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 properties; Determination of the quantity of dust deposited, by weight. WHAT IS DUST? Dust is a generic term used to describe fine particles that are suspended in the atmosphere. Dust comes from a wide variety of sources, including soil, vegetation (pollens and fungi), sea salt, fossil fuel combustion, burning of biomass, and industrial activities. It is formed when fine particles are taken up into the atmosphere (entrained) by the action of wind or other physical disturbances or through the release of particulate-rich gaseous emissions (primary particles). In addition, gases such as sulfur dioxide and oxides of nitrogen may react in the atmosphere over time to form fine particles, such as ammonium sulfate and ammonium nitrate (secondary particles).
Dust is typically classified according to its particle size, as follows:
- Deposited matter refers to any dust that falls out of suspension in the atmosphere.
- Total suspended particles (TSP) typically refers to particles 50μm (micrometers) (0.05mm diameter) in size or less.
- PM10 refers to particles 10μm (0.01mm) in size or less.
- PM2.5 refers to particles 2.5μm (0.0025mm) in size or less.
Particle size is an important factor influencing the dispersion and transport of dust in the atmosphere and the effects of dust on human health. Dust particles in the atmosphere can be as small as a few nanometres and as large as 100μm. Fine particles are typically defined as particles of size PM10 or less. Characteristics of these particles include:
- They are easily entrained by wind or disturbances and generally take a long time to settle once airborne, although they may be washed from the air by rain or snow.
- They may settle permanently on land or only temporarily before being picked up and moved again, and may settle on water, dissolve in water, or both.
- They may stick together or break apart changing the size distribution over time.
- They may undergo chemical changes and reactions with other substances.
Fine particles are not visible to the human eye, although a high concentration of fine particles may appear as a ‘haze’ in the atmosphere. Note: Particle size is not an absolute criterion as thin flakes or fibers larger than 10μm may be part of a PM10 sample because of their aerodynamic properties.
Dust Particle Size and Composition
Mining typically generates dust particles between 1μm to 100μm in size. According to a recent article, typical size ranges are:
- submicron size (less than 1μm): 0.2 percent (generally from diesel emissions) of total emissions
- PM2.5: 2 to 5 percent of total emissions
- PM2.5 to PM10: 15 to 45 percent of total emissions
- greater than 10μm: 50 to 70 percent of total emissions.
A survey of dust particle sizes from coal mines in the United States (sampled up to 100m from the source) found that particles had a median size (the midpoint in a range of sizes) of 24μm. Particles in the range PM10 or less comprised between 11 to 23 percent of total dust emissions. In relation to coal mining in the Hunter Valley, the Metropolitan Air Quality Study (MAQS) identified that mining is a significant source of TSP (50μm or less), contributing 26 percent of human sources in the greater MAQS region. In addition, the National Pollutant Inventory (NPI) estimated the mass of PM10 emissions due to coal mining in the Hunter Valley area for the period 2003 – 2004 was 42,000,000kg.
While mining does generate fine dust particles, it appears that the bulk of very fine particles in the atmosphere are typically derived from other sources. Studies in the early 1990s in the Sydney, Newcastle and Wollongong areas estimated that soil particles (sources include mining, but also other activities such as agriculture) make a relatively small contribution (6 percent) to the total ambient level of very fine particles (PM2.5) in the atmosphere. This compares to other particle types such as ammonium sulfate (23 percent) and elemental carbon (22 percent), which are derived from combustion processes.
A more recent study in the Hunter region identified that soil particles make an average contribution to the total ambient level of fine particles in the atmosphere of about 10 percent and 22 percent for PM2.5 and PM10 size fractions respectively. The percentage of soil particles was marginally higher in samples from the Hunter region than in samples from other rural areas in NSW.
The composition of dust particles generated by mining will depend on the geology of the site. The majority of dust generated by mining is typically derived from soil and rock. However, dust may also be derived from the material being mined (for example, coal dust or iron ore dust), or from activities associated with mining (for
example, diesel emissions).
Mining dust may contain various metals and other potentially hazardous substances. For example:
- The NPI estimated the mass of emissions to the air of toxic metals and their compounds due to coal mining in the Hunter Valley area for the period 2003 – 2004 as between 14kg and 55,000kg. These metals included arsenic, antimony, cadmium, chromium, cobalt, lead, manganese, mercury, nickel, selenium and zinc.
