Dust Defined – The following information comes from one of our own articles. What exactly is dust? Take a look below – for the full article please follow the link – http://dust-monitoring-equipment.com/training-presentation-dust-is-not-dust.htm
As dust is fine solids or, in some cases liquids, there needs to be a system of measuring the particulate size and then to categorise the various dusts by size to see to what extent the dust is ingested.
The particulate can be measured by various means but within the metric system of measurements, we use the term micrometer or micron, which is an exceedingly small measurement of one-one thousandth of the millimetre. The human eye will only see a profusion of dust in the air under certain conditions – predominantly if one is viewing the plume of dust against a blue sky against the light. This presumes of course, that the dust concerned has a low reflective index and the colour has the greatest contrast with the sky colour as possible.
A lime dust plume is far more visible than cement, which is grey – less of a contrast against the sky’s blue. Similarly, coal dust will be visible against the sky, whereas a light grey roadway dust will be less visible.
Any existence of moisture within the emission will also increase the dust visibility:-
| * | By physically wetting the particle, which then may become darker in colour. |
| * | By condensing and adding a visible dimension to the plume. Most observers will comment on how bad a dust plume looks when they are in fact seeing steam or water vapour, which they presume is white smoke rather than what it is – water vapour. As a rule of thumb, watch such a plume and if it suddenly starts to disappear, then you are seeing water vapour. What remains in the air is dust and this may just be visible at a distance further than the vapour plume extremity. |
Dust of differing size particulate has a system of descriptive classification, which, while rather subjective, does put a lot of light on the matter and enables us to obtain a good idea of the particle size range applicable for each category.
The following diagram 1, of which there are many examples with slight variations, is most handy to convey the concept of particulate sizing within each category by definition.
TABLE 01 – DESCRIPTIVE CLASSIFICATIONS FOR DUSTS
As soon as one starts to view the various dust sizes, further classification by various agencies come into the picture due to the need to monitor for health purposes or for other reasons.
Occupationally, in South Africa we need to be aware that there are categories for:
| * | Respirable dust |
| * | Thoracic dust |
| * | Inhaleable dust |
| * | Nuisance dust. |
While the last description may not be that official it is used by all and sundry as a “one size fits all” approach, as we all hate dust with one or two exceptions – “Gold Dust” or perhaps “Diamond Dust”. To define any dust one needs to specify the dust particulate size range not withstanding the reasonably hard and fast definitions outlined above. The American Conference of Governmental Industrial Hygienists (ACGIH), now considered to be one of the foremost authorities on industrial hygiene and contrary to its name, is a private not-for-profit non-governmental corporation, whose members are industrial and occupational hygienists and other safety and health professionals dedicated to the promotion of health, safety and health safety in the workplace, has established the indicated classifications based on the following criteria:-
| INHALABLE DUST/PARTICULATE | THORACIC DUST/PARTICULATE | RESPIRABLE DUST/PARTICULATE | ||||
| SIZE DISTRIBUTION | Aerodynamic diameter (d) | Mass % | Aerodynamic diameter (d) | Mass % | Aerodynamic diameter (d) | Mass % |
| 0
1 2 5 10 20 30 40 50 100 |
100
97 94 87 77 65 58 54,5 52,5 50 |
0
2 4 6 8 10 12 14 16 18 20 25 |
100
94 89 80,5 67 50 35 23 15 9,5 6 2 |
0
1 2 3 4 5 6 7 8
10 |
100
97 91 74 50 30 17 79 5
1 |
|
| DEFINED SIZE | d0,50 = 100µm | d0,50 = 10µm | d0,50 = 4µm | |||
DIAGRAM 01 – DEFINED DUST
The term or definition of total dust is “airborne material sampled with the 37mm closed face cassette traditionally used for aerosol sampling”. The term will ultimately need to be replaced occupationally by one of the above descriptions. Research using cassettes has broadly indicated it is scandalous that we still ‘assume’ this total dust is a risk or not after over 40 years. It can be ingested.
We point out that the three categories of particulate size sampling are achieved using the new ISO/CEN/ACGIH curve cyclone with a flow rate of 2,208 litres/min (say 2,2 litres/min). The previous BMRC curve cyclones were operated at a flow rate of 1,890 litres/min (say 1,9 litres/min).
