Examining and mitigating the risks to workers exposed to harmful emissions
The global challenge of dealing with the health effects of exposure to diesel exhaust is wide-ranging and encompasses regional health authorities concerned about persons living and/or working in close proximity to freeways and major thoroughfares, as well as corporations employing personnel who work alongside diesel powered equipment. Whilst accepting evidence of such serious community concerns one wonders how much more serious is the challenge for those working in our underground mines where diesel powered equipment predominate.
In this article, I will discuss an ongoing epidemiological study sponsored by the National Health and Medical Research Council (NHMRC) of the incidence of lung cancer contracted by mineworkers and their exposure to diesel exhaust which is being obtained from data held by the health authorities and the Western Australian Department of Mines and Petroleum. The principal aim of this study is to investigate the possible link between the two. The article will also attempt to review current knowledge of the nature of the challenge facing those responsible for the mining industry in Western Australia.
The scale and seriousness of the challenge became much clearer in 2012 following two significant events. Firstly in March of that year the National Institute for Occupational Health (NIOSH) and the National Cancer Institute (NCI) published their 20-year ‘Diesel Exhaust in Miners Study’ report which 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 and risk of lung cancer mortality to such an extent that the mortality rates for those at higher exposures were 3 to 5 times greater compared to those who had the lowest exposures (Attfield et al, 2012).
Secondly and soon thereafter the World Health Organisation’s (WHO) International Agency for Research on Cancer (IARC) 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 previous decades.
‘Diesel Exhaust Exposure in Western Australian (WA) Mines and Lung Cancer Risk’
This is an NHMRC sponsored epidemiological study involving miners employed in WA mines from 1996 to 2012 led by Dr Susan Peters, Research Fellow University of Western Australia.
The research team consists of national and international experts with specialised skills in epidemiology, clinical medicine, industrial hygiene and biostatistics, with access to a body of professionals in an Advisory Committee with knowledge of the mining industry, lung cancer and diesel exhaust.
It is the first such study carried out in Australia and its objectives include:
- estimating the risk of lung cancer among miners
- estimating current levels of exposure to diesel exhaust in WA mines
- investigating the exposure-response relationship between diesel exhaust and lung cancer for the full exposure range adjusting for various confounding risk factors such as smoking and exposure to asbestos, etc
- estimating the number of lung cancers attributable to occupational exposure each year.
Although there was an extensive body of evidence that convinced the IARC to re-categorise exposure to diesel exhaust as carcinogenic to humans, some questions still remain. For example, there were almost no female workers included in the US studies which raises the issue of the lung cancer risk among women exposed to diesel exhaust.
The researchers have ready access to a series of unique databases in WA which provide a fertile basis for this study. These include:
- health surveillance database of WA miners from 1996 to 2012 (n = approx 245 000; 12 per cent female)
- demographic characteristics and smoking habits
- WA cancer registry and mortality records
- CONTAM database of the Department of Mining and Petroleum containing results of diesel exposure monitoring from WA mines since 2003.
This research project is of three years duration, and it is anticipated the study will provide the WA mining industry with sound evidence to formulate a strategy for managing, ameliorating and/or overcoming the potential health effects to mine workers exposed to diesel exhaust.
Occasionally, the design of modern technology can solve one problem yet create another unexpected and unintended consequence. Whereas modern diesel engines with various after-treatment devices are significantly more efficient and produce less soot than in the past, their exhaust products have become more toxic and hazardous. The primary reason for this is the significant reduction in the discharge of carbonaceous (soot) particles which, although produced in larger quantities in the older engines, served the very useful purpose of adsorbing much of the toxic gases on their surface (Uhrner et al, 2011). Consequently the physical and chemical dynamics occurring in the dilution plume beyond the tailpipe with the newer engines is significantly different with the cooling of the discharged hot gases not only causing condensation on various surfaces, but as they become super-saturated the volatile organic compounds (VOCs) nucleate to form a new range of ultra-fine nanoparticles.
