Health Challenges facing Underground Hard-rock Mining

There are two vital ingredients for healthy living; clean air and pure water.  
Even though we live in a well organised, caring and well-resourced country we still have to be vigilant about the quality of the air we breathe and water we drink.  
Modern technology can, however, whilst solving one problem unwittingly create another unexpected and unintended problem, as it has in the case of the diesel engine on which we are so reliant. On the one hand modern diesel engines have become significantly more efficient than their predecessors whilst on the other their exhaust products have become more toxic and increasingly hazardous. 
In 2006 the California Air Resources Board estimated that diesel exhaust pollution directly accounted for 2400 deaths and annually and nearly 3000 hospital admissions for respiratory and cardiac related diseases at a cost of US$19 billion (Krishovko B.A., et al, 2008). More recently, Vermeulen et al, (2014) also concluded that Diesel Exhaust Emissions (DEE) at levels common in the workplace and in outdoor air appear to be prone to substantial excess lifetime risks of lung cancer above the usually acceptable limits in the United States and Europe. 
If these general statements reflect a serious community challenge one wonders how much more serious is the challenge for those responsible for the health and welfare of miners employed in underground mines where diesel powered equipment predominate. 
Responsible management of workplaces, regardless of the industry, is a risk management exercise and whilst the total elimination of risk is unrealistic, management is expected to have control strategies in place to reduce the risk to levels "as low as is reasonably practicable" (ALARP). 

Health Hazard associated with Diesel Exhausts 

Whereas the health effects of inhaling diesel exhaust products have been investigated and commented on over many decades the scale of the challenge became much clearer in 2012 following two significant events. 
In March of that year the National Institute for Occupational Safety and Health (NIOSH) and the National Cancer Institute (NCI) published their 20-year "Diesel Exhaust in Miners Study" report.  
This study involved a cohort mortality study of 12,315 mineworkers exposed to diesel exhaust at eight US non-metal mines.  
This study indicated a strong relationship between the level of exposure to diesel exhaust and risk of lung cancer mortality.  
At higher exposures to diesel exhaust, the mortality rates were about 3 to 5 times greater compared to those workers who had the lowest exposures (Attfield MD., et al, 2012). 
Soon thereafter a panel of experts, convened by the World Health Organisation's (WHO) "International Agency for Research on Cancer" (IARDC), changed the categorisation of diesel exhaust from Group 2A to "Carcinogenic to Humans, Group 1", based on the evidence of a number of epidemiological and toxicological studies carried out over the previous decades. 

The Nature of the Challenge 

The exhaust products from diesel engines include a mixture of high temperature carbonaceous particles (Soot) and gases, including oxides of carbon, nitrogen and sulphur as well as a wide range of Volatile Organic Compounds (VOCs). In the older, less efficient engines, more carbonaceous material or soot particles were produced which, whilst undesirable, served a very useful purpose in adsorbing much of the toxic gases on their surface. They also provided a very useful surface for the condensed gases which formulated in the ambient dilution zone beyond the tailpipe exit (Uhrner U., et al, 2011.). 
As the newer more efficient engines, with various after-treatment devices, came on stream the physical and chemical dynamics occurring in the dilution plume changed. With greatly reduced discharge of emitted primary carbonaceous particles the cooling of the hot gases not only caused condensation but, as they became super-saturated, the VOCs etc., nucleate and in so doing produce a new range of very fine particulates (Kittelson and Abdul- Khalek, 1999. Kittelson, et al., 2002. Ning et al, 2010. Kittelson and Kraft, 2014.)  
Indeed, researchers investigating such conditions have clearly identified a tri-modal distribution of particulates (Kittelson and Kraft, 2014) as shown in Fig 1.  
Nucleation mode: Particulates composed mainly of volatile organic and sulphur compounds which typically have aerodynamic diameters less than 50 nm with maximum concentrations occurring in the range 10-20 nm. These particulates typically make up less than 10% of the particle mass but more than 90% of the total particle number. 
Accumulation mode: Sub-micron particulates consisting mainly of carbonaceous agglomerates with condensates and adsorbed material having aerodynamic diameters ranging from 50-500 nm and maximum concentrations occurring between 100 and 200 nm. This mode contains most of the particle mass. 
Coarse mode: Soot particles with aerodynamic diameters in excess of one micron (1000 nm). 
 
Fig 2 demonstrates the likely sequence of particle formation taking place as the exhaust products move along the tailpipe and thereafter into the dilution and cooling plume in the ambient air for a heavy duty diesel engine in cruise conditions (Kittleson D, et al 2014). Within the dilution zone there is competition between the nucleation of ultrafine particulates and the adsorption and/or condensation processes with fast dilution and cooling in the presence of low concentrations of accumulation mode soot particles favouring nucleation (Ning et al, 2010). 
The nucleation mode particles due to their high number concentration, surface area, size and toxicity present the most serious health hazard since their ability to be transported to the lower levels of the lung and penetrate the epithelium make them the major cause of adverse health effects. 
Whereas high nanoparticle emissions are not a new development, what is new and unexpected is that nanoparticle emissions may not decrease along with mass emissions, but may actually increase. 

