments are normally mounted on the upper chest, close to the collar-bone
- Background Sampling:- Fixed point sampling may be used to determine background levels of dust in the workplace. The samplers should be mounted at approximately head height, away from obstructions, fresh air inlets or strong winds
The choice of filter type and filter media is dependent upon the specific type of inhalable or respirable dust that is to be collected. Once dust samples have been collected on to pre-weighed filters, they are reweighed to determine the mass of dust collected for a known volume of air. If required, the filters can also be submitted to a suitably accredited laboratory for analysis of particular substances in the dust sample such as heavy metals. The results can then be compared to the WELs outlined in EH40/2005.
WELs are set for long term exposure with an 8-hour time weighted average (TWA) reference period in both parts per million (ppm) and milligrams per cubic metre (mg.m-3) as well as a short term exposure limit over a 15 minute reference period. Limits for both inhalable and respirable dust are only given for an 8-hour TWA and are set at 10 and 4mg.m-3 respectively.
Nuisance dust
Nuisance dust can be described as that which is deposited from the atmosphere onto surfaces, such as land, buildings and property. Nuisance dust is generated by a wide range of anthropogenic activities, such as traffic, construction/demolition activities, mineral workings and general industry. It is also possible for dust from the atmosphere to be the result of natural causes, such as wind blown dust from arid environments or volcanic activity. Although the UK would not currently be classified as an arid environment it is possible for dust from arid lands to be carried long distances on the prevailing wind and subsequently deposited within other countries.
COSHH provides the statutory framework for the determination of dust that is generated or present within the workplace. However, there are currently no statutory or legislative standards that relate to the presence or production of dust in the wider environment. The development of land within the UK generally requires a risk-based approach to quantifying and managing potential pollution, harm to human health and detriment to the amenity. The impacts of a development are often assessed as part of an Environmental Impact Assessment (EIA), which will include potential impacts of increased dust generation.
With both urban and rural development there is a possibility that any dust generated will impact on “neighbours” to a site, whether residential premises or commercial offices. In order to quantify and assess the impacts of a site development, it is necessary to monitor dust levels within the vicinity of a development site. In our experience, the requirement for dust monitoring is now commonly being attached to planning conditions issued by local authority planning departments.
Such requirements are in place to protect those who may live or work in the vicinity of a site from an increase in the deposition of dust caused by activities associated with development. It is therefore necessary to quantify the amount of dust deposition. Generally, two different approaches are used to measure the amount of dust deposited on a surface;
- Determination of the soiling of a surface, by a change in its reflective properties
- Determination of the quantity of dust deposited, by weight
There is a range of equipment available for determining levels of ambient dust. For sites which are to be operational for a long period of time it is probably cost-effective to install a real-time particulate monitor. However, the deposit gauges provide a simple and yet accurate method for determining ambient dust deposition in the environment.
Deposit gauges
Dust is collected onto a horizontally mounted capture container, or in the case of the Directional Dust Gauge, into four vertical tubes aligned in different directions.
Dust samples collected are measured by washing the collecting container (and foam insert in the case of the Frisbee Gauge) through to a collecting bottle. The sample is passed through a pre-weighed filter, conditioned and re-weighed. The quantity collected over a known area and period can be calculated to give a deposition rate per unit area. It is also possible to look at the materials retained on the filter to make a determination as to the content of the dust or to subject the material to chemical analysis. Collection methods and devices include;
- The Frisbee Gauge – A dust sample is collected in a large capacity collection bottle at the base of the unit. Gauges are fitted with a stainless steel and nylon bird guard to avoid extraneous contamination and mounted on a tripod which can be attached to the ground for increased stability. The Frisbee Gauge is generally accepted to have the greatest sampling efficiency of those available
- The BS1747 Directional Dust Gauge – Uses four tubular collectors, positioned to distinguish between the relative amounts of dust deposited from the four compass points
- The Bergerhoff Gauge – The simplest of all the deposit gauges as they consist of a collecting container mounted on a pole
Soiling rate measurement
Another straight forward yet effective method for determining soiling rate is that of the Sticky Pad system which measures the soiling of a white adhesive strip (Fablon) over a known period and gives a measurement of the deposition (as percentage Effective Area Coverage (%EAC) per day) using a reflectometer
By mounting the Fablon pad on a flat surface, deposition by soiling rate is measured. By mounting the Fablon on a cylindrical bobbin, which can be attached to the Frisbee gauge, it is possible to measure deposition on a directional basis. By dividing the circular strip into sections it is possible to obtain soiling rate readings for the 8 sectors (N through to NW). After the monitoring period the Fablon is then subjected to reflectometry testing to determine the dust soiling rate.
The method is attributed to Beaman & Kingsbury (1981), who determined the sticky pad method, which is effective for monitoring periods of up to a maximum of circa 7 days. The method can be relatively non-intrusive and inconspicuous, which makes it popular in areas where conspicuous monitoring is likely to cause problems.