A number of researchers have recently reported on this increasing health hazard associated with the newer diesel engines.
These particles, due to their high number concentration, surface area, size and toxicity, present a very serious health hazard due to their ability to reach the deeper lung tissues and to thereafter penetrate the blood stream, thereby causing a range of adverse health effects.
A number of researchers have recently reported on this increasing health hazard associated with the newer diesel engines. Dr Patrick Glynn of CSIRO has stated that the newer diesel engine technology has resulted in ‘an unwanted outcome increasing the number of diesel particulates with more than 50 per cent reduction in average diesel particulate size. This reduction in DPM [diesel particulate matter] size is of particular concern as larger DPM coated with poly-aromatic hydrocarbons (known carcinogens) 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 on 100 per cent of the population’ (Glynn, 2011; Walker, 2014).
As Jones (2015) points out, the scale of the challenge is magnified many times over in Western Australia’s series-ventilated underground hard-rock mines where:
- the materials transport of ore and waste rock is carried out in the intake airways by diesel powered LHDs (up to 330 kW) loading diesel trucks (up to 600 kW) transporting ore/waste rock out of the mine and returning empty for another load
- other items of diesel powered equipment such as jumbo drill rigs and personnel carriers etc are also in regular use at various levels in the mine.
Consequently, the compounding and additive effect of the diesel exhaust products from these various sources creates an increasingly polluted atmosphere as the airstream proceeds along the intake airway to the lower levels of the mine where, normally, the major ore extraction occurs.
The primary objective of mine ventilation design should be to provide mine workers with the best possible quality of air. The options available to designers are either series or parallel ventilation, notwithstanding various hybrid systems. The advantages of parallel ventilation are paramount because the system is based on the provision of fresh air to each major production district. Due to the variability of most orebodies in location, extent and direction it is often easier and more cost effective to use series ventilation, albeit it has the disadvantage of decreased air quality with increased distance from the mine portal – especially so if transportation of waste rock and ore is carried out in the main intake airway.
In a recent paper (Brake, 2012) the author presents an even-handed account of both systems listing their advantages and disadvantages. Brake also emphasises the need to ensure the risk of illness to mine workers must be kept ‘as low as is reasonable practical (ALARP)’.
Bearing in mind the predominance of series ventilation at most metal mines in Western Australia, and the challenge of coping with diesel exhaust pollution, it is fair to ask how can this ALARP objective be achieved? On the face of it the easiest and most cost-effective means of alleviating the challenge of diesel exhaust pollution in our operating mines would appear to be the reversal of the ventilation system, whereby the main haulage decline becomes the main return airway. The fresh air from the forcing fan would be taken directly to the deepest production unit which otherwise endures the worst levels of particulate, gaseous and heat pollution. However, if the other production/development units continue to be ventilated primarily by the return air stream the challenge is only partially overcome.
However, retrofitting existing mines in this way has its downsides and includes challenges associated with re-entry requirements following blasting and the preference to have major infrastructure items, such as electrical transformers, in the clean main intake airway.
It should also be acknowledged that the more progressive mining companies, realising the imperative of maintaining good quality ventilation throughout the mine, have adopted a hybrid series/parallel ventilation system. Such hybrid systems may include two or three parallel circuits, each one servicing a number of production and/or development headings ventilated in series. Although such systems offer major improvements, they still must overcome the challenge of coping with the main intake airway also being the main decline haulage where diesel powered vehicles continuously traverse.
However, there is good reason for encouraging those designing new decline-access metal mines to think seriously of utilising electrically powered equipment in preference to their diesel powered equivalent. If this is not possible or preferable then every effort should be made to ensure that diesel powered trucks operate in the main return airway with fresh air access available to each parallel ventilation section from the main intake airway.