Mine Ventilation Systems. 

One of, if not the most important, requirement in the planning and management of underground mines is the provision of safe atmospheres within which workers can breathe and work. In some jurisdictions this requires each production unit within the mine to have its own supply of fresh air obtained from the main intake airway. Such systems, referred to as "Parallel Ventilation", ensure the pollutants generated in each section are returned directly to the main return airway and are isolated from other sections in the mine.  
However, Western Australian metal mines predominantly use the "Series Ventilation" system whereby the ventilating air traverses each section of the mine in sequence with the worst atmospheric conditions being experienced at the lowest point in the circuit where the atmosphere contains all the additive pollutants generated at all upstream locations. 
In a recent paper (Brake D.J., 2012) the author presents an even-handed comparison of both systems listing their advantages as well as emphasising the need to ensure that the risk of injury or illness to mineworkers must be within acceptable limits and be "as low as is reasonably practicable". 

Health Risks to Mineworkers 

The following statements made by Dr Patrick Glynn in recent articles (Glynn P., 2011, Walker S., 2014) highlights the challenges facing those responsible for the health and welfare of miners in WA's Underground Hard Rock Mines:  
"With the aim of reducing DPM mass, engine manufacturers have improved the combustion efficiency of diesel engines by the introduction of common rail and turbocharging". However, "an unwanted outcome of this development is the increase in the number of diesel particulates with a more than 50% reduction in average diesel particulate size. This reduction in DPM size is of particular concern as larger DPM (less than 2.5 microns) coated with polyaromatic hydrocarbons (a known carcinogen) will affect a minority of the population whereas the smaller (less than 100 nm) DPM can cross the lung membrane barrier into the bloodstream. This has the potential for health effects to 100% of the population."  
As stated by Ning et al, (2010) retrofitted vehicles with diesel particulate filters and other treatment control devices greatly reduce the emitted primary soot particles.  
However, this reduction in particle surface area facilitates the formation of nucleation mode particulates from the organic vapours under favourable temperature and dilution conditions. 
The scale of the problem is magnified many times over in our "Series Ventilated" underground hard rock mines by virtue of the fact that: 
· The materials handling of waste rock and ore is typically carried out in the intake airway by LHD's (up to 330 kW) loading diesel trucks (up to 600 kW) transporting rock/ore out of the mine for the full shift and running back empty ready for another full load. 
· All ancillary equipment such as jumbo drill rigs and LHDs etc., are also diesel powered and in regular use at various levels in the mine. 
Consequently it is not difficult to appreciate the compounding and additive effect of the diesel exhaust products from all these items of equipment as the intake air continues on its path from the mine portal to the lower levels of the mine.  

Exposure Metrics 

It has become increasingly clear to researchers that the current metric in use for assessing the level of DPM in underground workplaces based on mass of elemental carbon per unit volume (mg/m3) is inadequate.  
Other relevant factors such as size distribution of the particulates, their total surface area and their composition appear to be much more relevant in assessing the health hazards associated with the inhalation of DPM., Wierzbicka et al, (2014). 
As stated earlier ultrafine particulates can make up 90% of the total number of particulates in diesel exhausts' ambient dilution plumes and in general terms the smaller the particle inhaled the greater the health risk. As the average size of particles decrease their surface area per unit mass increases, thus making them more biologically active and allowing greater exposure of their surface chemistry to the alveoli tissue in the lung and their subsequent uptake across the lung membrane into the blood stream and lymph circulation enabling them to reach such sensitive targets as the lymph nodes, spleen, heart and possibly the central nervous system.  
Whereas current guidelines require the mass of elemental carbon to be measured, with the maximum level of exposure set at 0.1 mg/m3 for an 8 hour shift, both mass, particulate/aerosol size, number and surface area should be measured to better assess the adverse health impact of ultrafine aerosols in underground metal and non-metal mines ( Bugarski and Timko, 2007), not forgetting the gas phase components that also have carcinogenic and irritation effects. Ultimately it may be possible to develop a new metric based on a "Lung Deposited Dose" for exposure groups Wierzbicka et al, (2014). 

