The (dust) devil is in the detail

Nikky LaBranche Research Fellow - OHS, The University of Queensland

The resurgence of Silicosis, Pneumoconiosis and other mine dust lung diseases has placed the spotlight back on the management of worker exposure to particulate matter across all mining sectors.

Worker exposure to particulates is currently monitored on a total mass basis (gravimetric analysis). It is becoming evident that the size and shape of the particulate matter can affect its potential impact on human health.

While the contribution of respirable dust to respiratory disease is well known, and there is a substantial body of knowledge on particulate matter and its impacts on human health, it is also clear that there are major gaps in our understanding.

In part, those gaps relate to the contribution to adverse health effects of mineralogical constituents in the dust. The number of mine dust lung disease cases continues to climb in Queensland, where 317 people have been diagnosed as of 31 October 2021, with more than twice as many cases of silicosis (11) than CWP (5) being diagnosed in FY22. There were 29 cases of chronic obstructive pulmonary disease (COPD) diagnosed in FY22 alone, which is a significant increase. COPD is an umbrella term, including emphysema and chronic bronchitis.

Discovering the gaps

Despite its prevalence, there are major gaps in our understanding of particulate matter and its impacts on human health. In previous decades, disease severity was thought of only in terms of the magnitude of exposure and career duration. However, studies in both the United States and the United Kingdom have detailed regional differences in the workers diagnosed with different lung diseases.

For instance, the UK has found the CWP risk varied significantly by county, with chronic bronchitis and emphysema found to have less geographic variation and not correlate with CWP. Research suggested that the risk of chronic bronchitis and emphysema may not be directly determined by the measure of exposure but rather as the result of larger particles of dust, in the inhalable size fraction, depositing in the tracheobronchial region.

Digging into particle sizes and numbers

When sampling for personal exposure to respirable dust, it is important to know what you are measuring. Gravimetric sampling measures the total mass of dust, but the toxicity of the dust can change with the size, shape and mineralogy of the components.

Testing performed at the National Institute of Occupational Safety and Health (NIOSH) in the US found lung tissue was more reactive to ultrafine crystalline silica particles (mean particle size of 0.3 microns) than the commonly measured respirable fraction (mean particle size of 4.1 microns).

The number of particles present may also affect the health hazard of the dust. The gravimetric sampling techniques currently in use measure the total mass of the dust collected on a filter. This total dust mass does not consider the number of particles present and, assuming a constant density, it would take 2,578 particles of a 0.3µm diameter to equal the mass of one particle of 4.1µm diameter. This means it may be possible for a worker to be exposed to a significant number of ultrafine particles that do not add up to enough mass to exceed the eight-hour time-weighted average exposure limit, but still pose a significant hazard.

Despite its prevalence, there are major gaps in our understanding of particulate matter and its impacts on human health... it is becoming evident that the size and shape of the particulate matter can affect its potential impact on human health.

A new research tool – the Mineral Liberation Analyser

The University of Queensland has developed a methodology for characterising respirable and inhalable dust samples using scanning electron microscopy with energy dispersive spectra on the Mineral Liberation Analyser (MLA). The MLA was originally developed at the Julius Kruttschnitt Mineral Research Centre, part of the Sustainable Minerals Institute at UQ.

Multiple samples from numerous mining operations have already been analysed using the MLA. The process has been successful in showing the variety of mineralogical components and particle size distributions present in various areas of the mines.

Dust sources and characteristics

There can be several sources of dust in the mine besides the cutting of the coal seam, such as:

• vehicle traffic through the mine

• the mining of roof or floor rock or rider seams

• stone dusting activities

• other activities in the mine, such as shovelling

Performing sampling in actual mining conditions picks up the contributions from all these sources, which in some instances can be significant. Samples were taken in a variety of underground locations, including at the maingate and midface of the longwall, around the continuous miners, on the shuttle car, in belt roadways, and during secondary recovery activities.

The overall size distributions of the dust can be calculated by the MLA based on the individual particles. It should be noted that the MLA is measuring actual particle diameter, not aerodynamic equivalent diameter (AED). For the coal mines, many of the areas seem to follow a pattern, with the midface of the longwall being the finest PSD, while the maingate is somewhat coarser and the continuous miner section is the most coarse.

These variations in particle size distributions may be indicative of the number of particles that a person would be inhaling for a given mass of dust. There would be a larger number of small particles in the same mass of dust on the longwall than there would be on the continuous miner section. This would also mean that there would be more surface area on the longwall particles when compared to the coarser CM particles.

Agglomeration of particles – a new vector of research

The MLA characterisation data is also showing respirable particles are more complex than initially thought. The particles look to be agglomerating at this small size are producing multiple mineralogies in one particle, not just particles of single mineralogies, as was initially thought.

Ultimately, more work is needed to characterise the dust present in different mining environments in order to better understand how the chemical components, particle sizes and shape contribute to the health effects. The University of Queensland has begun this important characterisation work starting with underground coal and metals mines. This technique can be applied not only to different mining environments but also to more general applications dealing with silica, such as stone benchtop workers.

Future collaborations underway

Researchers in the Minerals Industry Safety and Health Centre within SMI have recently been awarded a $2.4 million research grant to collaborate with the University of Illinois Chicago and the University of New South Wales for further dust research. These projects include the characterisation of more underground coal mines as well as surface coal mines and the engineered stone industry. The work with UNSW includes lung tissue work which will allow the mineralogy of the dust to be compared with its toxicity.