The Future of Dust Monitoring: 2020 and beyond
By Nikky LaBranche, AusIMM Health and Safety Society Committee Member
In response to the ‘resurgence' of Coal Workers Pneumoconiosis (CWP) in Queensland, new national standards for the time-weighted average exposure limits are expected to be put in place for respirable coal mine dust and respirable crystalline silica before the end of 2019. Safe Work Australia (SWA), who sets workplace exposure standards for approximately 700 chemicals in Australia, has recommended a standard of 1.5 mg/m³ for respirable coal mine dust, which the state Minister for Natural Resources, Mines and Energy Dr Anthony Lynham committed to matching. This respirable coal mine dust limit was previously reduced from 3 mg/m³ to 2.5 mg/m³ on 1 November 2018. By contrast, the US reduced their designated operator exposure standard from 2 mg/m3 to 1.5 mg/m³ on 1 July 2016, while South Africa has a personal exposure limit of 2.0 mg/m³.
This reduction in exposure limit is in response to the mine dust lung diseases cases diagnosed in the last few years under the DNRME's improved health surveillance scheme. The number of mine dust lung disease cases continues to climb in Queensland, where 118 have been diagnosed as of 31 August 2019. Of those, 78 are pneumoconiosis cases, which itself includes 39 cases of coal workers pneumoconiosis, 25 cases of silicosis and 14 cases of mixed dust lung diseases. Nine of these reported cases have progressed to the most severe category of progressive massive fibrosis (PMF). There are also 28 cases of chronic obstructive pulmonary disease (COPD), which is an umbrella term including emphysema and chronic bronchitis.
This re-identification of mine dust lung diseases has prompted much work to be done to improve exposure monitoring and health surveillance in coal miners. In past decades CWP was thought of as a simple relationship of the mass of dust a coal mine worker is exposed to over their career duration. There are even some models based on the idea there was no exposure risk below 2 mg/m3. But it turns out the dose-response relationship is not that simple, and our understanding of particulate matter and its impact upon human health is more limited than we thought. The further one delves in to the literature, the more limitations to the the understanding of potential health impacts of respirable dust appear. It is now recognized as inadequate to talk about coal dust in general terms, because both size and chemical content can affect the adverse consequences of excessive exposure. To that end the term coal mine dust lung disease is now used, not simply coal dust lung disease, as silica and bio-available iron are examples of non-coal components of dust in mines. This term has now been shortened to simply mine dust lung disease (MDLD).
Regional differences in the prevalence rates of different lung diseases has been demonstrated by studies in both the United States and the United Kingdom. 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. UK research suggested the risk of chronic bronchitis and emphysema may not be directly determined by the measure of exposure but rather the result of larger particles of dust, in the inhalable size fraction, depositing in the tracheobronchial region. While the overall prevalence of CWP in the US is slightly above 10% for long tenured underground coal mine workers, the US has found similar patterns of regional variations in CWP. The prevalence of CWP in long tenured workers being four-fold higher in Central Appalachia than the rest of the country.
The measurement of the personal exposure of a miner to dust in the coal mine is currently performed in accordance with AS 2985 Workplace atmospheres – Method for sampling and gravimetric determination of respirable dust. Queensland legislation specified this standard. Gravimetric sampling for respirable coal dust is performed using cyclone elutriator and pumps to draw air through the sample. The cyclone collects a mass of dust on a filter of a size fraction determined by the cyclone type and flowrate of the pump. The amount of contaminant collected on the filter (in milligrams) is divided by the known quantity of airflow through the pump, which is the flowrate of the pump multiplied by the number of minutes over which the sample was collected, thus giving the units of mg/m3.
Historically Australian relied solely on manufactures claims that their equipment met the relevant standard. Cyclone elutriators were not required to have third party certification to the size-selective curve in the ISO standard they are meant to comply with. The problem with this approach was revealed in August 2018, one of the manufacturers of gravimetric cyclones for respirable dust sampling, announced their Higgins-Dewell type plastic cyclones, used by industry for compliance sampling, were collecting up to 30% more dust than intended. This over-collection of dust on the filter necessitated a change in the flowrate of the pump collecting the sample to collect the proper size fraction of dust. So far this is the only gravimetric sampler commonly used in Australia that has produced test results to show its compliance. The conformance of other cyclones currently in use in Australia is still in question. Similarly to cyclones, pumps have not been independently tested and overseas testing has identified a number of models that fail to meet the less than 10% pump pulsation criteria in the ISO standard. It is imperative that compliance monitoring equipment used in Australia is tested to ensure it meets the relevant standards.
The gravimetric sampling for compliance monitoring currently used in Australia can take up to three weeks to return results, because the filters must be sent to a laboratory to be weighed. The US has adopted mass based real-time monitoring, using a tapered element oscillating microbalance (TEOM) for their compliance monitoring, so miners know how much dust they are exposed to during their shift. Importantly, the BGI-4CP cyclone used in this monitor has been tested for conformance to the size-selective curve in the ISO standard.
Australia needs to embrace the latest technology by moving to real-time monitoring for compliance sampling. This would require a change in legislation from AS 2985 to a new standard. Environmental monitoring has a much broader range of standards for monitoring which include the use of TEOMs. There are also real-time monitors using optical techniques approved as intrinsically safe for underground use that can be extremely useful in identifying dust sources and their magnitudes and looking at the effectiveness of dust controls.
While Australia needs to embrace new technology for measuring the mass of dust in real-time, the size of particles may even be more important to the health hazard the dust poses. 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 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 measures 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 0.3 µm diameter to equal the mass of one particle of 4.1 µm 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 8-hour time-weighted average exposure limit, but still pose a significant hazard.
To better understand the relationship between dust exposure and the health hazard, more work is needed to characterise the dust present in different mining environments and understand the contribution of the chemical components, particle sizes and shape to the health hazard. The gravimetric sampling equipment currently used for compliance sampling needs to be testing to ensure that it meets the relevant standards. New real-time technology also need to be tested as it comes to the market and updates to legislation should be made to allow for dust monitoring technology to evolve. Technology will play an increasing role in identifying dust sources and putting effective controls in place.