Nitrogen Dioxide Issue 

Underground miners working with diesel powered equipment can potentially be exposed to a range of gases in addition to diesel particulates and the ventilation rates must always be sufficient to dilute these gases to below the threshold limit values (TLV). These gases include carbon monoxide, carbon dioxide and oxides of nitrogen and of these nitric oxide (NO) and nitrogen dioxide (NO2) along with diesel particulates are the most difficult engine emission products to control. 
Both NO and NO2 can cause short-term (acute) and long-term (chronic) health problems, mostly pulmonary and cardiac related (Cauda E.G., et al 2005). Of these NO2 is extremely toxic and its Time Weighted Average (TWA) for an eight hour shift was recently reduced by the American Conference of Government Industrial Hygienists (ACGIH) from 3 ppm to 0.2 ppm. In high concentrations NO2 forms nitric acid in the lungs causing pulmonary oedema.  
The after-treatment diesel exhaust devices usually include a filtration system to remove the particulates and an oxidation catalyst (DOC) to promote oxidation of Carbon Monoxide (CO), gas phase Hydrocarbons (HC) and the organic fraction of diesel particulates to harmless oxidised products. 
Oxidation catalysts whilst promoting oxidation of all compounds can generate unwanted products such as sulphuric acid from Sulphur Dioxide (SO2) which can nucleate as sulphate particulates and the more toxic NO2 from the oxidation of NO. Both these outcomes are highly undesirable and warrant further investigation. 
The European Commission's "Scientific Committee on Occupational Exposure limits for NO2" (SCOEL, 2014) recommends and 8 – hour TWA of 0.5 ppm (0.955 mg/m3) and a STEL of 1 ppm (1.91 mg/m3). As stated in a recent article ( Walker S, 2014) "hence the need for a better understanding of the conditions under which NO2 is produced during diesel-engine operation, with the aim of developing effective control strategies and technologies that are applicable to machines working underground". 
It is of course necessary to adjust the standard 8-hour/5-day week TWA Standards for non-standard shifts and rosters. Whilst there are numerous models available for doing so, the simplest and perhaps the most conservative one is the "Brief and Scala' model (1975) and the Reduction Factor (RF) for the case of a seven day working week is:  
RF = 40/h x ((168 – h)/128)  
Where: 
  • h = average hours worked per shift 
  • 168 – h = exposure free hours per week 
  • Risk Reduction 
Bearing in mind the potential health hazard to underground miners of consistent and long-term inhalation of diesel exhaust products surely the matter is deserving of a well-structured and organised industry-wide "Risk Assessment". This would normally involve a team of qualified and experienced personnel carrying out a HAZOP or similar analysis with the intention of identifying existing controls and those which should be adopted along with a list of recommended management actions, auditing procedures and timetable for implementation.  
Needless to say, to be successful, a whole mine approach would be required taking into account mining operations, maintenance practices, fuel quality, ventilation provisions, monitoring of air quality and the provision of well-structured education and training programs. In this regard the Resources Safety Division of the Western Australian Department of Mines and Petroleum (DMP) has through its Mining Industry Advisory Council published a useful and informative set of Guidelines; not to mention the New South Wales' MDG 29 and Queensland's QGN 21 guidelines which are equally informative and helpful.   
The hierarchy of controls worthy of consideration include: 
  • Replacement of diesel powered plant and equipment with electrically operated units. 
  • Use of lower emission fuel e.g. use ultra-low sulphur fuel (< 10 ppm) and/or biodiesel/blends instead of petroleum diesel. 
  • Improving the mine ventilation system, possibly using ventilation on demand (VOD) and/or use of alternative ventilation systems at workplaces and workshops etc. 
  • Isolate personnel as far as is humanly possible in enclosed pressure-controlled cabs and use respiratory protection for those working in high emission environments. 
  • Keep up to date with relevant and recent research work and consider full-scale trials of newly developed systems such as the combined CSIRO Acoustic Agglomeration system and efficient exhaust catalyser.  
  • Ensure high standard of education and training for management staff and mineworkers. 
 

Conclusions 

Bearing in mind the seriousness of this complex issue and its potential to adversely affect the health and wellbeing of underground hard-rock mineworkers where diesel powered equipment predominate, and whilst acknowledging the existence of well-structured "Diesel Emissions Management Plans" at most mines, a thorough review of the atmospheric quality conditions currently prevailing in a representative sample of our underground mines would be a good first step because an effective strategy to deal with the challenge can only be based on the facts.  
In this regard reference should be made to the Nanotechnology Safety and Health Research Program at the National Institute for Occupational Safety and Health (NIOSH), which has involved full scale studies characterising the physical, chemical and toxicological properties of diesel aerosols in an occupational setting at NIOSH's Lake Lynn Laboratory experimental mine in Pennsylvania (Bugarski and Timko, 2007). 
Thereafter the industry should take ownership of the challenge by fostering and financing well-targeted research as well as developing, in collaboration with equipment manufacturers and the various State Government Mines' Departments etc., appropriate Codes of Practice and Guidelines for the Ventilation and Air Quality Control in Underground Hard-rock Mines where diesel powered equipment predominate with such outcomes being embedded within the new "Harmonised Work Health and Safety Regulations". 
 

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