Diesel Emissions Management Plans
All underground mines in Western Australia where personnel are exposed to diesel emissions are encouraged by the Department of Mines and Petroleum to have in place ‘Diesel Emissions Management Plans’ based on the identification of sources, identification of transmission and dilution of contaminants, and strategies to minimise exposure and monitoring to ensure effective controls. Within such plans the focus is on new equipment utilising or conforming to Tier 3 engine specifications. In addition emphasis is placed on the use of ultra-low sulphur diesel fuel (ULSD), low sulphur lubricants, high-quality maintenance strategies, the use of air filters and their regular replacement and strong advice against excessive idling and throttling of equipment.
Such plans also stipulate time-weighted average (TWA) and short term exposure limit (STEL) gas exposure limits for CO, CO2, NO, NO2 and SO2, as well as the DPM exposure limit guideline of 0.1 mg/m3 for an 8 day 40 hour week. It also stipulates the adjusted values for DPM exposures for non-standard shifts and/or rosters.
While such plans and procedures are laudable they unfortunately miss addressing the challenges associated with the increasing production of ultra-fine particulates with the newer engines.
Health risks and exposure metrics
It has become increasingly clear that the current metric in use for assessing acceptable levels of DPM in underground mines based on the mass of elemental carbon per unit volume of ambient air is inadequate. Other relevant factors such as size distribution of particulates, their total surface area and composition appear to be much more relevant in assessing the health hazards of DPM in mine atmospheres (Wierzbicka et al, 2014).
Ultrafine particulates can account for over 90 per cent of the total number of particulates released into the ambient air, and the smaller the particle inhaled, the greater the health risk. As the size of particles decrease, their surface area per unit mass increases, making them more biologically active. This allows greater exposure of their surface chemistry to the lung’s alveoli tissue and their subsequent transfer across the lung membrane into the blood stream and lymph system. Following this, they may reach such sensitive targets as the lymph nodes, spleen, heart and even the central
The potential health hazard to underground miners of consistent and long-term exposure to diesel exhaust surely warrants a well-structured industry wide risk assessment.
Whereas the current guidelines require the mass of elemental carbon to be kept below 0.1 mg/m3 for an 8 hour shift, it is increasingly evident that in addition to mass, the particulate/aerosol size and surface area should be measured in order to better assess the adverse health impact of nano-particulates and ultrafine aerosols on underground mineworkers (Bugarski and Timko, 2007). Attention is now starting to be focussed on the use of the ‘Lung Deposited Surface Area’ (LDSA) dose as the preferred metric for defining the health hazard of inhaling diesel exhaust fumes.
Nitrogen dioxide issue
Diesel powered equipment discharge both carbonaceous particulates and a range of gases into the ambient air, and ideally the ventilation rates should be sufficient to dilute the discharged pollutants to below the threshold limit values (TLVs). Of these gases two of them are particularly difficult to control and manage. Both nitric oxide (NO) and nitrogen dioxide (NO2) present a serious challenge since both can cause short-term (acute) and long-term (chronic) health problems, mostly pulmonary and cardiac related (Caudia et al, 2005). Of these, NO2 is extremely toxic and its TWA for an eight hour shift was recently reduced by the American Conference of Government Hygienists (ACGIH) from 3ppm to 0.2 ppm. In high concentrations it can cause pulmonary oedema.
As stated earlier engineers must adjust the standard 8 hour/5 day week standards for non-standard shifts and rosters and whilst there are numerous models available for doing so, the simplest and perhaps the most conservative one is the ‘Brief and Scala’ model (1975) which provides a reduction factor for adjusting the TWA standard. See Figure 1.
Appraisal of current challenges and risk reduction controls
As previously recommended, (Jones, 2015) the potential health hazard to underground miners of consistent and long-term exposure to diesel exhaust surely warrants a well-structured industry wide risk assessment. However, before doing so it would be essential to have some factual evidence of the nature and scale of the challenges currently existing in our underground mines. Such evidence can only be achieved by monitoring the nature and extent of the situation prevailing at carefully identified locations in the primary ‘materials handling’ intake airway, including those sites where there is a predominance of miners exposed to diesel exhaust.