- Silica is a very common mineral found in most rocks. Dust generated from the crushing of rock that contains a lot of quartz (a form of crystalline silica) may generate silica dust, which is potentially hazardous.
Note: Industrial facilities are required to report emissions to the NPI if they use more than a certain amount of one or more substances on the NPI reporting list.
Emissions are estimated by each facility using techniques outlined in industry handbooks. The pollution exposure to humans and the environment cannot be determined solely from the NPI. Many additional factors determine whether a pollutant emission is felt as ground level pollution, such as the nature of the receiving environment, the chemical reactivity of the substance and the prevailing weather conditions. Since the NPI does not attempt to collect information about these additional factors, NPI data can only give an indication of pollutant emissions at the source of the emission.
HEALTH EFFECTS OF DUST
The factors that influence the health effects of dust are:
- size of the dust particles
- composition of the dust particles
- concentration of the dust particles in the air
- duration of exposure (possibly in years) to the dust particles.
There is considerable debate over the relative importance of these factors in relation to health effects. Generally only fine particles of dust are of health concern. Dust particles PM10 or less in size are likely to have the greatest health effects because they may be drawn deep into the lungs. Particles larger than PM10 tend to be trapped in the nose, mouth, throat or major bronchi and are typically expelled from the body.
Dust particles may be termed ‘inhalable’ or ‘respirable’. Inhalable particles are usually less than PM10 and greater than PM2.5 in size. These may be deposited in the upper sections of the lungs. Respirable dust particles are less than PM2.5 and may be deposited in the lower sections of the lungs, including the alveoli.
The NPI has given particulate matter of size PM10 or less a health hazard rating of 1.2 out of 3. A score of 3 represents a very high hazard to health, a score of 2 represents a medium hazard and a score of 1 is harmful to health.
The health effects of fine particles include:
- Absorption of dust into the blood through the lungs, with potentially toxic effects. Dust particles may contain various metals. Dusts containing mercury, arsenic or cadmium are particularly hazardous to human health.
- Diseases of the lungs, including cancer. Particles that penetrate deeply into the lungs may be permanently lodged, which may result in diseases of the lung.
- Long term negative effect on lung function causing marginally increased death rates and sickness in sensitive people.
- Allergic or hypersensitivity effects.
- Irritation of mucous membranes.
According to NPI, recent health research in relation to dust particles of size PM10 or less suggests there is no threshold at which health effects do not occur. Dust generated from the crushing of rock that contains a lot of quartz may generate silica dust. Long term inhalation of silica dust may lead to the formation
of scar tissue in the lungs and can result in silicosis, a serious lung disease. It appears that silicosis is solely a work place issue. A recent Australian Government Senate Committee (2005) report identified that there are no reports in the international literature of individuals developing silicosis as a result of exposure to
non-occupational levels (i.e. levels outside the work place) of silica dust, and an expert appearing before the committee confirmed the potential for such an occurrence as being very remote. A United States Environment Protection Authority study (1996), which specifically addressed this issue, also supports this
The main methods for monitoring dust levels are:
- Dust deposition gauges: This method measures dust deposition rate and involves the passive deposition and capture of dust within a funnel and bottle arrangement. Data is usually collected over monthly periods and results are expressed in g/m2/month (ie. the mass of dust deposited per m2 per month). This method enables determination of the relative ‘dustiness’ of sampling locations. It does not provide data on dust concentrations or enable determination of dust levels from a particular event or source. It does not give an indication of the potential health effects of the dust because it does not measure the amount of fine and very fine particles in the atmosphere.
- High volume samplers: This method determines average dust concentrations and comprises the collection of dust by drawing a constant flow rate of ambient air through a filter. Data is usually collected over a 24
hour period and results are expressed in g/m3/24hr (ie. mass of dust per volume of air per 24 hrs). A selective inlet may be fitted to a high volume sampler to restrict the particle size being sampled (for example, to ensure only PM10 particles are sampled).
When coupled with a wind direction vane or matched with records of wind data, this method enables determination of dust levels from a particular event or source. It also gives an indication of the potential health effects of the dust because it allows measurement of fine and very fine particles in the atmosphere.