Guidelines for assessing dust deposition on soiling rates
Although there is no statutory framework within which assessments of nuisance dust can be made, a methodology for measuring the deposition of nuisance dust does exist, which has been published by the Stockholm Environment Institute (1995) (SEI).
Case study
The site Churngold Remediation Limited carried out a remediation excavation programme at a site in Corsham, Wiltshire, prior to development, which was eventually to include the construction of houses and associated infra-structure. The earthworks continued for a period of 17 weeks and included the excavation, stockpiling and removal off-site of soil material required as part of the remediation works and also for the creation of a construction base.
The site itself was circa 13 hectares (ha) in size and comprised predominantly open spaces, accessible to the general public for leisure activities. It had formerly been used by the Ministry of Defence (MoD). Residential properties were present along several of the site boundaries and were therefore considered to be potentially at risk from the generation of dust, particularly during the earthworks phase of the project.
Aims and objectives
The aims and objective of the work was to determine the dust deposition and soiling rates/conditions around the perimeter of the site. This was carried out both preceding and during the excavation works. A directional element was included, to determine possible sources of dust from the surrounding environment, should any elevated levels be encountered. A limit for dust deposition of 200mg. m-3day-1 was set for the site.
In order to determine the impact of the development in terms of dust generation it was of vital importance to obtain baseline data. Monitoring at this site therefore commenced 2 weeks prior to the earthworks programme. By doing this it was possible to take into account the impacts of ‘background’ levels of dust within the atmosphere.
Methodology
Frisbee deposition collectors were set up at eleven pre-determined locations around the perimeter of the site. The collectors were supplemented with a cyclindrical attachment upon which an adhesive Fablon pad was mounted in order to make a determination on the direction of dust. The locations were chosen in accordance with the advice provided by the SEI (1995) in so far as was reasonably practicable. The gauges were located away from obstructions such as buildings and trees or over-head power cables, which may have interfered with the integrity of the results. The standard protocol for selecting locations requires that no large objects are within 5 metres of the monitoring location and that the top of any obstructions should subtend less than a 30° angle with the horizontal at the sampling point. The sample point itself should be 1.7 metres off the ground.
With open sites, as was the case in Corsham at the outset, problems are commonly experienced with damage to the equipment. Therefore as is shown in the illustration below, until the site was secured the gauges were locked within Heras fencing to ensure that the instruments were not vandalised and that the integrity of the monitoring was maintained. Once a site is operational, especially a construction site, there are also other potential problems associated with plant movements and access to specific areas. It was therefore important that at the outset of the project there was full liaison with representatives from Churngold to determine suitable locations, which were not going to have to be moved due to operational requirements later on during the programme.
Representatives of Voelcker Consultants visited the site on a weekly basis to collect samples from the gauges. Samples were submitted to a UKAS accredited laboratory to make a gravimetric determination of the weight of particles and conduct a reflectometry test to determine the dust soiling rate on a directional basis.
Assessment of the results
With an ongoing project it was important to look for trends within the results. As with any similar type of testing the results are reported retrospective to the generation of any dust.
Our remit was to report any trends in dust generation or anomalies that were identified to Churngold such that any problems could be mitigated. In this instance, the use of standard operational procedures and favourable weather during the project ensured that the 200mg.m- 2day-1 was not exceeded throughout the excavation works. In general the levels of dust deposition were well within the limits set for the site, the majority being below 20 mg.m-2day-1 and as such would conform to the requirements set as part of the planning conditions.
When comparison was made to the baseline monitoring results for soiling rates at each location there were no significant increases in the soiling rate during the earthworks activities compared to the monitoring undertaken in the period preceding the project. In general the results ranged from <0.1% EAC to 0.7% EAC, which was comparable to the results obtained during the baseline monitoring.
In general the soiling rate encountered was below that at which complaints are probable and in our experience were no higher than would typically be expected ‘naturally’ within rural settings.
Conclusions
Primarily involved in the contaminated land sector, our involvement in dust monitoring is primarily at construction, demolition or remediation sites, whereby a flexible, accurate and efficient method for dust monitoring is required. We have used the Frisbee Deposit Gauge, supplemented with sticky Fablon pads to provide a directional facet to the monitoring and have employed this technique in both rural and commercial settings. To date it has proven to be a reliable means of monitoring ambient dust and in instances where the contractors on site ‘buy-in’ to the scheme has proven to be an effective method of satisfying local authority planning conditions.
Further Information
The National Physical Laboratory develops and promotes measurement standards and technology to enhance productivity, facilitate trade and improve the quality of life.
Good measurement enables improved quality, consumer confi dence and new products and processes.
Published: 10th Sep 2005 in AWE International