Thankfully we have some indication of how to approach this task from previous work by NIOSH personnel in monitoring the quality of the atmospheres in occupational settings at their NIOSH Lake Lynn Diesel Laboratory in an underground limestone experimental mine in Pennsylvania, USA.
In a paper describing such a recent study (Bugarski and Timko, 2007) reference is made to the fact that ‘the primary constituents of DPM are elemental carbon (EC), numerous organic carbon species (including some that are known carcinogens), sulfates and transitional metals. A large portion of these aerosols fall into the nanometer (less than 50 nm) and ultrafine (less than 100 nm) size ranges (Kittleson, 1998) which comprise more than 90 per cent of the total particle number and surface area.’
The NIOSH Diesel Laboratory field tests were carried out in a drift approximately 530 m long, 6 m wide and 2 m high in an established limestone mine which incorporated a dynamometer/engine system, three sampling and measurement stations, and a ventilation measurement control system. Ambient concentrations of selected gases and aerosols were sampled in the mine air some 60 m upstream and downstream of the dynamometer. The downstream station is designed to record aerosol concentrations, carbon monoxide (CO), carbon dioxide (CO2), nitric oxide (NO), nitrogen dioxide (NO2), sulphur dioxide (SO2), and total hydrocarbons (HC). At both stations ambient concentrations of total particulate matter was monitored using a Tapered Element Oscillating Microbalance (TEOM) whereas the downstream aerosol size distribution and count concentration was measured by a Scanning Mobility Particle Sizer (SMPS) capable of recording aerosols of between 10 and 408 nm (Bugarski et al, 2008 ). A schematic view of the laboratory layout is shown in Figure 2.
These tests indicated a wide range of interesting findings, such as that the increase in percentages of NO2 in total NOx resulting from the use of DOC was strongly dependent on engine operation mode/exhaust temperature. As the authors indicate, such in-mine investigations should contribute greatly to the development of technical control mechanisms for reducing the exposure of underground miners to nano- and ultrafine particulates without potential increase in exposures to NO2.
To be successful in overcoming these challenges, a whole mine approach would be required including taking into account of mining operations, maintenance procedures, fuel quality, ventilation provisions, monitoring protocols and well-organised education and training programs. In this regard due recognition must be accorded to the advice and guidance provided in publications produced by the various State Mines Departments including the ‘Guideline for management of diesel emissions in Western Australian mining operations’ produced by the Minerals Industry Advisory Council (MIAC) of the Western Australian Department of Mines and Petroleum, not to mention the equally informative Guidelines of New South Wales’ MDG 29 and Queensland’s QGN 21.
Included in these and other documents references are made to a hierarchy of controls such as:
- replacement of diesel powered plant and equipment with electrically operated equivalents
- use of lower emission fuel eg ultra-low sulphur fuel and/or bio-diesel blends
- improving mine ventilation systems eg using the main return airway for material transport and/or use Ventilation on Demand to improve localised conditions when appropriate
- isolate personnel wherever possible in enclosed pressure controlled cabs or alternatively use respiratory protection for those working in high exhaust emission environments
- keep up to date with relevant/recent research outcomes and consider full-scale trials of newly developed systems such as CSIRO’s Acoustic Agglomeration system and Exhaust Catalyser
- ensure the highest level of communication with, and education and training of, all employees.
As indicated throughout this article, there is a growing body of evidence indicating that the inhalation of diesel exhaust products can adversely and possibly seriously affect human health, and this must especially be the case for those employed in underground hard-rock mines where diesel equipment predominate.
Consequently, it is suggested that in parallel with the ongoing NHMRC sponsored epidemiological study, a thorough review of the atmospheric conditions prevailing in Western Australian underground mines would be an appropriate first step to assess the scale of the challenge we face. We know the equipment is available as indicated by NIOSH’s research work at its experimental mine and the recent laboratory tests carried out at Queensland University of Technology. Consequently, it is up to the industry to take ownership of this challenge and in doing so lead the way to demonstrate, once again, that safety and cost-effective mining can always be complementary, providing the management strategy is soundly evidence based.
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