We are all familiar with dust as a nuisance, whether it is in the home, environmentally from natural deposition sources such as the Saharan winds; or from other more anthropogenic sources such as mining, waste processing or quarrying. Of these, the production of the latter falls into the duty of care of the producer of the dust and as such they are required to monitor the dust production and deposition; where relevant using this data to inform dust suppression techniques employed on site.
Dust is defined as solid particulate matter between 1 to 75 μm in diameter (BS 6069) and can become suspended in the atmosphere and deposited, a process which can be repeated many times. For the purposes of this article, we are considering “nuisance” dust (typically greater than 20 μm in diameter) rather than dusts that are of interest due to their health effects (PM2.5, PM10).
Dust deposition from construction activities and earth works is one of the more obvious nuisances to the general public; along with noise pollution. As such, it is becoming increasingly common to find requirements for site dust monitoring within the specifications for the works. These requirements for analysis seek to answer two questions:
- Where is the dust coming from?
- What dust levels are present?
There are obvious ways of identifying, without quantifying, the levels of dust present – deposited dust soiling surfaces on site and nearby are visually apparent; as are any dust plumes generated on site. The presence of these are good visual indicators that more informed data are required and in this case typically there are two types of analysis commonly specified: dust deposition rate and directional analysis.
Within these two classes there are a couple of different ways of capturing the dust for analysis.
Dust deposition rate
Dust deposition rate is the amount of dust (in mg) deposited on a specific area (m2) over a number of days. The dust deposition rate is thus recorded and reported as mg/m2/day. There are two types of gauge commonly used to determine this – they are the Bergerhoff Gauge and the Frisbee Gauge.
The Bergerhoff Gauge
The collector is a 1.5 litre PET (Polyethylene terephthalate) bottle with a neck diameter of 90 mm, so its area is 0.00636 m2. The dust falls into the bottle, assisted by rainfall. This size is as specified in the German DIN standard 2199 and with the 1.5 litre bottle this allows for up to 225 mm of rainfall.
If the gauge collects a mass of dust (W) (mg) in days (D), the deposition rate is calculated as follows:
Deposition = W/0.00636/D mg/m2/day
The main assembly consists of a plastic-coated stand with removable bird guard and bottle holder. The bird guard is critical to prevent contamination from bird strikes.
The dust collected in the bottle is measured by filtration and gravimetric analysis of the filter.
Dry Foam Frisbee Gauge
As with the Bergerhoff Gauge, this instrument is designed to measure dust deposition, usually quoted as mg/sq m/day. The collector is an anodised aluminium device looking like an inverted Frisbee. It has an operational diameter of 227mm so its area is 0.04 m2.
“the mean monthly deposition rate can be calculated and compared with guideline values to give an indication as to the likelihood of complaints”
If the gauge collects W mg in D days, then the deposition rate is given by:
Deposition = [24.7*W]/D mg/m2/day
The dust collected in this bottle is also measured by filtration and gravimetric analysis of the filter.
What does the data mean?
By looking at the data produced over a period of time, the mean monthly deposition rate can be calculated and compared with guideline values to give an indication as to the likelihood of complaints.
Directional dust analysis
Having identified how much dust is being generated, it’s also important to get some understanding of the direction the dust is coming from. Where several activities are taking place on site this can help differentiate sources, as well as understanding air flow and wind direction across the site.
The most common test method of choice is the use of the directional “sticky pad” gauges, which are used to measure Effective Area Coverage per day (%EAC/day) originally developed by Beaman and Kingsbury.
A sticky pad typically of 317 mm x 148 mm is affixed sticky side outwards on a drum, the join line aligning with north. The drum and the sticky pad can be mounted on a Frisbee or Bergerhoff Gauge meaning that multiple types of information can be obtained from the same observation post.
At the end of the sampling period, the sticky pad is removed, covered over with the original wrapping paper and submitted to a laboratory for analysis.
What does the data mean?
A typical sticky pad submitted for testing is shown below. The dust intensity is clearly greater at around 3/5 of the way across the pad which corresponds with SSE.
The sticky pad reader takes reflectance readings at intervals across the pad and from this a graph is produced showing the EAC% /day versus orientation.
The work carried out by Beaman and Kingsbury gives the following possible implications for different EAC% levels.
Currently no statutory or official air quality criterion for dust annoyance has been set at a UK, European or WHO level. However, in England and Wales, a ‘custom and practice’ limit of 200 mg/m2/day is used for measurements with dust deposition gauges.
“currently no statutory or official air quality criterion for dust annoyance has been set at a UK, European or WHO level”
With no other guidance available this unofficial guideline has been used widely in environmental assessments. Whilst the data that has been derived from this guideline are not particularly robust, the values are similar to those in other countries:
- USA, Washington has set a state standard of 187 mg/m2/day for residential areas
- In Germany ‘possible nuisance’ and ‘very likely nuisance’ levels are 350 mg/m2/day and 650 mg/m2/day, respectively
- Western Australia also sets a two-stage standard, with ‘loss of amenity first perceived’ at 133 mg/m2/day and ‘unacceptable reduction in air quality’ at 333 mg/m2/day
- Sweden – Stockholm Environment Institute – 140 mg/m2/day for rural areas to 260 mg/m2/day for town centres
Other aspects of dust pollution
Whilst the main area of discussion has been around dust itself and its mass deposition and direction of deposition; what must not be over looked is the potential health impact of these particulates. With each of the types of collector detailed herein, it is possible to obtain some chemical analysis data such as metals or polyaromatic hydrocarbons. In some cases, it may even be possible to see if asbestos fibres have been collected. Together with specific targeted dust analysis, using special air sampling techniques and subsequent analytical testing, a more comprehensive picture can be obtained.