During the initial stages of the swap-over to ISO/CEN/ACGIH cyclones, we noted that paired rigs yielded a d0,50 cut off of 4,00µm for the BRMC and 5,0µm for the latter. This was largely due to the differing flow rates and if the ISO/CEN/ACGIH was operated at 1,9 litres/min then a value closer to the BRMC 4,00µm was achieved. So why increase the flow rate? Research has now found larger particulate trapped in lung tissue.
The above information is handy for occupational hygienists to determine PM10 levels using ISO/CEN/ACGIH cyclones and 37mm cassettes and monitoring for environmental purposes.
When using the cassette without the cyclone at 1,90 litres/min, one achieves a PM10 result, but to improve the distribution of dust on to the filter, the distance between the cassette entrance hole and filter needs to be increased so a more lamina distribution can be achieved as well as a more consistent loading of the filter. Any material entering and remaining loosely in the bottom of the cassettes must not be retained in the sample as this will be average oversize and considerably so.
It is possible to take a larger cut-off at perhaps d0,50 – 20µm, 30µm or even 50µm, but if we bear in mind that the limiting factor after 2,0 litres/min becomes the cassette opening, which needs to be drilled out to 6mm for 20µm and 30µm and to 10mm for 50µm dust, then one is sacrificing cassettes. The flow rates also become increasingly critical the larger particulate we wish to capture and one then needs to consider the density of the dust material to arrive at a flow rate.
HOW DOES PRECIPITANT DUST FIT INTO THE EQUATION?
Well, to start off, we need to go back and notice how we accepted the particulate sizing and flow rates so easily and assumed that these are finite, cut and dried and cast in stone as it were!!!
No, life is never that simple and air density played a massive role in the amount of air that our gravimetric sampler was handling, and in fact the altitude will also have played a massive role, as well as barometric pressure, so at the end of the day, how accurate are those Respirable, Thoracic or Inhalable dust samples, and while we are at it, how accurate is your high volume constant flow sampler determining PM10 sample results? It has become question after question with fewer and fewer answers being available, which means that the environmental auditors who check your reports will only specify and check that your methodology was to regulation or method. Where has reality gone? Dust is not dust, is not dust, or is it?
While we are on Question Time, let us select a few more to look at:-
| * | If your PM10 or gravimetric sampling rig sampler is operating to spec and the dust is mainly organic, are you over reading or under reading? Does Durban and Johannesburg make a difference? |
| * | The same question needs to be asked for gravimetric sampling, but let us add common pollutants to make the answer more difficult. If your gravimetric sampler and ISO/CEN/ACGIH cyclone and cassette are running at 2,2 litres/min with cellulose wood fibre dust and coal dust, will the results be the same and will both be representative bearing in mind that the density of wood fibre could be as low as 20 kg/m3, while the coal dust will have a density of over 2 ton/m3? I have used bulk density and not material density. Is this correct? |
Having questioned convention surrounding capturing the airborne dust for your sample, let us look at how the dust is scavenged or collected.
The inlet on an ISO/CEN/ACGIH cyclone is exceedingly small and is directional and far too many assumptions are made around the acceptance of accuracy.
We need to be aware of directional airflow in the sampling area or over the sampling rig and this airflow needs to be stabilised before we can assume that the result is correct. The bell or impactor on a PM10 or PM2,5 rig can scavenge windblown dust and affect the dynamics of collection to the point where accuracy is affected in the same way as the cyclone rig, so we need to ask what we are sampling for and work within the limitation of the equipment we are using.
Finally, the cost of equipment and the labour needed to run sampling exercises, means that we try to minimise the number of samples that are taken as well as the position and we erroneously assume that these are representative.
| * | Sampling in one or two positions is not representative of conditions on a plant, surrounding a property, in a township or industrial area. |
| * | Sampling on one or two days, a week or even a month is as inaccurate as a total guess when viewing permanent conditions. |
| * | Most analysis methods demand samples of substance, more than the couple of micrograms collected in a PM10 or gravimetric dust sample with the result that inaccuracies of scale are being accepted. |
To illustrate this, the City of Cape Town has about seven permanent monitoring stations around the Peninsula, which run at best erratically and often not at all and the results are accepted without question.
On mines, the dust levels come down all the time but silicosis cases increase – there is something wrong. There are many cases out there with persons never having worked at a mine or lived near one.
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Dust Monitoring Equipment – providing equipment, services and training in dust fallout management to the mining industry.
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